3 Basic Usage (with examples) for each of the Yocto Tracing Tools


This chapter presents basic usage examples for each of the tracing tools.

3.1 perf

The ‘perf’ tool is the profiling and tracing tool that comes bundled with the Linux kernel.

Don’t let the fact that it’s part of the kernel fool you into thinking that it’s only for tracing and profiling the kernel - you can indeed use it to trace and profile just the kernel, but you can also use it to profile specific applications separately (with or without kernel context), and you can also use it to trace and profile the kernel and all applications on the system simultaneously to gain a system-wide view of what’s going on.

In many ways, perf aims to be a superset of all the tracing and profiling tools available in Linux today, including all the other tools covered in this HOWTO. The past couple of years have seen perf subsume a lot of the functionality of those other tools and, at the same time, those other tools have removed large portions of their previous functionality and replaced it with calls to the equivalent functionality now implemented by the perf subsystem. Extrapolation suggests that at some point those other tools will simply become completely redundant and go away; until then, we’ll cover those other tools in these pages and in many cases show how the same things can be accomplished in perf and the other tools when it seems useful to do so.

The coverage below details some of the most common ways you’ll likely want to apply the tool; full documentation can be found either within the tool itself or in the man pages at perf(1).

3.1.1 Perf Setup

For this section, we’ll assume you’ve already performed the basic setup outlined in the “General Setup” section.

In particular, you’ll get the most mileage out of perf if you profile an image built with the following in your local.conf file:

INHIBIT_PACKAGE_STRIP = "1"

perf runs on the target system for the most part. You can archive profile data and copy it to the host for analysis, but for the rest of this document we assume you’ve ssh’ed to the host and will be running the perf commands on the target.

3.1.2 Basic Perf Usage

The perf tool is pretty much self-documenting. To remind yourself of the available commands, simply type ‘perf’, which will show you basic usage along with the available perf subcommands:

root@crownbay:~# perf

usage: perf [--version] [--help] COMMAND [ARGS]

The most commonly used perf commands are:
  annotate        Read perf.data (created by perf record) and display annotated code
  archive         Create archive with object files with build-ids found in perf.data file
  bench           General framework for benchmark suites
  buildid-cache   Manage build-id cache.
  buildid-list    List the buildids in a perf.data file
  diff            Read two perf.data files and display the differential profile
  evlist          List the event names in a perf.data file
  inject          Filter to augment the events stream with additional information
  kmem            Tool to trace/measure kernel memory(slab) properties
  kvm             Tool to trace/measure kvm guest os
  list            List all symbolic event types
  lock            Analyze lock events
  probe           Define new dynamic tracepoints
  record          Run a command and record its profile into perf.data
  report          Read perf.data (created by perf record) and display the profile
  sched           Tool to trace/measure scheduler properties (latencies)
  script          Read perf.data (created by perf record) and display trace output
  stat            Run a command and gather performance counter statistics
  test            Runs sanity tests.
  timechart       Tool to visualize total system behavior during a workload
  top             System profiling tool.

See 'perf help COMMAND' for more information on a specific command.

3.1.2.1 Using perf to do Basic Profiling

As a simple test case, we’ll profile the ‘wget’ of a fairly large file, which is a minimally interesting case because it has both file and network I/O aspects, and at least in the case of standard Yocto images, it’s implemented as part of BusyBox, so the methods we use to analyze it can be used in a very similar way to the whole host of supported BusyBox applets in Yocto.

root@crownbay:~# rm linux-2.6.19.2.tar.bz2; \
                 wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2

The quickest and easiest way to get some basic overall data about what’s going on for a particular workload is to profile it using ‘perf stat’. ‘perf stat’ basically profiles using a few default counters and displays the summed counts at the end of the run:

root@crownbay:~# perf stat wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |***************************************************| 41727k  0:00:00 ETA

Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2':

      4597.223902 task-clock                #    0.077 CPUs utilized
            23568 context-switches          #    0.005 M/sec
               68 CPU-migrations            #    0.015 K/sec
              241 page-faults               #    0.052 K/sec
       3045817293 cycles                    #    0.663 GHz
  <not supported> stalled-cycles-frontend
  <not supported> stalled-cycles-backend
        858909167 instructions              #    0.28  insns per cycle
        165441165 branches                  #   35.987 M/sec
         19550329 branch-misses             #   11.82% of all branches

     59.836627620 seconds time elapsed

Many times such a simple-minded test doesn’t yield much of interest, but sometimes it does (see Real-world Yocto bug (slow loop-mounted write speed)).

Also, note that ‘perf stat’ isn’t restricted to a fixed set of counters - basically any event listed in the output of ‘perf list’ can be tallied by ‘perf stat’. For example, suppose we wanted to see a summary of all the events related to kernel memory allocation/freeing along with cache hits and misses:

root@crownbay:~# perf stat -e kmem:* -e cache-references -e cache-misses wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |***************************************************| 41727k  0:00:00 ETA

Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2':

             5566 kmem:kmalloc
           125517 kmem:kmem_cache_alloc
                0 kmem:kmalloc_node
                0 kmem:kmem_cache_alloc_node
            34401 kmem:kfree
            69920 kmem:kmem_cache_free
              133 kmem:mm_page_free
               41 kmem:mm_page_free_batched
            11502 kmem:mm_page_alloc
            11375 kmem:mm_page_alloc_zone_locked
                0 kmem:mm_page_pcpu_drain
                0 kmem:mm_page_alloc_extfrag
         66848602 cache-references
          2917740 cache-misses              #    4.365 % of all cache refs

     44.831023415 seconds time elapsed

So ‘perf stat’ gives us a nice easy way to get a quick overview of what might be happening for a set of events, but normally we’d need a little more detail in order to understand what’s going on in a way that we can act on in a useful way.

To dive down into a next level of detail, we can use ‘perf record’/’perf report’ which will collect profiling data and present it to use using an interactive text-based UI (or simply as text if we specify –stdio to ‘perf report’).

As our first attempt at profiling this workload, we’ll simply run ‘perf record’, handing it the workload we want to profile (everything after ‘perf record’ and any perf options we hand it - here none - will be executed in a new shell). perf collects samples until the process exits and records them in a file named ‘perf.data’ in the current working directory.

root@crownbay:~# perf record wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2

Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |************************************************| 41727k  0:00:00 ETA
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.176 MB perf.data (~7700 samples) ]

To see the results in a ‘text-based UI’ (tui), simply run ‘perf report’, which will read the perf.data file in the current working directory and display the results in an interactive UI:

root@crownbay:~# perf report
../_images/perf-wget-flat-stripped.png

The above screenshot displays a ‘flat’ profile, one entry for each ‘bucket’ corresponding to the functions that were profiled during the profiling run, ordered from the most popular to the least (perf has options to sort in various orders and keys as well as display entries only above a certain threshold and so on - see the perf documentation for details). Note that this includes both userspace functions (entries containing a [.]) and kernel functions accounted to the process (entries containing a [k]). (perf has command-line modifiers that can be used to restrict the profiling to kernel or userspace, among others).

Notice also that the above report shows an entry for ‘busybox’, which is the executable that implements ‘wget’ in Yocto, but that instead of a useful function name in that entry, it displays a not-so-friendly hex value instead. The steps below will show how to fix that problem.

Before we do that, however, let’s try running a different profile, one which shows something a little more interesting. The only difference between the new profile and the previous one is that we’ll add the -g option, which will record not just the address of a sampled function, but the entire callchain to the sampled function as well:

root@crownbay:~# perf record -g wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% |************************************************| 41727k  0:00:00 ETA
[ perf record: Woken up 3 times to write data ]
[ perf record: Captured and wrote 0.652 MB perf.data (~28476 samples) ]


root@crownbay:~# perf report
../_images/perf-wget-g-copy-to-user-expanded-stripped.png

Using the callgraph view, we can actually see not only which functions took the most time, but we can also see a summary of how those functions were called and learn something about how the program interacts with the kernel in the process.

Notice that each entry in the above screenshot now contains a ‘+’ on the left-hand side. This means that we can expand the entry and drill down into the callchains that feed into that entry. Pressing ‘enter’ on any one of them will expand the callchain (you can also press ‘E’ to expand them all at the same time or ‘C’ to collapse them all).

In the screenshot above, we’ve toggled the __copy_to_user_ll() entry and several subnodes all the way down. This lets us see which callchains contributed to the profiled __copy_to_user_ll() function which contributed 1.77% to the total profile.

As a bit of background explanation for these callchains, think about what happens at a high level when you run wget to get a file out on the network. Basically what happens is that the data comes into the kernel via the network connection (socket) and is passed to the userspace program ‘wget’ (which is actually a part of BusyBox, but that’s not important for now), which takes the buffers the kernel passes to it and writes it to a disk file to save it.

The part of this process that we’re looking at in the above call stacks is the part where the kernel passes the data it has read from the socket down to wget i.e. a copy-to-user.

Notice also that here there’s also a case where the hex value is displayed in the callstack, here in the expanded sys_clock_gettime() function. Later we’ll see it resolve to a userspace function call in busybox.

../_images/perf-wget-g-copy-from-user-expanded-stripped.png

The above screenshot shows the other half of the journey for the data - from the wget program’s userspace buffers to disk. To get the buffers to disk, the wget program issues a write(2), which does a copy-from-user to the kernel, which then takes care via some circuitous path (probably also present somewhere in the profile data), to get it safely to disk.

Now that we’ve seen the basic layout of the profile data and the basics of how to extract useful information out of it, let’s get back to the task at hand and see if we can get some basic idea about where the time is spent in the program we’re profiling, wget. Remember that wget is actually implemented as an applet in BusyBox, so while the process name is ‘wget’, the executable we’re actually interested in is BusyBox. So let’s expand the first entry containing BusyBox:

../_images/perf-wget-busybox-expanded-stripped.png

Again, before we expanded we saw that the function was labeled with a hex value instead of a symbol as with most of the kernel entries. Expanding the BusyBox entry doesn’t make it any better.

The problem is that perf can’t find the symbol information for the busybox binary, which is actually stripped out by the Yocto build system.

One way around that is to put the following in your local.conf file when you build the image:

INHIBIT_PACKAGE_STRIP = "1"

However, we already have an image with the binaries stripped, so what can we do to get perf to resolve the symbols? Basically we need to install the debuginfo for the BusyBox package.

To generate the debug info for the packages in the image, we can add dbg-pkgs to EXTRA_IMAGE_FEATURES in local.conf. For example:

EXTRA_IMAGE_FEATURES = "debug-tweaks tools-profile dbg-pkgs"

Additionally, in order to generate the type of debuginfo that perf understands, we also need to set PACKAGE_DEBUG_SPLIT_STYLE in the local.conf file:

PACKAGE_DEBUG_SPLIT_STYLE = 'debug-file-directory'

Once we’ve done that, we can install the debuginfo for BusyBox. The debug packages once built can be found in build/tmp/deploy/rpm/* on the host system. Find the busybox-dbg-…rpm file and copy it to the target. For example:

[trz@empanada core2]$ scp /home/trz/yocto/crownbay-tracing-dbg/build/tmp/deploy/rpm/core2_32/busybox-dbg-1.20.2-r2.core2_32.rpm root@192.168.1.31:
busybox-dbg-1.20.2-r2.core2_32.rpm                     100% 1826KB   1.8MB/s   00:01

Now install the debug rpm on the target:

root@crownbay:~# rpm -i busybox-dbg-1.20.2-r2.core2_32.rpm

Now that the debuginfo is installed, we see that the BusyBox entries now display their functions symbolically:

../_images/perf-wget-busybox-debuginfo.png

If we expand one of the entries and press ‘enter’ on a leaf node, we’re presented with a menu of actions we can take to get more information related to that entry:

../_images/perf-wget-busybox-dso-zoom-menu.png

One of these actions allows us to show a view that displays a busybox-centric view of the profiled functions (in this case we’ve also expanded all the nodes using the ‘E’ key):

../_images/perf-wget-busybox-dso-zoom.png

Finally, we can see that now that the BusyBox debuginfo is installed, the previously unresolved symbol in the sys_clock_gettime() entry mentioned previously is now resolved, and shows that the sys_clock_gettime system call that was the source of 6.75% of the copy-to-user overhead was initiated by the handle_input() BusyBox function:

../_images/perf-wget-g-copy-to-user-expanded-debuginfo.png

At the lowest level of detail, we can dive down to the assembly level and see which instructions caused the most overhead in a function. Pressing ‘enter’ on the ‘udhcpc_main’ function, we’re again presented with a menu:

../_images/perf-wget-busybox-annotate-menu.png

Selecting ‘Annotate udhcpc_main’, we get a detailed listing of percentages by instruction for the udhcpc_main function. From the display, we can see that over 50% of the time spent in this function is taken up by a couple tests and the move of a constant (1) to a register:

../_images/perf-wget-busybox-annotate-udhcpc.png

As a segue into tracing, let’s try another profile using a different counter, something other than the default ‘cycles’.

The tracing and profiling infrastructure in Linux has become unified in a way that allows us to use the same tool with a completely different set of counters, not just the standard hardware counters that traditional tools have had to restrict themselves to (of course the traditional tools can also make use of the expanded possibilities now available to them, and in some cases have, as mentioned previously).

