Ftrace

리눅스 커널의 내부 동작을 trace

주요 기능

  • 1) dynamic kernel function tracing
    .stack usage, call graph, counting, filters
  • 2) latency tracing
    .interrupts, wake-up
  • 3) static tracepoints를 사용하여 event tracing
    .scheduling, interrupts, file systems, virtual guest connections

특징

  • debugfs 파일시스템을 /sys/kernel/debug에 마운트하여 사용
  • gcc 컴파일러의 프로파일링 옵션(-pg)로 빌드된 커널 사용
  • linux kernel 2.6.27에서 mainline에 합류
  • 적은(10%) overhead
  • buffered(trace) vs live tracing(trace_pipe)
  • trace source로 tracepoints, kprobes를 사용

Tracer Plugins

  • function: 임의의 기간동안 커널 내부 함수의 호출관계를 분석
  • function_graph: 그래프 형식의 함수 관계 분석
  • wakeup, wakeup_dl, wakeup_rt: wake up latency 분석
  • mmiotrace: 메모리 맵의 I/O에 대한 분석
  • irqsoff: interrupt latency 분석
  • nop: 디버깅 기능 off

preemptoff, preemptirqsoff 기능은 커널 3.0.x 에서 제거됨.
sched_switch의 경우 deprecated되어 wakeup 기능으로 동작

1) Function Tracing

function filter: 모든 함수 → 특정 함수

# cd /sys/kernel/debug/tracing
# cat set_ftrace_filter
#### all functions enabled ####

# echo schedule_tail >> set_ftrace_filter
# cat set_ftrace_filter
schedule_tail

# echo 'sched*' > set_ftrace_filter
# cat set_ftrace_filter
schedule_on_each_cpu
sched_feat_open
sched_feat_show
(…)

 

제외시킬 function: 없음 → 특정 함수

# cat set_ftrace_notrace
#### no functions disabled ####

# cat set_ftrace_notrace
xen_pte_unlock
update_persistent_clock
read_persistent_clock
(…)

 

사용가능한 tracer 목록

# cat current_tracer
nop

# cat available_tracers
blk mmiotrace function_graph wakeup_dl wakeup_rt wakeup function nop

 

사용가능한 function 목록

# cat available_filter_functions
run_init_process
try_to_run_init_process
do_one_initcall
match_dev_by_uuid
rootfs_mount
name_to_dev_t
init_linuxrc
init_linuxrc
calibration_delay_done
calibrate_delay
(...)

 

function trace

# cat trace | head -30
# tracer: function
# entries-in-buffer/entries-written: 62862/62862   #P:4
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
            bash-11785 [000] d... 172870.350417: sched_stat_sleep: comm=rcuos/0 pid=9 ...
            bash-11785 [000] d... 172870.350418: sched_wake_idle_without_ipi: cpu=1
            bash-11785 [000] d... 172870.350419: sched_wakeup: comm=rcuos/0 pid=9 ...
          <idle>-0     [001] .N.. 172870.350436: schedule <-schedule_preempt_disabled
          <idle>-0     [001] dN.. 172870.350438: sched_stat_wait: comm=rcuos/0 pid=9 ...

 

function_graph trace

# cat trace | head -30
# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
 3) + 11.008 us   |  }
 1)               |  schedule() {
 1)               |  /* sched_stat