We can get a list of the available events that can be used to profile a workload via ‘perf list’:

root@crownbay:~# perf list

List of pre-defined events (to be used in -e):
 cpu-cycles OR cycles                               [Hardware event]
 stalled-cycles-frontend OR idle-cycles-frontend    [Hardware event]
 stalled-cycles-backend OR idle-cycles-backend      [Hardware event]
 instructions                                       [Hardware event]
 cache-references                                   [Hardware event]
 cache-misses                                       [Hardware event]
 branch-instructions OR branches                    [Hardware event]
 branch-misses                                      [Hardware event]
 bus-cycles                                         [Hardware event]
 ref-cycles                                         [Hardware event]

 cpu-clock                                          [Software event]
 task-clock                                         [Software event]
 page-faults OR faults                              [Software event]
 minor-faults                                       [Software event]
 major-faults                                       [Software event]
 context-switches OR cs                             [Software event]
 cpu-migrations OR migrations                       [Software event]
 alignment-faults                                   [Software event]
 emulation-faults                                   [Software event]

 L1-dcache-loads                                    [Hardware cache event]
 L1-dcache-load-misses                              [Hardware cache event]
 L1-dcache-prefetch-misses                          [Hardware cache event]
 L1-icache-loads                                    [Hardware cache event]
 L1-icache-load-misses                              [Hardware cache event]
 .
 .
 .
 rNNN                                               [Raw hardware event descriptor]
 cpu/t1=v1[,t2=v2,t3 ...]/modifier                  [Raw hardware event descriptor]
  (see 'perf list --help' on how to encode it)

 mem:<addr>[:access]                                [Hardware breakpoint]

 sunrpc:rpc_call_status                             [Tracepoint event]
 sunrpc:rpc_bind_status                             [Tracepoint event]
 sunrpc:rpc_connect_status                          [Tracepoint event]
 sunrpc:rpc_task_begin                              [Tracepoint event]
 skb:kfree_skb                                      [Tracepoint event]
 skb:consume_skb                                    [Tracepoint event]
 skb:skb_copy_datagram_iovec                        [Tracepoint event]
 net:net_dev_xmit                                   [Tracepoint event]
 net:net_dev_queue                                  [Tracepoint event]
 net:netif_receive_skb                              [Tracepoint event]
 net:netif_rx                                       [Tracepoint event]
 napi:napi_poll                                     [Tracepoint event]
 sock:sock_rcvqueue_full                            [Tracepoint event]
 sock:sock_exceed_buf_limit                         [Tracepoint event]
 udp:udp_fail_queue_rcv_skb                         [Tracepoint event]
 hda:hda_send_cmd                                   [Tracepoint event]
 hda:hda_get_response                               [Tracepoint event]
 hda:hda_bus_reset                                  [Tracepoint event]
 scsi:scsi_dispatch_cmd_start                       [Tracepoint event]
 scsi:scsi_dispatch_cmd_error                       [Tracepoint event]
 scsi:scsi_eh_wakeup                                [Tracepoint event]
 drm:drm_vblank_event                               [Tracepoint event]
 drm:drm_vblank_event_queued                        [Tracepoint event]
 drm:drm_vblank_event_delivered                     [Tracepoint event]
 random:mix_pool_bytes                              [Tracepoint event]
 random:mix_pool_bytes_nolock                       [Tracepoint event]
 random:credit_entropy_bits                         [Tracepoint event]
 gpio:gpio_direction                                [Tracepoint event]
 gpio:gpio_value                                    [Tracepoint event]
 block:block_rq_abort                               [Tracepoint event]
 block:block_rq_requeue                             [Tracepoint event]
 block:block_rq_issue                               [Tracepoint event]
 block:block_bio_bounce                             [Tracepoint event]
 block:block_bio_complete                           [Tracepoint event]
 block:block_bio_backmerge                          [Tracepoint event]
 .
 .
 writeback:writeback_wake_thread                    [Tracepoint event]
 writeback:writeback_wake_forker_thread             [Tracepoint event]
 writeback:writeback_bdi_register                   [Tracepoint event]
 .
 .
 writeback:writeback_single_inode_requeue           [Tracepoint event]
 writeback:writeback_single_inode                   [Tracepoint event]
 kmem:kmalloc                                       [Tracepoint event]
 kmem:kmem_cache_alloc                              [Tracepoint event]
 kmem:mm_page_alloc                                 [Tracepoint event]
 kmem:mm_page_alloc_zone_locked                     [Tracepoint event]
 kmem:mm_page_pcpu_drain                            [Tracepoint event]
 kmem:mm_page_alloc_extfrag                         [Tracepoint event]
 vmscan:mm_vmscan_kswapd_sleep                      [Tracepoint event]
 vmscan:mm_vmscan_kswapd_wake                       [Tracepoint event]
 vmscan:mm_vmscan_wakeup_kswapd                     [Tracepoint event]
 vmscan:mm_vmscan_direct_reclaim_begin              [Tracepoint event]
 .
 .
 module:module_get                                  [Tracepoint event]
 module:module_put                                  [Tracepoint event]
 module:module_request                              [Tracepoint event]
 sched:sched_kthread_stop                           [Tracepoint event]
 sched:sched_wakeup                                 [Tracepoint event]
 sched:sched_wakeup_new                             [Tracepoint event]
 sched:sched_process_fork                           [Tracepoint event]
 sched:sched_process_exec                           [Tracepoint event]
 sched:sched_stat_runtime                           [Tracepoint event]
 rcu:rcu_utilization                                [Tracepoint event]
 workqueue:workqueue_queue_work                     [Tracepoint event]
 workqueue:workqueue_execute_end                    [Tracepoint event]
 signal:signal_generate                             [Tracepoint event]
 signal:signal_deliver                              [Tracepoint event]
 timer:timer_init                                   [Tracepoint event]
 timer:timer_start                                  [Tracepoint event]
 timer:hrtimer_cancel                               [Tracepoint event]
 timer:itimer_state                                 [Tracepoint event]
 timer:itimer_expire                                [Tracepoint event]
 irq:irq_handler_entry                              [Tracepoint event]
 irq:irq_handler_exit                               [Tracepoint event]
 irq:softirq_entry                                  [Tracepoint event]
 irq:softirq_exit                                   [Tracepoint event]
 irq:softirq_raise                                  [Tracepoint event]
 printk:console                                     [Tracepoint event]
 task:task_newtask                                  [Tracepoint event]
 task:task_rename                                   [Tracepoint event]
 syscalls:sys_enter_socketcall                      [Tracepoint event]
 syscalls:sys_exit_socketcall                       [Tracepoint event]
 .
 .
 .
 syscalls:sys_enter_unshare                         [Tracepoint event]
 syscalls:sys_exit_unshare                          [Tracepoint event]
 raw_syscalls:sys_enter                             [Tracepoint event]
 raw_syscalls:sys_exit                              [Tracepoint event]

Tying it Together

These are exactly the same set of events defined by the trace event subsystem and exposed by ftrace/tracecmd/kernelshark as files in /sys/kernel/debug/tracing/events, by SystemTap as kernel.trace(“tracepoint_name”) and (partially) accessed by LTTng.

Only a subset of these would be of interest to us when looking at this workload, so let’s choose the most likely subsystems (identified by the string before the colon in the Tracepoint events) and do a ‘perf stat’ run using only those wildcarded subsystems:

root@crownbay:~# perf stat -e skb:* -e net:* -e napi:* -e sched:* -e workqueue:* -e irq:* -e syscalls:* wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
Performance counter stats for 'wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2':

            23323 skb:kfree_skb
                0 skb:consume_skb
            49897 skb:skb_copy_datagram_iovec
             6217 net:net_dev_xmit
             6217 net:net_dev_queue
             7962 net:netif_receive_skb
                2 net:netif_rx
             8340 napi:napi_poll
                0 sched:sched_kthread_stop
                0 sched:sched_kthread_stop_ret
             3749 sched:sched_wakeup
                0 sched:sched_wakeup_new
                0 sched:sched_switch
               29 sched:sched_migrate_task
                0 sched:sched_process_free
                1 sched:sched_process_exit
                0 sched:sched_wait_task
                0 sched:sched_process_wait
                0 sched:sched_process_fork
                1 sched:sched_process_exec
                0 sched:sched_stat_wait
    2106519415641 sched:sched_stat_sleep
                0 sched:sched_stat_iowait
        147453613 sched:sched_stat_blocked
      12903026955 sched:sched_stat_runtime
                0 sched:sched_pi_setprio
             3574 workqueue:workqueue_queue_work
             3574 workqueue:workqueue_activate_work
                0 workqueue:workqueue_execute_start
                0 workqueue:workqueue_execute_end
            16631 irq:irq_handler_entry
            16631 irq:irq_handler_exit
            28521 irq:softirq_entry
            28521 irq:softirq_exit
            28728 irq:softirq_raise
                1 syscalls:sys_enter_sendmmsg
                1 syscalls:sys_exit_sendmmsg
                0 syscalls:sys_enter_recvmmsg
                0 syscalls:sys_exit_recvmmsg
               14 syscalls:sys_enter_socketcall
               14 syscalls:sys_exit_socketcall
                  .
                  .
                  .
            16965 syscalls:sys_enter_read
            16965 syscalls:sys_exit_read
            12854 syscalls:sys_enter_write
            12854 syscalls:sys_exit_write
                  .
                  .
                  .

     58.029710972 seconds time elapsed

Let’s pick one of these tracepoints and tell perf to do a profile using it as the sampling event:

root@crownbay:~# perf record -g -e sched:sched_wakeup wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
../_images/sched-wakeup-profile.png

The screenshot above shows the results of running a profile using sched:sched_switch tracepoint, which shows the relative costs of various paths to sched_wakeup (note that sched_wakeup is the name of the tracepoint - it’s actually defined just inside ttwu_do_wakeup(), which accounts for the function name actually displayed in the profile:

/*
 * Mark the task runnable and perform wakeup-preemption.
 */
static void
ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
{
     trace_sched_wakeup(p, true);
     .
     .
     .
}

A couple of the more interesting callchains are expanded and displayed above, basically some network receive paths that presumably end up waking up wget (busybox) when network data is ready.

Note that because tracepoints are normally used for tracing, the default sampling period for tracepoints is 1 i.e. for tracepoints perf will sample on every event occurrence (this can be changed using the -c option). This is in contrast to hardware counters such as for example the default ‘cycles’ hardware counter used for normal profiling, where sampling periods are much higher (in the thousands) because profiling should have as low an overhead as possible and sampling on every cycle would be prohibitively expensive.

3.1.2.2 Using perf to do Basic Tracing

Profiling is a great tool for solving many problems or for getting a high-level view of what’s going on with a workload or across the system. It is however by definition an approximation, as suggested by the most prominent word associated with it, ‘sampling’. On the one hand, it allows a representative picture of what’s going on in the system to be cheaply taken, but on the other hand, that cheapness limits its utility when that data suggests a need to ‘dive down’ more deeply to discover what’s really going on. In such cases, the only way to see what’s really going on is to be able to look at (or summarize more intelligently) the individual steps that go into the higher-level behavior exposed by the coarse-grained profiling data.

As a concrete example, we can trace all the events we think might be applicable to our workload:

root@crownbay:~# perf record -g -e skb:* -e net:* -e napi:* -e sched:sched_switch -e sched:sched_wakeup -e irq:*
 -e syscalls:sys_enter_read -e syscalls:sys_exit_read -e syscalls:sys_enter_write -e syscalls:sys_exit_write
 wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2

We can look at the raw trace output using ‘perf script’ with no arguments:

root@crownbay:~# perf script

      perf  1262 [000] 11624.857082: sys_exit_read: 0x0
      perf  1262 [000] 11624.857193: sched_wakeup: comm=migration/0 pid=6 prio=0 success=1 target_cpu=000
      wget  1262 [001] 11624.858021: softirq_raise: vec=1 [action=TIMER]
      wget  1262 [001] 11624.858074: softirq_entry: vec=1 [action=TIMER]
      wget  1262 [001] 11624.858081: softirq_exit: vec=1 [action=TIMER]
      wget  1262 [001] 11624.858166: sys_enter_read: fd: 0x0003, buf: 0xbf82c940, count: 0x0200
      wget  1262 [001] 11624.858177: sys_exit_read: 0x200
      wget  1262 [001] 11624.858878: kfree_skb: skbaddr=0xeb248d80 protocol=0 location=0xc15a5308
      wget  1262 [001] 11624.858945: kfree_skb: skbaddr=0xeb248000 protocol=0 location=0xc15a5308
      wget  1262 [001] 11624.859020: softirq_raise: vec=1 [action=TIMER]
      wget  1262 [001] 11624.859076: softirq_entry: vec=1 [action=TIMER]
      wget  1262 [001] 11624.859083: softirq_exit: vec=1 [action=TIMER]
      wget  1262 [001] 11624.859167: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
      wget  1262 [001] 11624.859192: sys_exit_read: 0x1d7
      wget  1262 [001] 11624.859228: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
      wget  1262 [001] 11624.859233: sys_exit_read: 0x0
      wget  1262 [001] 11624.859573: sys_enter_read: fd: 0x0003, buf: 0xbf82c580, count: 0x0200
      wget  1262 [001] 11624.859584: sys_exit_read: 0x200
      wget  1262 [001] 11624.859864: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
      wget  1262 [001] 11624.859888: sys_exit_read: 0x400
      wget  1262 [001] 11624.859935: sys_enter_read: fd: 0x0003, buf: 0xb7720000, count: 0x0400
      wget  1262 [001] 11624.859944: sys_exit_read: 0x400

This gives us a detailed timestamped sequence of events that occurred within the workload with respect to those events.

In many ways, profiling can be viewed as a subset of tracing - theoretically, if you have a set of trace events that’s sufficient to capture all the important aspects of a workload, you can derive any of the results or views that a profiling run can.

Another aspect of traditional profiling is that while powerful in many ways, it’s limited by the granularity of the underlying data. Profiling tools offer various ways of sorting and presenting the sample data, which make it much more useful and amenable to user experimentation, but in the end it can’t be used in an open-ended way to extract data that just isn’t present as a consequence of the fact that conceptually, most of it has been thrown away.

Full-blown detailed tracing data does however offer the opportunity to manipulate and present the information collected during a tracing run in an infinite variety of ways.

Another way to look at it is that there are only so many ways that the ‘primitive’ counters can be used on their own to generate interesting output; to get anything more complicated than simple counts requires some amount of additional logic, which is typically very specific to the problem at hand. For example, if we wanted to make use of a ‘counter’ that maps to the value of the time difference between when a process was scheduled to run on a processor and the time it actually ran, we wouldn’t expect such a counter to exist on its own, but we could derive one called say ‘wakeup_latency’ and use it to extract a useful view of that metric from trace data. Likewise, we really can’t figure out from standard profiling tools how much data every process on the system reads and writes, along with how many of those reads and writes fail completely. If we have sufficient trace data, however, we could with the right tools easily extract and present that information, but we’d need something other than pre-canned profiling tools to do that.

Luckily, there is a general-purpose way to handle such needs, called ‘programming languages’. Making programming languages easily available to apply to such problems given the specific format of data is called a ‘programming language binding’ for that data and language. Perf supports two programming language bindings, one for Python and one for Perl.

Tying it Together

Language bindings for manipulating and aggregating trace data are of course not a new idea. One of the first projects to do this was IBM’s DProbes dpcc compiler, an ANSI C compiler which targeted a low-level assembly language running on an in-kernel interpreter on the target system. This is exactly analogous to what Sun’s DTrace did, except that DTrace invented its own language for the purpose. Systemtap, heavily inspired by DTrace, also created its own one-off language, but rather than running the product on an in-kernel interpreter, created an elaborate compiler-based machinery to translate its language into kernel modules written in C.

Now that we have the trace data in perf.data, we can use ‘perf script -g’ to generate a skeleton script with handlers for the read/write entry/exit events we recorded:

root@crownbay:~# perf script -g python
generated Python script: perf-script.py

The skeleton script simply creates a python function for each event type in the perf.data file. The body of each function simply prints the event name along with its parameters. For example:

def net__netif_rx(event_name, context, common_cpu,
       common_secs, common_nsecs, common_pid, common_comm,
       skbaddr, len, name):
               print_header(event_name, common_cpu, common_secs, common_nsecs,
                       common_pid, common_comm)

               print "skbaddr=%u, len=%u, name=%s\n" % (skbaddr, len, name),

We can run that script directly to print all of the events contained in the perf.data file:

root@crownbay:~# perf script -s perf-script.py

in trace_begin
syscalls__sys_exit_read     0 11624.857082795     1262 perf                  nr=3, ret=0
sched__sched_wakeup      0 11624.857193498     1262 perf                  comm=migration/0, pid=6, prio=0,      success=1, target_cpu=0
irq__softirq_raise       1 11624.858021635     1262 wget                  vec=TIMER
irq__softirq_entry       1 11624.858074075     1262 wget                  vec=TIMER
irq__softirq_exit        1 11624.858081389     1262 wget                  vec=TIMER
syscalls__sys_enter_read     1 11624.858166434     1262 wget                  nr=3, fd=3, buf=3213019456,      count=512
syscalls__sys_exit_read     1 11624.858177924     1262 wget                  nr=3, ret=512
skb__kfree_skb           1 11624.858878188     1262 wget                  skbaddr=3945041280,           location=3243922184, protocol=0
skb__kfree_skb           1 11624.858945608     1262 wget                  skbaddr=3945037824,      location=3243922184, protocol=0
irq__softirq_raise       1 11624.859020942     1262 wget                  vec=TIMER
irq__softirq_entry       1 11624.859076935     1262 wget                  vec=TIMER
irq__softirq_exit        1 11624.859083469     1262 wget                  vec=TIMER
syscalls__sys_enter_read     1 11624.859167565     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
syscalls__sys_exit_read     1 11624.859192533     1262 wget                  nr=3, ret=471
syscalls__sys_enter_read     1 11624.859228072     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
syscalls__sys_exit_read     1 11624.859233707     1262 wget                  nr=3, ret=0
syscalls__sys_enter_read     1 11624.859573008     1262 wget                  nr=3, fd=3, buf=3213018496,      count=512
syscalls__sys_exit_read     1 11624.859584818     1262 wget                  nr=3, ret=512
syscalls__sys_enter_read     1 11624.859864562     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
syscalls__sys_exit_read     1 11624.859888770     1262 wget                  nr=3, ret=1024
syscalls__sys_enter_read     1 11624.859935140     1262 wget                  nr=3, fd=3, buf=3077701632,      count=1024
syscalls__sys_exit_read     1 11624.859944032     1262 wget                  nr=3, ret=1024

That in itself isn’t very useful; after all, we can accomplish pretty much the same thing by simply running ‘perf script’ without arguments in the same directory as the perf.data file.

We can however replace the print statements in the generated function bodies with whatever we want, and thereby make it infinitely more useful.

As a simple example, let’s just replace the print statements in the function bodies with a simple function that does nothing but increment a per-event count. When the program is run against a perf.data file, each time a particular event is encountered, a tally is incremented for that event. For example:

def net__netif_rx(event_name, context, common_cpu,
       common_secs, common_nsecs, common_pid, common_comm,
       skbaddr, len, name):
           inc_counts(event_name)

Each event handler function in the generated code is modified to do this. For convenience, we define a common function called inc_counts() that each handler calls; inc_counts() simply tallies a count for each event using the ‘counts’ hash, which is a specialized hash function that does Perl-like autovivification, a capability that’s extremely useful for kinds of multi-level aggregation commonly used in processing traces (see perf’s documentation on the Python language binding for details):

counts = autodict()

def inc_counts(event_name):
       try:
               counts[event_name] += 1
       except TypeError:
               counts[event_name] = 1

Finally, at the end of the trace processing run, we want to print the result of all the per-event tallies. For that, we use the special ‘trace_end()’ function:

def trace_end():
       for event_name, count in counts.iteritems():
               print "%-40s %10s\n" % (event_name, count)

The end result is a summary of all the events recorded in the trace:

skb__skb_copy_datagram_iovec                  13148
irq__softirq_entry                             4796
irq__irq_handler_exit                          3805
irq__softirq_exit                              4795
syscalls__sys_enter_write                      8990
net__net_dev_xmit                               652
skb__kfree_skb                                 4047
sched__sched_wakeup                            1155
irq__irq_handler_entry                         3804
irq__softirq_raise                             4799
net__net_dev_queue                              652
syscalls__sys_enter_read                      17599
net__netif_receive_skb                         1743
syscalls__sys_exit_read                       17598
net__netif_rx                                     2
napi__napi_poll                                1877
syscalls__sys_exit_write                       8990

Note that this is pretty much exactly the same information we get from ‘perf stat’, which goes a little way to support the idea mentioned previously that given the right kind of trace data, higher-level profiling-type summaries can be derived from it.

Documentation on using the ‘perf script’ python binding.

3.1.2.3 System-Wide Tracing and Profiling

The examples so far have focused on tracing a particular program or workload - in other words, every profiling run has specified the program to profile in the command-line e.g. ‘perf record wget …’.

It’s also possible, and more interesting in many cases, to run a system-wide profile or trace while running the workload in a separate shell.

To do system-wide profiling or tracing, you typically use the -a flag to ‘perf record’.

To demonstrate this, open up one window and start the profile using the -a flag (press Ctrl-C to stop tracing):

root@crownbay:~# perf record -g -a
^C[ perf record: Woken up 6 times to write data ]
[ perf record: Captured and wrote 1.400 MB perf.data (~61172 samples) ]

In another window, run the wget test:

root@crownbay:~# wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% \|*******************************\| 41727k 0:00:00 ETA

Here we see entries not only for our wget load, but for other processes running on the system as well:

../_images/perf-systemwide.png

In the snapshot above, we can see callchains that originate in libc, and a callchain from Xorg that demonstrates that we’re using a proprietary X driver in userspace (notice the presence of ‘PVR’ and some other unresolvable symbols in the expanded Xorg callchain).

Note also that we have both kernel and userspace entries in the above snapshot. We can also tell perf to focus on userspace but providing a modifier, in this case ‘u’, to the ‘cycles’ hardware counter when we record a profile:

root@crownbay:~# perf record -g -a -e cycles:u
^C[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 0.376 MB perf.data (~16443 samples) ]
../_images/perf-report-cycles-u.png

Notice in the screenshot above, we see only userspace entries ([.])

Finally, we can press ‘enter’ on a leaf node and select the ‘Zoom into DSO’ menu item to show only entries associated with a specific DSO. In the screenshot below, we’ve zoomed into the ‘libc’ DSO which shows all the entries associated with the libc-xxx.so DSO.

../_images/perf-systemwide-libc.png

We can also use the system-wide -a switch to do system-wide tracing. Here we’ll trace a couple of scheduler events:

root@crownbay:~# perf record -a -e sched:sched_switch -e sched:sched_wakeup
^C[ perf record: Woken up 38 times to write data ]
[ perf record: Captured and wrote 9.780 MB perf.data (~427299 samples) ]

We can look at the raw output using ‘perf script’ with no arguments:

root@crownbay:~# perf script

           perf  1383 [001]  6171.460045: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1383 [001]  6171.460066: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
    kworker/1:1    21 [001]  6171.460093: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120
        swapper     0 [000]  6171.468063: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000
        swapper     0 [000]  6171.468107: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
    kworker/0:3  1209 [000]  6171.468143: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
           perf  1383 [001]  6171.470039: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1383 [001]  6171.470058: sched_switch: prev_comm=perf prev_pid=1383 prev_prio=120 prev_state=R+ ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
    kworker/1:1    21 [001]  6171.470082: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=perf next_pid=1383 next_prio=120
           perf  1383 [001]  6171.480035: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
3.1.2.3.1 Filtering

Notice that there are a lot of events that don’t really have anything to do with what we’re interested in, namely events that schedule ‘perf’ itself in and out or that wake perf up. We can get rid of those by using the ‘–filter’ option - for each event we specify using -e, we can add a –filter after that to filter out trace events that contain fields with specific values:

root@crownbay:~# perf record -a -e sched:sched_switch --filter 'next_comm != perf && prev_comm != perf' -e sched:sched_wakeup --filter 'comm != perf'
^C[ perf record: Woken up 38 times to write data ]
[ perf record: Captured and wrote 9.688 MB perf.data (~423279 samples) ]


root@crownbay:~# perf script

        swapper     0 [000]  7932.162180: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
    kworker/0:3  1209 [000]  7932.162236: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120
           perf  1407 [001]  7932.170048: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1407 [001]  7932.180044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1407 [001]  7932.190038: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1407 [001]  7932.200044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1407 [001]  7932.210044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
           perf  1407 [001]  7932.220044: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
        swapper     0 [001]  7932.230111: sched_wakeup: comm=kworker/1:1 pid=21 prio=120 success=1 target_cpu=001
        swapper     0 [001]  7932.230146: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/1:1 next_pid=21 next_prio=120
    kworker/1:1    21 [001]  7932.230205: sched_switch: prev_comm=kworker/1:1 prev_pid=21 prev_prio=120 prev_state=S ==> next_comm=swapper/1 next_pid=0 next_prio=120
        swapper     0 [000]  7932.326109: sched_wakeup: comm=kworker/0:3 pid=1209 prio=120 success=1 target_cpu=000
        swapper     0 [000]  7932.326171: sched_switch: prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/0:3 next_pid=1209 next_prio=120
    kworker/0:3  1209 [000]  7932.326214: sched_switch: prev_comm=kworker/0:3 prev_pid=1209 prev_prio=120 prev_state=S ==> next_comm=swapper/0 next_pid=0 next_prio=120

In this case, we’ve filtered out all events that have ‘perf’ in their ‘comm’ or ‘comm_prev’ or ‘comm_next’ fields. Notice that there are still events recorded for perf, but notice that those events don’t have values of ‘perf’ for the filtered fields. To completely filter out anything from perf will require a bit more work, but for the purpose of demonstrating how to use filters, it’s close enough.

Tying it Together

These are exactly the same set of event filters defined by the trace event subsystem. See the ftrace/tracecmd/kernelshark section for more discussion about these event filters.

Tying it Together

These event filters are implemented by a special-purpose pseudo-interpreter in the kernel and are an integral and indispensable part of the perf design as it relates to tracing. kernel-based event filters provide a mechanism to precisely throttle the event stream that appears in user space, where it makes sense to provide bindings to real programming languages for postprocessing the event stream. This architecture allows for the intelligent and flexible partitioning of processing between the kernel and user space. Contrast this with other tools such as SystemTap, which does all of its processing in the kernel and as such requires a special project-defined language in order to accommodate that design, or LTTng, where everything is sent to userspace and as such requires a super-efficient kernel-to-userspace transport mechanism in order to function properly. While perf certainly can benefit from for instance advances in the design of the transport, it doesn’t fundamentally depend on them. Basically, if you find that your perf tracing application is causing buffer I/O overruns, it probably means that you aren’t taking enough advantage of the kernel filtering engine.

3.1.2.4 Using Dynamic Tracepoints

perf isn’t restricted to the fixed set of static tracepoints listed by ‘perf list’. Users can also add their own ‘dynamic’ tracepoints anywhere in the kernel. For instance, suppose we want to define our own tracepoint on do_fork(). We can do that using the ‘perf probe’ perf subcommand:

root@crownbay:~# perf probe do_fork
Added new event:
  probe:do_fork        (on do_fork)

You can now use it in all perf tools, such as:

  perf record -e probe:do_fork -aR sleep 1

Adding a new tracepoint via ‘perf probe’ results in an event with all the expected files and format in /sys/kernel/debug/tracing/events, just the same as for static tracepoints (as discussed in more detail in the trace events subsystem section:

root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# ls -al
drwxr-xr-x    2 root     root             0 Oct 28 11:42 .
drwxr-xr-x    3 root     root             0 Oct 28 11:42 ..
-rw-r--r--    1 root     root             0 Oct 28 11:42 enable
-rw-r--r--    1 root     root             0 Oct 28 11:42 filter
-r--r--r--    1 root     root             0 Oct 28 11:42 format
-r--r--r--    1 root     root             0 Oct 28 11:42 id

root@crownbay:/sys/kernel/debug/tracing/events/probe/do_fork# cat format
name: do_fork
ID: 944
format:
        field:unsigned short common_type;    offset:0;       size:2; signed:0;
        field:unsigned char common_flags;    offset:2;       size:1; signed:0;
        field:unsigned char common_preempt_count;    offset:3;       size:1; signed:0;
        field:int common_pid;        offset:4;       size:4; signed:1;
        field:int common_padding;    offset:8;       size:4; signed:1;

        field:unsigned long __probe_ip;      offset:12;      size:4; signed:0;

print fmt: "(%lx)", REC->__probe_ip

We can list all dynamic tracepoints currently in existence:

root@crownbay:~# perf probe -l
 probe:do_fork (on do_fork)
 probe:schedule (on schedule)

Let’s record system-wide (‘sleep 30’ is a trick for recording system-wide but basically do nothing and then wake up after 30 seconds):

root@crownbay:~# perf record -g -a -e probe:do_fork sleep 30
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.087 MB perf.data (~3812 samples) ]

Using ‘perf script’ we can see each do_fork event that fired:

root@crownbay:~# perf script

# ========
# captured on: Sun Oct 28 11:55:18 2012
# hostname : crownbay
# os release : 3.4.11-yocto-standard
# perf version : 3.4.11
# arch : i686
# nrcpus online : 2
# nrcpus avail : 2
# cpudesc : Intel(R) Atom(TM) CPU E660 @ 1.30GHz
# cpuid : GenuineIntel,6,38,1
# total memory : 1017184 kB
# cmdline : /usr/bin/perf record -g -a -e probe:do_fork sleep 30
# event : name = probe:do_fork, type = 2, config = 0x3b0, config1 = 0x0, config2 = 0x0, excl_usr = 0, excl_kern
 = 0, id = { 5, 6 }
# HEADER_CPU_TOPOLOGY info available, use -I to display
# ========
#
 matchbox-deskto  1197 [001] 34211.378318: do_fork: (c1028460)
 matchbox-deskto  1295 [001] 34211.380388: do_fork: (c1028460)
         pcmanfm  1296 [000] 34211.632350: do_fork: (c1028460)
         pcmanfm  1296 [000] 34211.639917: do_fork: (c1028460)
 matchbox-deskto  1197 [001] 34217.541603: do_fork: (c1028460)
 matchbox-deskto  1299 [001] 34217.543584: do_fork: (c1028460)
          gthumb  1300 [001] 34217.697451: do_fork: (c1028460)
          gthumb  1300 [001] 34219.085734: do_fork: (c1028460)
          gthumb  1300 [000] 34219.121351: do_fork: (c1028460)
          gthumb  1300 [001] 34219.264551: do_fork: (c1028460)
         pcmanfm  1296 [000] 34219.590380: do_fork: (c1028460)
 matchbox-deskto  1197 [001] 34224.955965: do_fork: (c1028460)
 matchbox-deskto  1306 [001] 34224.957972: do_fork: (c1028460)
 matchbox-termin  1307 [000] 34225.038214: do_fork: (c1028460)
 matchbox-termin  1307 [001] 34225.044218: do_fork: (c1028460)
 matchbox-termin  1307 [000] 34225.046442: do_fork: (c1028460)
 matchbox-deskto  1197 [001] 34237.112138: do_fork: (c1028460)
 matchbox-deskto  1311 [001] 34237.114106: do_fork: (c1028460)
            gaku  1312 [000] 34237.202388: do_fork: (c1028460)

And using ‘perf report’ on the same file, we can see the callgraphs from starting a few programs during those 30 seconds:

../_images/perf-probe-do_fork-profile.png

Tying it Together

The trace events subsystem accommodate static and dynamic tracepoints in exactly the same way - there’s no difference as far as the infrastructure is concerned. See the ftrace section for more details on the trace event subsystem.

Tying it Together

Dynamic tracepoints are implemented under the covers by kprobes and uprobes. kprobes and uprobes are also used by and in fact are the main focus of SystemTap.

3.1.3 Perf Documentation

Online versions of the man pages for the commands discussed in this section can be found here:

Normally, you should be able to invoke the man pages via perf itself e.g. ‘perf help’ or ‘perf help record’.

To have the perf manpages installed on your target, modify your configuration as follows:

IMAGE_INSTALL:append = " perf perf-doc"
DISTRO_FEATURES:append = " api-documentation"

The man pages in text form, along with some other files, such as a set of examples, can also be found in the ‘perf’ directory of the kernel tree:

tools/perf/Documentation

There’s also a nice perf tutorial on the perf wiki that goes into more detail than we do here in certain areas: Perf Tutorial

3.2 ftrace

‘ftrace’ literally refers to the ‘ftrace function tracer’ but in reality this encompasses a number of related tracers along with the infrastructure that they all make use of.

3.2.1 ftrace Setup

For this section, we’ll assume you’ve already performed the basic setup outlined in the “General Setup” section.

ftrace, trace-cmd, and kernelshark run on the target system, and are ready to go out-of-the-box - no additional setup is necessary. For the rest of this section we assume you’ve ssh’ed to the host and will be running ftrace on the target. kernelshark is a GUI application and if you use the ‘-X’ option to ssh you can have the kernelshark GUI run on the target but display remotely on the host if you want.

3.2.2 Basic ftrace usage

‘ftrace’ essentially refers to everything included in the /tracing directory of the mounted debugfs filesystem (Yocto follows the standard convention and mounts it at /sys/kernel/debug). Here’s a listing of all the files found in /sys/kernel/debug/tracing on a Yocto system:

root@sugarbay:/sys/kernel/debug/tracing# ls
README                      kprobe_events               trace
available_events            kprobe_profile              trace_clock
available_filter_functions  options                     trace_marker
available_tracers           per_cpu                     trace_options
buffer_size_kb              printk_formats              trace_pipe
buffer_total_size_kb        saved_cmdlines              tracing_cpumask
current_tracer              set_event                   tracing_enabled
dyn_ftrace_total_info       set_ftrace_filter           tracing_on
enabled_functions           set_ftrace_notrace          tracing_thresh
events                      set_ftrace_pid
free_buffer                 set_graph_function

The files listed above are used for various purposes - some relate directly to the tracers themselves, others are used to set tracing options, and yet others actually contain the tracing output when a tracer is in effect. Some of the functions can be guessed from their names, others need explanation; in any case, we’ll cover some of the files we see here below but for an explanation of the others, please see the ftrace documentation.

We’ll start by looking at some of the available built-in tracers.

cat’ing the ‘available_tracers’ file lists the set of available tracers:

root@sugarbay:/sys/kernel/debug/tracing# cat available_tracers
blk function_graph function nop

The ‘current_tracer’ file contains the tracer currently in effect:

root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer
nop

The above listing of current_tracer shows that the ‘nop’ tracer is in effect, which is just another way of saying that there’s actually no tracer currently in effect.

echo’ing one of the available_tracers into current_tracer makes the specified tracer the current tracer:

root@sugarbay:/sys/kernel/debug/tracing# echo function > current_tracer
root@sugarbay:/sys/kernel/debug/tracing# cat current_tracer
function

The above sets the current tracer to be the ‘function tracer’. This tracer traces every function call in the kernel and makes it available as the contents of the ‘trace’ file. Reading the ‘trace’ file lists the currently buffered function calls that have been traced by the function tracer:

root@sugarbay:/sys/kernel/debug/tracing# cat trace | less

# tracer: function
#
# entries-in-buffer/entries-written: 310629/766471   #P:8
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
         <idle>-0     [004] d..1   470.867169: ktime_get_real <-intel_idle
         <idle>-0     [004] d..1   470.867170: getnstimeofday <-ktime_get_real
         <idle>-0     [004] d..1   470.867171: ns_to_timeval <-intel_idle
         <idle>-0     [004] d..1   470.867171: ns_to_timespec <-ns_to_timeval
         <idle>-0     [004] d..1   470.867172: smp_apic_timer_interrupt <-apic_timer_interrupt
         <idle>-0     [004] d..1   470.867172: native_apic_mem_write <-smp_apic_timer_interrupt
         <idle>-0     [004] d..1   470.867172: irq_enter <-smp_apic_timer_interrupt
         <idle>-0     [004] d..1   470.867172: rcu_irq_enter <-irq_enter
         <idle>-0     [004] d..1   470.867173: rcu_idle_exit_common.isra.33 <-rcu_irq_enter
         <idle>-0     [004] d..1   470.867173: local_bh_disable <-irq_enter
         <idle>-0     [004] d..1   470.867173: add_preempt_count <-local_bh_disable
         <idle>-0     [004] d.s1   470.867174: tick_check_idle <-irq_enter
         <idle>-0     [004] d.s1   470.867174: tick_check_oneshot_broadcast <-tick_check_idle
         <idle>-0     [004] d.s1   470.867174: ktime_get <-tick_check_idle
         <idle>-0     [004] d.s1   470.867174: tick_nohz_stop_idle <-tick_check_idle
         <idle>-0     [004] d.s1   470.867175: update_ts_time_stats <-tick_nohz_stop_idle
         <idle>-0     [004] d.s1   470.867175: nr_iowait_cpu <-update_ts_time_stats
         <idle>-0     [004] d.s1   470.867175: tick_do_update_jiffies64 <-tick_check_idle
         <idle>-0     [004] d.s1   470.867175: _raw_spin_lock <-tick_do_update_jiffies64
         <idle>-0     [004] d.s1   470.867176: add_preempt_count <-_raw_spin_lock
         <idle>-0     [004] d.s2   470.867176: do_timer <-tick_do_update_jiffies64
         <idle>-0     [004] d.s2   470.867176: _raw_spin_lock <-do_timer
         <idle>-0     [004] d.s2   470.867176: add_preempt_count <-_raw_spin_lock
         <idle>-0     [004] d.s3   470.867177: ntp_tick_length <-do_timer
         <idle>-0     [004] d.s3   470.867177: _raw_spin_lock_irqsave <-ntp_tick_length
         .
         .
         .

Each line in the trace above shows what was happening in the kernel on a given cpu, to the level of detail of function calls. Each entry shows the function called, followed by its caller (after the arrow).

The function tracer gives you an extremely detailed idea of what the kernel was doing at the point in time the trace was taken, and is a great way to learn about how the kernel code works in a dynamic sense.

Tying it Together

The ftrace function tracer is also available from within perf, as the ftrace:function tracepoint.

It is a little more difficult to follow the call chains than it needs to be - luckily there’s a variant of the function tracer that displays the callchains explicitly, called the ‘function_graph’ tracer:

root@sugarbay:/sys/kernel/debug/tracing# echo function_graph > current_tracer
root@sugarbay:/sys/kernel/debug/tracing# cat trace | less

 tracer: function_graph

 CPU  DURATION                  FUNCTION CALLS
 |     |   |                     |   |   |   |
7)   0.046 us    |      pick_next_task_fair();
7)   0.043 us    |      pick_next_task_stop();
7)   0.042 us    |      pick_next_task_rt();
7)   0.032 us    |      pick_next_task_fair();
7)   0.030 us    |      pick_next_task_idle();
7)               |      _raw_spin_unlock_irq() {
7)   0.033 us    |        sub_preempt_count();
7)   0.258 us    |      }
7)   0.032 us    |      sub_preempt_count();
7) + 13.341 us   |    } /* __schedule */
7)   0.095 us    |  } /* sub_preempt_count */
7)               |  schedule() {
7)               |    __schedule() {
7)   0.060 us    |      add_preempt_count();
7)   0.044 us    |      rcu_note_context_switch();
7)               |      _raw_spin_lock_irq() {
7)   0.033 us    |        add_preempt_count();
7)   0.247 us    |      }
7)               |      idle_balance() {
7)               |        _raw_spin_unlock() {
7)   0.031 us    |          sub_preempt_count();
7)   0.246 us    |        }
7)               |        update_shares() {
7)   0.030 us    |          __rcu_read_lock();
7)   0.029 us    |          __rcu_read_unlock();
7)   0.484 us    |        }
7)   0.030 us    |        __rcu_read_lock();
7)               |        load_balance() {
7)               |          find_busiest_group() {
7)   0.031 us    |            idle_cpu();
7)   0.029 us    |            idle_cpu();
7)   0.035 us    |            idle_cpu();
7)   0.906 us    |          }
7)   1.141 us    |        }
7)   0.022 us    |        msecs_to_jiffies();
7)               |        load_balance() {
7)               |          find_busiest_group() {
7)   0.031 us    |            idle_cpu();
.
.
.
4)   0.062 us    |        msecs_to_jiffies();
4)   0.062 us    |        __rcu_read_unlock();
4)               |        _raw_spin_lock() {
4)   0.073 us    |          add_preempt_count();
4)   0.562 us    |        }
4) + 17.452 us   |      }
4)   0.108 us    |      put_prev_task_fair();
4)   0.102 us    |      pick_next_task_fair();
4)   0.084 us    |      pick_next_task_stop();
4)   0.075 us    |      pick_next_task_rt();
4)   0.062 us    |      pick_next_task_fair();
4)   0.066 us    |      pick_next_task_idle();
------------------------------------------
4)   kworker-74   =>    <idle>-0
------------------------------------------

4)               |      finish_task_switch() {
4)               |        _raw_spin_unlock_irq() {
4)   0.100 us    |          sub_preempt_count();
4)   0.582 us    |        }
4)   1.105 us    |      }
4)   0.088 us    |      sub_preempt_count();
4) ! 100.066 us  |    }
.
.
.
3)               |  sys_ioctl() {
3)   0.083 us    |    fget_light();
3)               |    security_file_ioctl() {
3)   0.066 us    |      cap_file_ioctl();
3)   0.562 us    |    }
3)               |    do_vfs_ioctl() {
3)               |      drm_ioctl() {
3)   0.075 us    |        drm_ut_debug_printk();
3)               |        i915_gem_pwrite_ioctl() {
3)               |          i915_mutex_lock_interruptible() {
3)   0.070 us    |            mutex_lock_interruptible();
3)   0.570 us    |          }
3)               |          drm_gem_object_lookup() {
3)               |            _raw_spin_lock() {
3)   0.080 us    |              add_preempt_count();
3)   0.620 us    |            }
3)               |            _raw_spin_unlock() {
3)   0.085 us    |              sub_preempt_count();
3)   0.562 us    |            }
3)   2.149 us    |          }
3)   0.133 us    |          i915_gem_object_pin();
3)               |          i915_gem_object_set_to_gtt_domain() {
3)   0.065 us    |            i915_gem_object_flush_gpu_write_domain();
3)   0.065 us    |            i915_gem_object_wait_rendering();
3)   0.062 us    |            i915_gem_object_flush_cpu_write_domain();
3)   1.612 us    |          }
3)               |          i915_gem_object_put_fence() {
3)   0.097 us    |            i915_gem_object_flush_fence.constprop.36();
3)   0.645 us    |          }
3)   0.070 us    |          add_preempt_count();
3)   0.070 us    |          sub_preempt_count();
3)   0.073 us    |          i915_gem_object_unpin();
3)   0.068 us    |          mutex_unlock();
3)   9.924 us    |        }
3) + 11.236 us   |      }
3) + 11.770 us   |    }
3) + 13.784 us   |  }
3)               |  sys_ioctl() {

As you can see, the function_graph display is much easier to follow. Also note that in addition to the function calls and associated braces, other events such as scheduler events are displayed in context. In fact, you can freely include any tracepoint available in the trace events subsystem described in the next section by simply enabling those events, and they’ll appear in context in the function graph display. Quite a powerful tool for understanding kernel dynamics.

Also notice that there are various annotations on the left hand side of the display. For example if the total time it took for a given function to execute is above a certain threshold, an exclamation point or plus sign appears on the left hand side. Please see the ftrace documentation for details on all these fields.

3.2.3 The ‘trace events’ Subsystem

One especially important directory contained within the /sys/kernel/debug/tracing directory is the ‘events’ subdirectory, which contains representations of every tracepoint in the system. Listing out the contents of the ‘events’ subdirectory, we see mainly another set of subdirectories:

root@sugarbay:/sys/kernel/debug/tracing# cd events
root@sugarbay:/sys/kernel/debug/tracing/events# ls -al
drwxr-xr-x   38 root     root             0 Nov 14 23:19 .
drwxr-xr-x    5 root     root             0 Nov 14 23:19 ..
drwxr-xr-x   19 root     root             0 Nov 14 23:19 block
drwxr-xr-x   32 root     root             0 Nov 14 23:19 btrfs
drwxr-xr-x    5 root     root             0 Nov 14 23:19 drm
-rw-r--r--    1 root     root             0 Nov 14 23:19 enable
drwxr-xr-x   40 root     root             0 Nov 14 23:19 ext3
drwxr-xr-x   79 root     root             0 Nov 14 23:19 ext4
drwxr-xr-x   14 root     root             0 Nov 14 23:19 ftrace
drwxr-xr-x    8 root     root             0 Nov 14 23:19 hda
-r--r--r--    1 root     root             0 Nov 14 23:19 header_event
-r--r--r--    1 root     root             0 Nov 14 23:19 header_page
drwxr-xr-x   25 root     root             0 Nov 14 23:19 i915
drwxr-xr-x    7 root     root             0 Nov 14 23:19 irq
drwxr-xr-x   12 root     root             0 Nov 14 23:19 jbd
drwxr-xr-x   14 root     root             0 Nov 14 23:19 jbd2
drwxr-xr-x   14 root     root             0 Nov 14 23:19 kmem
drwxr-xr-x    7 root     root             0 Nov 14 23:19 module
drwxr-xr-x    3 root     root             0 Nov 14 23:19 napi
drwxr-xr-x    6 root     root             0 Nov 14 23:19 net
drwxr-xr-x    3 root     root             0 Nov 14 23:19 oom
drwxr-xr-x   12 root     root             0 Nov 14 23:19 power
drwxr-xr-x    3 root     root             0 Nov 14 23:19 printk
drwxr-xr-x    8 root     root             0 Nov 14 23:19 random
drwxr-xr-x    4 root     root             0 Nov 14 23:19 raw_syscalls
drwxr-xr-x    3 root     root             0 Nov 14 23:19 rcu
drwxr-xr-x    6 root     root             0 Nov 14 23:19 rpm
drwxr-xr-x   20 root     root             0 Nov 14 23:19 sched
drwxr-xr-x    7 root     root             0 Nov 14 23:19 scsi
drwxr-xr-x    4 root     root             0 Nov 14 23:19 signal
drwxr-xr-x    5 root     root             0 Nov 14 23:19 skb
drwxr-xr-x    4 root     root             0 Nov 14 23:19 sock
drwxr-xr-x   10 root     root             0 Nov 14 23:19 sunrpc
drwxr-xr-x  538 root     root             0 Nov 14 23:19 syscalls
drwxr-xr-x    4 root     root             0 Nov 14 23:19 task
drwxr-xr-x   14 root     root             0 Nov 14 23:19 timer
drwxr-xr-x    3 root     root             0 Nov 14 23:19 udp
drwxr-xr-x   21 root     root             0 Nov 14 23:19 vmscan
drwxr-xr-x    3 root     root             0 Nov 14 23:19 vsyscall
drwxr-xr-x    6 root     root             0 Nov 14 23:19 workqueue
drwxr-xr-x   26 root     root             0 Nov 14 23:19 writeback

Each one of these subdirectories corresponds to a ‘subsystem’ and contains yet again more subdirectories, each one of those finally corresponding to a tracepoint. For example, here are the contents of the ‘kmem’ subsystem:

root@sugarbay:/sys/kernel/debug/tracing/events# cd kmem
root@sugarbay:/sys/kernel/debug/tracing/events/kmem# ls -al
drwxr-xr-x   14 root     root             0 Nov 14 23:19 .
drwxr-xr-x   38 root     root             0 Nov 14 23:19 ..
-rw-r--r--    1 root     root             0 Nov 14 23:19 enable
-rw-r--r--    1 root     root             0 Nov 14 23:19 filter
drwxr-xr-x    2 root     root             0 Nov 14 23:19 kfree
drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmalloc
drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmalloc_node
drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmem_cache_alloc
drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmem_cache_alloc_node
drwxr-xr-x    2 root     root             0 Nov 14 23:19 kmem_cache_free
drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_alloc
drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_alloc_extfrag
drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_alloc_zone_locked
drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_free
drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_free_batched
drwxr-xr-x    2 root     root             0 Nov 14 23:19 mm_page_pcpu_drain

Let’s see what’s inside the subdirectory for a specific tracepoint, in this case the one for kmalloc:

root@sugarbay:/sys/kernel/debug/tracing/events/kmem# cd kmalloc
root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# ls -al
drwxr-xr-x    2 root     root             0 Nov 14 23:19 .
drwxr-xr-x   14 root     root             0 Nov 14 23:19 ..
-rw-r--r--    1 root     root             0 Nov 14 23:19 enable
-rw-r--r--    1 root     root             0 Nov 14 23:19 filter
-r--r--r--    1 root     root             0 Nov 14 23:19 format
-r--r--r--    1 root     root             0 Nov 14 23:19 id

The ‘format’ file for the tracepoint describes the event in memory, which is used by the various tracing tools that now make use of these tracepoint to parse the event and make sense of it, along with a ‘print fmt’ field that allows tools like ftrace to display the event as text. Here’s what the format of the kmalloc event looks like:

root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# cat format
name: kmalloc
ID: 313
format:
        field:unsigned short common_type;    offset:0;       size:2; signed:0;
        field:unsigned char common_flags;    offset:2;       size:1; signed:0;
        field:unsigned char common_preempt_count;    offset:3;       size:1; signed:0;
        field:int common_pid;        offset:4;       size:4; signed:1;
        field:int common_padding;    offset:8;       size:4; signed:1;

        field:unsigned long call_site;       offset:16;      size:8; signed:0;
        field:const void * ptr;      offset:24;      size:8; signed:0;
        field:size_t bytes_req;      offset:32;      size:8; signed:0;
        field:size_t bytes_alloc;    offset:40;      size:8; signed:0;
        field:gfp_t gfp_flags;       offset:48;      size:4; signed:0;

print fmt: "call_site=%lx ptr=%p bytes_req=%zu bytes_alloc=%zu gfp_flags=%s", REC->call_site, REC->ptr, REC->bytes_req, REC->bytes_alloc,
(REC->gfp_flags) ? __print_flags(REC->gfp_flags, "|", {(unsigned long)(((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
gfp_t)0x20000u) | (( gfp_t)0x02u) | (( gfp_t)0x08u)) | (( gfp_t)0x4000u) | (( gfp_t)0x10000u) | (( gfp_t)0x1000u) | (( gfp_t)0x200u) | ((
gfp_t)0x400000u)), "GFP_TRANSHUGE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x20000u) | ((
gfp_t)0x02u) | (( gfp_t)0x08u)), "GFP_HIGHUSER_MOVABLE"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
gfp_t)0x20000u) | (( gfp_t)0x02u)), "GFP_HIGHUSER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | ((
gfp_t)0x20000u)), "GFP_USER"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u) | (( gfp_t)0x80000u)), GFP_TEMPORARY"},
{(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u) | (( gfp_t)0x80u)), "GFP_KERNEL"}, {(unsigned long)((( gfp_t)0x10u) | (( gfp_t)0x40u)),
"GFP_NOFS"}, {(unsigned long)((( gfp_t)0x20u)), "GFP_ATOMIC"}, {(unsigned long)((( gfp_t)0x10u)), "GFP_NOIO"}, {(unsigned long)((
gfp_t)0x20u), "GFP_HIGH"}, {(unsigned long)(( gfp_t)0x10u), "GFP_WAIT"}, {(unsigned long)(( gfp_t)0x40u), "GFP_IO"}, {(unsigned long)((
gfp_t)0x100u), "GFP_COLD"}, {(unsigned long)(( gfp_t)0x200u), "GFP_NOWARN"}, {(unsigned long)(( gfp_t)0x400u), "GFP_REPEAT"}, {(unsigned
long)(( gfp_t)0x800u), "GFP_NOFAIL"}, {(unsigned long)(( gfp_t)0x1000u), "GFP_NORETRY"},      {(unsigned long)(( gfp_t)0x4000u), "GFP_COMP"},
{(unsigned long)(( gfp_t)0x8000u), "GFP_ZERO"}, {(unsigned long)(( gfp_t)0x10000u), "GFP_NOMEMALLOC"}, {(unsigned long)(( gfp_t)0x20000u),
"GFP_HARDWALL"}, {(unsigned long)(( gfp_t)0x40000u), "GFP_THISNODE"}, {(unsigned long)(( gfp_t)0x80000u), "GFP_RECLAIMABLE"}, {(unsigned
long)(( gfp_t)0x08u), "GFP_MOVABLE"}, {(unsigned long)(( gfp_t)0), "GFP_NOTRACK"}, {(unsigned long)(( gfp_t)0x400000u), "GFP_NO_KSWAPD"},
{(unsigned long)(( gfp_t)0x800000u), "GFP_OTHER_NODE"} ) : "GFP_NOWAIT"

The ‘enable’ file in the tracepoint directory is what allows the user (or tools such as trace-cmd) to actually turn the tracepoint on and off. When enabled, the corresponding tracepoint will start appearing in the ftrace ‘trace’ file described previously. For example, this turns on the kmalloc tracepoint:

root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 1 > enable

At the moment, we’re not interested in the function tracer or some other tracer that might be in effect, so we first turn it off, but if we do that, we still need to turn tracing on in order to see the events in the output buffer:

root@sugarbay:/sys/kernel/debug/tracing# echo nop > current_tracer
root@sugarbay:/sys/kernel/debug/tracing# echo 1 > tracing_on

Now, if we look at the ‘trace’ file, we see nothing but the kmalloc events we just turned on:

root@sugarbay:/sys/kernel/debug/tracing# cat trace | less
# tracer: nop
#
# entries-in-buffer/entries-written: 1897/1897   #P:8
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
       dropbear-1465  [000] ...1 18154.620753: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
         <idle>-0     [000] ..s3 18154.621640: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
         <idle>-0     [000] ..s3 18154.621656: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
matchbox-termin-1361  [001] ...1 18154.755472: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f0e00 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT
           Xorg-1264  [002] ...1 18154.755581: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
           Xorg-1264  [002] ...1 18154.755583: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
           Xorg-1264  [002] ...1 18154.755589: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
matchbox-termin-1361  [001] ...1 18155.354594: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db35400 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT
           Xorg-1264  [002] ...1 18155.354703: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
           Xorg-1264  [002] ...1 18155.354705: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
           Xorg-1264  [002] ...1 18155.354711: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
         <idle>-0     [000] ..s3 18155.673319: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
       dropbear-1465  [000] ...1 18155.673525: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
         <idle>-0     [000] ..s3 18155.674821: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
         <idle>-0     [000] ..s3 18155.793014: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
       dropbear-1465  [000] ...1 18155.793219: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
         <idle>-0     [000] ..s3 18155.794147: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
         <idle>-0     [000] ..s3 18155.936705: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
       dropbear-1465  [000] ...1 18155.936910: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
         <idle>-0     [000] ..s3 18155.937869: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
matchbox-termin-1361  [001] ...1 18155.953667: kmalloc: call_site=ffffffff81614050 ptr=ffff88006d5f2000 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_KERNEL|GFP_REPEAT
           Xorg-1264  [002] ...1 18155.953775: kmalloc: call_site=ffffffff8141abe8 ptr=ffff8800734f4cc0 bytes_req=168 bytes_alloc=192 gfp_flags=GFP_KERNEL|GFP_NOWARN|GFP_NORETRY
           Xorg-1264  [002] ...1 18155.953777: kmalloc: call_site=ffffffff814192a3 ptr=ffff88001f822520 bytes_req=24 bytes_alloc=32 gfp_flags=GFP_KERNEL|GFP_ZERO
           Xorg-1264  [002] ...1 18155.953783: kmalloc: call_site=ffffffff81419edb ptr=ffff8800721a2f00 bytes_req=64 bytes_alloc=64 gfp_flags=GFP_KERNEL|GFP_ZERO
         <idle>-0     [000] ..s3 18156.176053: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
       dropbear-1465  [000] ...1 18156.176257: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
         <idle>-0     [000] ..s3 18156.177717: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
         <idle>-0     [000] ..s3 18156.399229: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d555800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
       dropbear-1465  [000] ...1 18156.399434: kmalloc: call_site=ffffffff816650d4 ptr=ffff8800729c3000 bytes_http://rostedt.homelinux.com/kernelshark/req=2048 bytes_alloc=2048 gfp_flags=GFP_KERNEL
         <idle>-0     [000] ..s3 18156.400660: kmalloc: call_site=ffffffff81619b36 ptr=ffff88006d554800 bytes_req=512 bytes_alloc=512 gfp_flags=GFP_ATOMIC
matchbox-termin-1361  [001] ...1 18156.552800: kmalloc: call_site=ffffffff81614050 ptr=ffff88006db34800 bytes_req=576 bytes_alloc=1024 gfp_flags=GFP_KERNEL|GFP_REPEAT

To again disable the kmalloc event, we need to send 0 to the enable file:

root@sugarbay:/sys/kernel/debug/tracing/events/kmem/kmalloc# echo 0 > enable

You can enable any number of events or complete subsystems (by using the ‘enable’ file in the subsystem directory) and get an arbitrarily fine-grained idea of what’s going on in the system by enabling as many of the appropriate tracepoints as applicable.

A number of the tools described in this HOWTO do just that, including trace-cmd and kernelshark in the next section.

Tying it Together

These tracepoints and their representation are used not only by ftrace, but by many of the other tools covered in this document and they form a central point of integration for the various tracers available in Linux. They form a central part of the instrumentation for the following tools: perf, lttng, ftrace, blktrace and SystemTap

Tying it Together

Eventually all the special-purpose tracers currently available in /sys/kernel/debug/tracing will be removed and replaced with equivalent tracers based on the ‘trace events’ subsystem.

3.2.4 trace-cmd/kernelshark

trace-cmd is essentially an extensive command-line ‘wrapper’ interface that hides the details of all the individual files in /sys/kernel/debug/tracing, allowing users to specify specific particular events within the /sys/kernel/debug/tracing/events/ subdirectory and to collect traces and avoid having to deal with those details directly.

As yet another layer on top of that, kernelshark provides a GUI that allows users to start and stop traces and specify sets of events using an intuitive interface, and view the output as both trace events and as a per-CPU graphical display. It directly uses ‘trace-cmd’ as the plumbing that accomplishes all that underneath the covers (and actually displays the trace-cmd command it uses, as we’ll see).

To start a trace using kernelshark, first start kernelshark:

root@sugarbay:~# kernelshark

Then bring up the ‘Capture’ dialog by choosing from the kernelshark menu:

Capture | Record

That will display the following dialog, which allows you to choose one or more events (or even one or more complete subsystems) to trace:

../_images/kernelshark-choose-events.png

Note that these are exactly the same sets of events described in the previous trace events subsystem section, and in fact is where trace-cmd gets them for kernelshark.

In the above screenshot, we’ve decided to explore the graphics subsystem a bit and so have chosen to trace all the tracepoints contained within the ‘i915’ and ‘drm’ subsystems.

After doing that, we can start and stop the trace using the ‘Run’ and ‘Stop’ button on the lower right corner of the dialog (the same button will turn into the ‘Stop’ button after the trace has started):

../_images/kernelshark-output-display.png

Notice that the right-hand pane shows the exact trace-cmd command-line that’s used to run the trace, along with the results of the trace-cmd run.

Once the ‘Stop’ button is pressed, the graphical view magically fills up with a colorful per-cpu display of the trace data, along with the detailed event listing below that:

../_images/kernelshark-i915-display.png

Here’s another example, this time a display resulting from tracing ‘all events’:

../_images/kernelshark-all.png

The tool is pretty self-explanatory, but for more detailed information on navigating through the data, see the kernelshark website.

3.2.5 ftrace Documentation

The documentation for ftrace can be found in the kernel Documentation directory:

Documentation/trace/ftrace.txt

The documentation for the trace event subsystem can also be found in the kernel Documentation directory:

Documentation/trace/events.txt

There is a nice series of articles on using ftrace and trace-cmd at LWN:

There’s more detailed documentation kernelshark usage here: KernelShark

An amusing yet useful README (a tracing mini-HOWTO) can be found in /sys/kernel/debug/tracing/README.

3.3 systemtap

SystemTap is a system-wide script-based tracing and profiling tool.

SystemTap scripts are C-like programs that are executed in the kernel to gather/print/aggregate data extracted from the context they end up being invoked under.

For example, this probe from the SystemTap tutorial simply prints a line every time any process on the system open()s a file. For each line, it prints the executable name of the program that opened the file, along with its PID, and the name of the file it opened (or tried to open), which it extracts from the open syscall’s argstr.

probe syscall.open
{
        printf ("%s(%d) open (%s)\n", execname(), pid(), argstr)
}

probe timer.ms(4000) # after 4 seconds
{
        exit ()
}

Normally, to execute this probe, you’d simply install systemtap on the system you want to probe, and directly run the probe on that system e.g. assuming the name of the file containing the above text is trace_open.stp:

# stap trace_open.stp

What systemtap does under the covers to run this probe is 1) parse and convert the probe to an equivalent ‘C’ form, 2) compile the ‘C’ form into a kernel module, 3) insert the module into the kernel, which arms it, and 4) collect the data generated by the probe and display it to the user.

In order to accomplish steps 1 and 2, the ‘stap’ program needs access to the kernel build system that produced the kernel that the probed system is running. In the case of a typical embedded system (the ‘target’), the kernel build system unfortunately isn’t typically part of the image running on the target. It is normally available on the ‘host’ system that produced the target image however; in such cases, steps 1 and 2 are executed on the host system, and steps 3 and 4 are executed on the target system, using only the systemtap ‘runtime’.

The systemtap support in Yocto assumes that only steps 3 and 4 are run on the target; it is possible to do everything on the target, but this section assumes only the typical embedded use-case.

So basically what you need to do in order to run a systemtap script on the target is to 1) on the host system, compile the probe into a kernel module that makes sense to the target, 2) copy the module onto the target system and 3) insert the module into the target kernel, which arms it, and 4) collect the data generated by the probe and display it to the user.

3.3.1 systemtap Setup

Those are a lot of steps and a lot of details, but fortunately Yocto includes a script called ‘crosstap’ that will take care of those details, allowing you to simply execute a systemtap script on the remote target, with arguments if necessary.

In order to do this from a remote host, however, you need to have access to the build for the image you booted. The ‘crosstap’ script provides details on how to do this if you run the script on the host without having done a build:

$ crosstap root@192.168.1.88 trace_open.stp

Error: No target kernel build found.
Did you forget to create a local build of your image?

'crosstap' requires a local sdk build of the target system
(or a build that includes 'tools-profile') in order to build
kernel modules that can probe the target system.

Practically speaking, that means you need to do the following:
 - If you're running a pre-built image, download the release
   and/or BSP tarballs used to build the image.
 - If you're working from git sources, just clone the metadata
   and BSP layers needed to build the image you'll be booting.
 - Make sure you're properly set up to build a new image (see
   the BSP README and/or the widely available basic documentation
   that discusses how to build images).
 - Build an -sdk version of the image e.g.:
     $ bitbake core-image-sato-sdk
 OR
 - Build a non-sdk image but include the profiling tools:
     [ edit local.conf and add 'tools-profile' to the end of
       the EXTRA_IMAGE_FEATURES variable ]
     $ bitbake core-image-sato

Once you've build the image on the host system, you're ready to
boot it (or the equivalent pre-built image) and use 'crosstap'
to probe it (you need to source the environment as usual first):

   $ source oe-init-build-env
   $ cd ~/my/systemtap/scripts
   $ crosstap root@192.168.1.xxx myscript.stp

Note

SystemTap, which uses ‘crosstap’, assumes you can establish an ssh connection to the remote target. Please refer to the crosstap wiki page for details on verifying ssh connections at . Also, the ability to ssh into the target system is not enabled by default in *-minimal images.

So essentially what you need to do is build an SDK image or image with ‘tools-profile’ as detailed in the “General Setup” section of this manual, and boot the resulting target image.

Note

If you have a build directory containing multiple machines, you need to have the MACHINE you’re connecting to selected in local.conf, and the kernel in that machine’s build directory must match the kernel on the booted system exactly, or you’ll get the above ‘crosstap’ message when you try to invoke a script.

3.3.2 Running a Script on a Target

Once you’ve done that, you should be able to run a systemtap script on the target:

$ cd /path/to/yocto
$ source oe-init-build-env

### Shell environment set up for builds. ###

You can now run 'bitbake <target>'

Common targets are:
         core-image-minimal
         core-image-sato
         meta-toolchain
         meta-ide-support

You can also run generated QEMU images with a command like 'runqemu qemux86-64'

Once you’ve done that, you can cd to whatever directory contains your scripts and use ‘crosstap’ to run the script:

$ cd /path/to/my/systemap/script
$ crosstap root@192.168.7.2 trace_open.stp

If you get an error connecting to the target e.g.:

$ crosstap root@192.168.7.2 trace_open.stp
error establishing ssh connection on remote 'root@192.168.7.2'

Try ssh’ing to the target and see what happens:

$ ssh root@192.168.7.2

A lot of the time, connection problems are due specifying a wrong IP address or having a ‘host key verification error’.

If everything worked as planned, you should see something like this (enter the password when prompted, or press enter if it’s set up to use no password):

$ crosstap root@192.168.7.2 trace_open.stp
root@192.168.7.2's password:
matchbox-termin(1036) open ("/tmp/vte3FS2LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600)
matchbox-termin(1036) open ("/tmp/vteJMC7LW", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600)

3.3.3 systemtap Documentation

The SystemTap language reference can be found here: SystemTap Language Reference

Links to other SystemTap documents, tutorials, and examples can be found here: SystemTap documentation page

3.4 Sysprof

Sysprof is a very easy to use system-wide profiler that consists of a single window with three panes and a few buttons which allow you to start, stop, and view the profile from one place.

3.4.1 Sysprof Setup

For this section, we’ll assume you’ve already performed the basic setup outlined in the “General Setup” section.

Sysprof is a GUI-based application that runs on the target system. For the rest of this document we assume you’ve ssh’ed to the host and will be running Sysprof on the target (you can use the ‘-X’ option to ssh and have the Sysprof GUI run on the target but display remotely on the host if you want).

3.4.2 Basic Sysprof Usage

To start profiling the system, you simply press the ‘Start’ button. To stop profiling and to start viewing the profile data in one easy step, press the ‘Profile’ button.

Once you’ve pressed the profile button, the three panes will fill up with profiling data:

../_images/sysprof-copy-to-user.png

The left pane shows a list of functions and processes. Selecting one of those expands that function in the right pane, showing all its callees. Note that this caller-oriented display is essentially the inverse of perf’s default callee-oriented callchain display.

In the screenshot above, we’re focusing on __copy_to_user_ll() and looking up the callchain we can see that one of the callers of __copy_to_user_ll is sys_read() and the complete callpath between them. Notice that this is essentially a portion of the same information we saw in the perf display shown in the perf section of this page.

../_images/sysprof-copy-from-user.png

Similarly, the above is a snapshot of the Sysprof display of a copy-from-user callchain.

Finally, looking at the third Sysprof pane in the lower left, we can see a list of all the callers of a particular function selected in the top left pane. In this case, the lower pane is showing all the callers of __mark_inode_dirty:

../_images/sysprof-callers.png

Double-clicking on one of those functions will in turn change the focus to the selected function, and so on.

Tying it Together

If you like sysprof’s ‘caller-oriented’ display, you may be able to approximate it in other tools as well. For example, ‘perf report’ has the -g (–call-graph) option that you can experiment with; one of the options is ‘caller’ for an inverted caller-based callgraph display.

3.4.3 Sysprof Documentation

There doesn’t seem to be any documentation for Sysprof, but maybe that’s because it’s pretty self-explanatory. The Sysprof website, however, is here: Sysprof, System-wide Performance Profiler for Linux

3.5 LTTng (Linux Trace Toolkit, next generation)

3.5.1 LTTng Setup

For this section, we’ll assume you’ve already performed the basic setup outlined in the “General Setup” section. LTTng is run on the target system by ssh’ing to it.

3.5.2 Collecting and Viewing Traces

Once you’ve applied the above commits and built and booted your image (you need to build the core-image-sato-sdk image or use one of the other methods described in the “General Setup” section), you’re ready to start tracing.

3.5.2.1 Collecting and viewing a trace on the target (inside a shell)

First, from the host, ssh to the target:

$ ssh -l root 192.168.1.47
The authenticity of host '192.168.1.47 (192.168.1.47)' can't be established.
RSA key fingerprint is 23:bd:c8:b1:a8:71:52:00:ee:00:4f:64:9e:10:b9:7e.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.47' (RSA) to the list of known hosts.
root@192.168.1.47's password:

Once on the target, use these steps to create a trace:

root@crownbay:~# lttng create
Spawning a session daemon
Session auto-20121015-232120 created.
Traces will be written in /home/root/lttng-traces/auto-20121015-232120

Enable the events you want to trace (in this case all kernel events):

root@crownbay:~# lttng enable-event --kernel --all
All kernel events are enabled in channel channel0

Start the trace:

root@crownbay:~# lttng start
Tracing started for session auto-20121015-232120

And then stop the trace after awhile or after running a particular workload that you want to trace:

root@crownbay:~# lttng stop
Tracing stopped for session auto-20121015-232120

You can now view the trace in text form on the target:

root@crownbay:~# lttng view
[23:21:56.989270399] (+?.?????????) sys_geteuid: { 1 }, { }
[23:21:56.989278081] (+0.000007682) exit_syscall: { 1 }, { ret = 0 }
[23:21:56.989286043] (+0.000007962) sys_pipe: { 1 }, { fildes = 0xB77B9E8C }
[23:21:56.989321802] (+0.000035759) exit_syscall: { 1 }, { ret = 0 }
[23:21:56.989329345] (+0.000007543) sys_mmap_pgoff: { 1 }, { addr = 0x0, len = 10485760, prot = 3, flags = 131362, fd = 4294967295, pgoff = 0 }
[23:21:56.989351694] (+0.000022349) exit_syscall: { 1 }, { ret = -1247805440 }
[23:21:56.989432989] (+0.000081295) sys_clone: { 1 }, { clone_flags = 0x411, newsp = 0xB5EFFFE4, parent_tid = 0xFFFFFFFF, child_tid = 0x0 }
[23:21:56.989477129] (+0.000044140) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 681660, vruntime = 43367983388 }
[23:21:56.989486697] (+0.000009568) sched_migrate_task: { 1 }, { comm = "lttng-consumerd", tid = 1193, prio = 20, orig_cpu = 1, dest_cpu = 1 }
[23:21:56.989508418] (+0.000021721) hrtimer_init: { 1 }, { hrtimer = 3970832076, clockid = 1, mode = 1 }
[23:21:56.989770462] (+0.000262044) hrtimer_cancel: { 1 }, { hrtimer = 3993865440 }
[23:21:56.989771580] (+0.000001118) hrtimer_cancel: { 0 }, { hrtimer = 3993812192 }
[23:21:56.989776957] (+0.000005377) hrtimer_expire_entry: { 1 }, { hrtimer = 3993865440, now = 79815980007057, function = 3238465232 }
[23:21:56.989778145] (+0.000001188) hrtimer_expire_entry: { 0 }, { hrtimer = 3993812192, now = 79815980008174, function = 3238465232 }
[23:21:56.989791695] (+0.000013550) softirq_raise: { 1 }, { vec = 1 }
[23:21:56.989795396] (+0.000003701) softirq_raise: { 0 }, { vec = 1 }
[23:21:56.989800635] (+0.000005239) softirq_raise: { 0 }, { vec = 9 }
[23:21:56.989807130] (+0.000006495) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 330710, vruntime = 43368314098 }
[23:21:56.989809993] (+0.000002863) sched_stat_runtime: { 0 }, { comm = "lttng-sessiond", tid = 1181, runtime = 1015313, vruntime = 36976733240 }
[23:21:56.989818514] (+0.000008521) hrtimer_expire_exit: { 0 }, { hrtimer = 3993812192 }
[23:21:56.989819631] (+0.000001117) hrtimer_expire_exit: { 1 }, { hrtimer = 3993865440 }
[23:21:56.989821866] (+0.000002235) hrtimer_start: { 0 }, { hrtimer = 3993812192, function = 3238465232, expires = 79815981000000, softexpires = 79815981000000 }
[23:21:56.989822984] (+0.000001118) hrtimer_start: { 1 }, { hrtimer = 3993865440, function = 3238465232, expires = 79815981000000, softexpires = 79815981000000 }
[23:21:56.989832762] (+0.000009778) softirq_entry: { 1 }, { vec = 1 }
[23:21:56.989833879] (+0.000001117) softirq_entry: { 0 }, { vec = 1 }
[23:21:56.989838069] (+0.000004190) timer_cancel: { 1 }, { timer = 3993871956 }
[23:21:56.989839187] (+0.000001118) timer_cancel: { 0 }, { timer = 3993818708 }
[23:21:56.989841492] (+0.000002305) timer_expire_entry: { 1 }, { timer = 3993871956, now = 79515980, function = 3238277552 }
[23:21:56.989842819] (+0.000001327) timer_expire_entry: { 0 }, { timer = 3993818708, now = 79515980, function = 3238277552 }
[23:21:56.989854831] (+0.000012012) sched_stat_runtime: { 1 }, { comm = "lttng-consumerd", tid = 1193, runtime = 49237, vruntime = 43368363335 }
[23:21:56.989855949] (+0.000001118) sched_stat_runtime: { 0 }, { comm = "lttng-sessiond", tid = 1181, runtime = 45121, vruntime = 36976778361 }
[23:21:56.989861257] (+0.000005308) sched_stat_sleep: { 1 }, { comm = "kworker/1:1", tid = 21, delay = 9451318 }
[23:21:56.989862374] (+0.000001117) sched_stat_sleep: { 0 }, { comm = "kworker/0:0", tid = 4, delay = 9958820 }
[23:21:56.989868241] (+0.000005867) sched_wakeup: { 0 }, { comm = "kworker/0:0", tid = 4, prio = 120, success = 1, target_cpu = 0 }
[23:21:56.989869358] (+0.000001117) sched_wakeup: { 1 }, { comm = "kworker/1:1", tid = 21, prio = 120, success = 1, target_cpu = 1 }
[23:21:56.989877460] (+0.000008102) timer_expire_exit: { 1 }, { timer = 3993871956 }
[23:21:56.989878577] (+0.000001117) timer_expire_exit: { 0 }, { timer = 3993818708 }
.
.
.

You can now safely destroy the trace session (note that this doesn’t delete the trace - it’s still there in ~/lttng-traces):

root@crownbay:~# lttng destroy
Session auto-20121015-232120 destroyed at /home/root

Note that the trace is saved in a directory of the same name as returned by ‘lttng create’, under the ~/lttng-traces directory (note that you can change this by supplying your own name to ‘lttng create’):

root@crownbay:~# ls -al ~/lttng-traces
drwxrwx---    3 root     root          1024 Oct 15 23:21 .
drwxr-xr-x    5 root     root          1024 Oct 15 23:57 ..
drwxrwx---    3 root     root          1024 Oct 15 23:21 auto-20121015-232120

3.5.2.2 Collecting and viewing a userspace trace on the target (inside a shell)

For LTTng userspace tracing, you need to have a properly instrumented userspace program. For this example, we’ll use the ‘hello’ test program generated by the lttng-ust build.

The ‘hello’ test program isn’t installed on the rootfs by the lttng-ust build, so we need to copy it over manually. First cd into the build directory that contains the hello executable:

$ cd build/tmp/work/core2_32-poky-linux/lttng-ust/2.0.5-r0/git/tests/hello/.libs

Copy that over to the target machine:

$ scp hello root@192.168.1.20:

You now have the instrumented lttng ‘hello world’ test program on the target, ready to test.

First, from the host, ssh to the target:

$ ssh -l root 192.168.1.47
The authenticity of host '192.168.1.47 (192.168.1.47)' can't be established.
RSA key fingerprint is 23:bd:c8:b1:a8:71:52:00:ee:00:4f:64:9e:10:b9:7e.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.47' (RSA) to the list of known hosts.
root@192.168.1.47's password:

Once on the target, use these steps to create a trace:

root@crownbay:~# lttng create
Session auto-20190303-021943 created.
Traces will be written in /home/root/lttng-traces/auto-20190303-021943

Enable the events you want to trace (in this case all userspace events):

root@crownbay:~# lttng enable-event --userspace --all
All UST events are enabled in channel channel0

Start the trace:

root@crownbay:~# lttng start
Tracing started for session auto-20190303-021943

Run the instrumented hello world program:

root@crownbay:~# ./hello
Hello, World!
Tracing... done.

And then stop the trace after awhile or after running a particular workload that you want to trace:

root@crownbay:~# lttng stop
Tracing stopped for session auto-20190303-021943

You can now view the trace in text form on the target:

root@crownbay:~# lttng view
[02:31:14.906146544] (+?.?????????) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 0, intfield2 = 0x0, longfield = 0, netintfield = 0, netintfieldhex = 0x0, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4,  seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
[02:31:14.906170360] (+0.000023816) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 1, intfield2 = 0x1, longfield = 1, netintfield = 1, netintfieldhex = 0x1, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
[02:31:14.906183140] (+0.000012780) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 2, intfield2 = 0x2, longfield = 2, netintfield = 2, netintfieldhex = 0x2, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
[02:31:14.906194385] (+0.000011245) hello:1424 ust_tests_hello:tptest: { cpu_id = 1 }, { intfield = 3, intfield2 = 0x3, longfield = 3, netintfield = 3, netintfieldhex = 0x3, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2, boolfield = 1 }
.
.
.

You can now safely destroy the trace session (note that this doesn’t delete the trace - it’s still there in ~/lttng-traces):

root@crownbay:~# lttng destroy
Session auto-20190303-021943 destroyed at /home/root

3.5.3 LTTng Documentation

You can find the primary LTTng Documentation on the LTTng Documentation site. The documentation on this site is appropriate for intermediate to advanced software developers who are working in a Linux environment and are interested in efficient software tracing.

For information on LTTng in general, visit the LTTng Project site. You can find a “Getting Started” link on this site that takes you to an LTTng Quick Start.

3.6 blktrace

blktrace is a tool for tracing and reporting low-level disk I/O. blktrace provides the tracing half of the equation; its output can be piped into the blkparse program, which renders the data in a human-readable form and does some basic analysis:

3.6.1 blktrace Setup

For this section, we’ll assume you’ve already performed the basic setup outlined in the “General Setup” section.

blktrace is an application that runs on the target system. You can run the entire blktrace and blkparse pipeline on the target, or you can run blktrace in ‘listen’ mode on the target and have blktrace and blkparse collect and analyze the data on the host (see the “Using blktrace Remotely” section below). For the rest of this section we assume you’ve ssh’ed to the host and will be running blkrace on the target.

3.6.2 Basic blktrace Usage

To record a trace, simply run the ‘blktrace’ command, giving it the name of the block device you want to trace activity on:

root@crownbay:~# blktrace /dev/sdc

In another shell, execute a workload you want to trace.

root@crownbay:/media/sdc# rm linux-2.6.19.2.tar.bz2; wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2; sync
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% \|*******************************\| 41727k 0:00:00 ETA

Press Ctrl-C in the blktrace shell to stop the trace. It will display how many events were logged, along with the per-cpu file sizes (blktrace records traces in per-cpu kernel buffers and simply dumps them to userspace for blkparse to merge and sort later).

^C=== sdc ===
 CPU  0:                 7082 events,      332 KiB data
 CPU  1:                 1578 events,       74 KiB data
 Total:                  8660 events (dropped 0),      406 KiB data

If you examine the files saved to disk, you see multiple files, one per CPU and with the device name as the first part of the filename:

root@crownbay:~# ls -al
drwxr-xr-x    6 root     root          1024 Oct 27 22:39 .
drwxr-sr-x    4 root     root          1024 Oct 26 18:24 ..
-rw-r--r--    1 root     root        339938 Oct 27 22:40 sdc.blktrace.0
-rw-r--r--    1 root     root         75753 Oct 27 22:40 sdc.blktrace.1

To view the trace events, simply invoke ‘blkparse’ in the directory containing the trace files, giving it the device name that forms the first part of the filenames:

root@crownbay:~# blkparse sdc

 8,32   1        1     0.000000000  1225  Q  WS 3417048 + 8 [jbd2/sdc-8]
 8,32   1        2     0.000025213  1225  G  WS 3417048 + 8 [jbd2/sdc-8]
 8,32   1        3     0.000033384  1225  P   N [jbd2/sdc-8]
 8,32   1        4     0.000043301  1225  I  WS 3417048 + 8 [jbd2/sdc-8]
 8,32   1        0     0.000057270     0  m   N cfq1225 insert_request
 8,32   1        0     0.000064813     0  m   N cfq1225 add_to_rr
 8,32   1        5     0.000076336  1225  U   N [jbd2/sdc-8] 1
 8,32   1        0     0.000088559     0  m   N cfq workload slice:150
 8,32   1        0     0.000097359     0  m   N cfq1225 set_active wl_prio:0 wl_type:1
 8,32   1        0     0.000104063     0  m   N cfq1225 Not idling. st->count:1
 8,32   1        0     0.000112584     0  m   N cfq1225 fifo=  (null)
 8,32   1        0     0.000118730     0  m   N cfq1225 dispatch_insert
 8,32   1        0     0.000127390     0  m   N cfq1225 dispatched a request
 8,32   1        0     0.000133536     0  m   N cfq1225 activate rq, drv=1
 8,32   1        6     0.000136889  1225  D  WS 3417048 + 8 [jbd2/sdc-8]
 8,32   1        7     0.000360381  1225  Q  WS 3417056 + 8 [jbd2/sdc-8]
 8,32   1        8     0.000377422  1225  G  WS 3417056 + 8 [jbd2/sdc-8]
 8,32   1        9     0.000388876  1225  P   N [jbd2/sdc-8]
 8,32   1       10     0.000397886  1225  Q  WS 3417064 + 8 [jbd2/sdc-8]
 8,32   1       11     0.000404800  1225  M  WS 3417064 + 8 [jbd2/sdc-8]
 8,32   1       12     0.000412343  1225  Q  WS 3417072 + 8 [jbd2/sdc-8]
 8,32   1       13     0.000416533  1225  M  WS 3417072 + 8 [jbd2/sdc-8]
 8,32   1       14     0.000422121  1225  Q  WS 3417080 + 8 [jbd2/sdc-8]
 8,32   1       15     0.000425194  1225  M  WS 3417080 + 8 [jbd2/sdc-8]
 8,32   1       16     0.000431968  1225  Q  WS 3417088 + 8 [jbd2/sdc-8]
 8,32   1       17     0.000435251  1225  M  WS 3417088 + 8 [jbd2/sdc-8]
 8,32   1       18     0.000440279  1225  Q  WS 3417096 + 8 [jbd2/sdc-8]
 8,32   1       19     0.000443911  1225  M  WS 3417096 + 8 [jbd2/sdc-8]
 8,32   1       20     0.000450336  1225  Q  WS 3417104 + 8 [jbd2/sdc-8]
 8,32   1       21     0.000454038  1225  M  WS 3417104 + 8 [jbd2/sdc-8]
 8,32   1       22     0.000462070  1225  Q  WS 3417112 + 8 [jbd2/sdc-8]
 8,32   1       23     0.000465422  1225  M  WS 3417112 + 8 [jbd2/sdc-8]
 8,32   1       24     0.000474222  1225  I  WS 3417056 + 64 [jbd2/sdc-8]
 8,32   1        0     0.000483022     0  m   N cfq1225 insert_request
 8,32   1       25     0.000489727  1225  U   N [jbd2/sdc-8] 1
 8,32   1        0     0.000498457     0  m   N cfq1225 Not idling. st->count:1
 8,32   1        0     0.000503765     0  m   N cfq1225 dispatch_insert
 8,32   1        0     0.000512914     0  m   N cfq1225 dispatched a request
 8,32   1        0     0.000518851     0  m   N cfq1225 activate rq, drv=2
 .
 .
 .
 8,32   0        0    58.515006138     0  m   N cfq3551 complete rqnoidle 1
 8,32   0     2024    58.516603269     3  C  WS 3156992 + 16 [0]
 8,32   0        0    58.516626736     0  m   N cfq3551 complete rqnoidle 1
 8,32   0        0    58.516634558     0  m   N cfq3551 arm_idle: 8 group_idle: 0
 8,32   0        0    58.516636933     0  m   N cfq schedule dispatch
 8,32   1        0    58.516971613     0  m   N cfq3551 slice expired t=0
 8,32   1        0    58.516982089     0  m   N cfq3551 sl_used=13 disp=6 charge=13 iops=0 sect=80
 8,32   1        0    58.516985511     0  m   N cfq3551 del_from_rr
 8,32   1        0    58.516990819     0  m   N cfq3551 put_queue

CPU0 (sdc):
 Reads Queued:           0,        0KiB       Writes Queued:         331,   26,284KiB
 Read Dispatches:        0,        0KiB       Write Dispatches:      485,   40,484KiB
 Reads Requeued:         0            Writes Requeued:         0
 Reads Completed:        0,        0KiB       Writes Completed:      511,   41,000KiB
 Read Merges:            0,        0KiB       Write Merges:           13,      160KiB
 Read depth:             0            Write depth:             2
 IO unplugs:            23            Timer unplugs:           0
CPU1 (sdc):
 Reads Queued:           0,        0KiB       Writes Queued:         249,   15,800KiB
 Read Dispatches:        0,        0KiB       Write Dispatches:       42,    1,600KiB
 Reads Requeued:         0            Writes Requeued:         0
 Reads Completed:        0,        0KiB       Writes Completed:       16,    1,084KiB
 Read Merges:            0,        0KiB       Write Merges:           40,      276KiB
 Read depth:             0            Write depth:             2
 IO unplugs:            30            Timer unplugs:           1

Total (sdc):
 Reads Queued:           0,        0KiB       Writes Queued:         580,   42,084KiB
 Read Dispatches:        0,        0KiB       Write Dispatches:      527,   42,084KiB
 Reads Requeued:         0            Writes Requeued:         0
 Reads Completed:        0,        0KiB       Writes Completed:      527,   42,084KiB
 Read Merges:            0,        0KiB       Write Merges:           53,      436KiB
 IO unplugs:            53            Timer unplugs:           1

Throughput (R/W): 0KiB/s / 719KiB/s
Events (sdc): 6,592 entries
Skips: 0 forward (0 -   0.0%)
Input file sdc.blktrace.0 added
Input file sdc.blktrace.1 added

The report shows each event that was found in the blktrace data, along with a summary of the overall block I/O traffic during the run. You can look at the blkparse manpage to learn the meaning of each field displayed in the trace listing.

3.6.2.1 Live Mode

blktrace and blkparse are designed from the ground up to be able to operate together in a ‘pipe mode’ where the stdout of blktrace can be fed directly into the stdin of blkparse:

root@crownbay:~# blktrace /dev/sdc -o - | blkparse -i -

This enables long-lived tracing sessions to run without writing anything to disk, and allows the user to look for certain conditions in the trace data in ‘real-time’ by viewing the trace output as it scrolls by on the screen or by passing it along to yet another program in the pipeline such as grep which can be used to identify and capture conditions of interest.

There’s actually another blktrace command that implements the above pipeline as a single command, so the user doesn’t have to bother typing in the above command sequence:

root@crownbay:~# btrace /dev/sdc

3.6.2.2 Using blktrace Remotely

Because blktrace traces block I/O and at the same time normally writes its trace data to a block device, and in general because it’s not really a great idea to make the device being traced the same as the device the tracer writes to, blktrace provides a way to trace without perturbing the traced device at all by providing native support for sending all trace data over the network.

To have blktrace operate in this mode, start blktrace on the target system being traced with the -l option, along with the device to trace:

root@crownbay:~# blktrace -l /dev/sdc
server: waiting for connections...

On the host system, use the -h option to connect to the target system, also passing it the device to trace:

$ blktrace -d /dev/sdc -h 192.168.1.43
blktrace: connecting to 192.168.1.43
blktrace: connected!

On the target system, you should see this:

server: connection from 192.168.1.43

In another shell, execute a workload you want to trace.

root@crownbay:/media/sdc# rm linux-2.6.19.2.tar.bz2; wget http://downloads.yoctoproject.org/mirror/sources/linux-2.6.19.2.tar.bz2; sync
Connecting to downloads.yoctoproject.org (140.211.169.59:80)
linux-2.6.19.2.tar.b 100% \|*******************************\| 41727k 0:00:00 ETA

When it’s done, do a Ctrl-C on the host system to stop the trace:

^C=== sdc ===
 CPU  0:                 7691 events,      361 KiB data
 CPU  1:                 4109 events,      193 KiB data
 Total:                 11800 events (dropped 0),      554 KiB data

On the target system, you should also see a trace summary for the trace just ended:

server: end of run for 192.168.1.43:sdc
=== sdc ===
 CPU  0:                 7691 events,      361 KiB data
 CPU  1:                 4109 events,      193 KiB data
 Total:                 11800 events (dropped 0),      554 KiB data

The blktrace instance on the host will save the target output inside a hostname-timestamp directory:

$ ls -al
drwxr-xr-x   10 root     root          1024 Oct 28 02:40 .
drwxr-sr-x    4 root     root          1024 Oct 26 18:24 ..
drwxr-xr-x    2 root     root          1024 Oct 28 02:40 192.168.1.43-2012-10-28-02:40:56

cd into that directory to see the output files:

$ ls -l
-rw-r--r--    1 root     root        369193 Oct 28 02:44 sdc.blktrace.0
-rw-r--r--    1 root     root        197278 Oct 28 02:44 sdc.blktrace.1

And run blkparse on the host system using the device name:

$ blkparse sdc

 8,32   1        1     0.000000000  1263  Q  RM 6016 + 8 [ls]
 8,32   1        0     0.000036038     0  m   N cfq1263 alloced
 8,32   1        2     0.000039390  1263  G  RM 6016 + 8 [ls]
 8,32   1        3     0.000049168  1263  I  RM 6016 + 8 [ls]
 8,32   1        0     0.000056152     0  m   N cfq1263 insert_request
 8,32   1        0     0.000061600     0  m   N cfq1263 add_to_rr
 8,32   1        0     0.000075498     0  m   N cfq workload slice:300
 .
 .
 .
 8,32   0        0   177.266385696     0  m   N cfq1267 arm_idle: 8 group_idle: 0
 8,32   0        0   177.266388140     0  m   N cfq schedule dispatch
 8,32   1        0   177.266679239     0  m   N cfq1267 slice expired t=0
 8,32   1        0   177.266689297     0  m   N cfq1267 sl_used=9 disp=6 charge=9 iops=0 sect=56
 8,32   1        0   177.266692649     0  m   N cfq1267 del_from_rr
 8,32   1        0   177.266696560     0  m   N cfq1267 put_queue

CPU0 (sdc):
 Reads Queued:           0,        0KiB       Writes Queued:         270,   21,708KiB
 Read Dispatches:       59,    2,628KiB       Write Dispatches:      495,   39,964KiB
 Reads Requeued:         0            Writes Requeued:         0
 Reads Completed:       90,    2,752KiB       Writes Completed:      543,   41,596KiB
 Read Merges:            0,        0KiB       Write Merges:            9,      344KiB
 Read depth:             2            Write depth:             2
 IO unplugs:            20            Timer unplugs:           1
CPU1 (sdc):
 Reads Queued:         688,    2,752KiB       Writes Queued:         381,   20,652KiB
 Read Dispatches:       31,      124KiB       Write Dispatches:       59,    2,396KiB
 Reads Requeued:         0            Writes Requeued:         0
 Reads Completed:        0,        0KiB       Writes Completed:       11,      764KiB
 Read Merges:          598,    2,392KiB       Write Merges:           88,      448KiB
 Read depth:             2            Write depth:             2
 IO unplugs:            52            Timer unplugs:           0

Total (sdc):
 Reads Queued:         688,    2,752KiB       Writes Queued:         651,   42,360KiB
 Read Dispatches:       90,    2,752KiB       Write Dispatches:      554,   42,360KiB
 Reads Requeued:         0            Writes Requeued:         0
 Reads Completed:       90,    2,752KiB       Writes Completed:      554,   42,360KiB
 Read Merges:          598,    2,392KiB       Write Merges:           97,      792KiB
 IO unplugs:            72            Timer unplugs:           1

Throughput (R/W): 15KiB/s / 238KiB/s
Events (sdc): 9,301 entries
Skips: 0 forward (0 -   0.0%)

You should see the trace events and summary just as you would have if you’d run the same command on the target.

3.6.2.3 Tracing Block I/O via ‘ftrace’

It’s also possible to trace block I/O using only The ‘trace events’ Subsystem, which can be useful for casual tracing if you don’t want to bother dealing with the userspace tools.

To enable tracing for a given device, use /sys/block/xxx/trace/enable, where xxx is the device name. This for example enables tracing for /dev/sdc:

root@crownbay:/sys/kernel/debug/tracing# echo 1 > /sys/block/sdc/trace/enable

Once you’ve selected the device(s) you want to trace, selecting the ‘blk’ tracer will turn the blk tracer on:

root@crownbay:/sys/kernel/debug/tracing# cat available_tracers
blk function_graph function nop

root@crownbay:/sys/kernel/debug/tracing# echo blk > current_tracer

Execute the workload you’re interested in:

root@crownbay:/sys/kernel/debug/tracing# cat /media/sdc/testfile.txt

And look at the output (note here that we’re using ‘trace_pipe’ instead of trace to capture this trace - this allows us to wait around on the pipe for data to appear):

root@crownbay:/sys/kernel/debug/tracing# cat trace_pipe
            cat-3587  [001] d..1  3023.276361:   8,32   Q   R 1699848 + 8 [cat]
            cat-3587  [001] d..1  3023.276410:   8,32   m   N cfq3587 alloced
            cat-3587  [001] d..1  3023.276415:   8,32   G   R 1699848 + 8 [cat]
            cat-3587  [001] d..1  3023.276424:   8,32   P   N [cat]
            cat-3587  [001] d..2  3023.276432:   8,32   I   R 1699848 + 8 [cat]
            cat-3587  [001] d..1  3023.276439:   8,32   m   N cfq3587 insert_request
            cat-3587  [001] d..1  3023.276445:   8,32   m   N cfq3587 add_to_rr
            cat-3587  [001] d..2  3023.276454:   8,32   U   N [cat] 1
            cat-3587  [001] d..1  3023.276464:   8,32   m   N cfq workload slice:150
            cat-3587  [001] d..1  3023.276471:   8,32   m   N cfq3587 set_active wl_prio:0 wl_type:2
            cat-3587  [001] d..1  3023.276478:   8,32   m   N cfq3587 fifo=  (null)
            cat-3587  [001] d..1  3023.276483:   8,32   m   N cfq3587 dispatch_insert
            cat-3587  [001] d..1  3023.276490:   8,32   m   N cfq3587 dispatched a request
            cat-3587  [001] d..1  3023.276497:   8,32   m   N cfq3587 activate rq, drv=1
            cat-3587  [001] d..2  3023.276500:   8,32   D   R 1699848 + 8 [cat]

And this turns off tracing for the specified device:

root@crownbay:/sys/kernel/debug/tracing# echo 0 > /sys/block/sdc/trace/enable

3.6.3 blktrace Documentation

Online versions of the man pages for the commands discussed in this section can be found here:

The above manpages, along with manpages for the other blktrace utilities (btt, blkiomon, etc) can be found in the /doc directory of the blktrace tools git repo:

$ git clone git://git.kernel.dk/blktrace.git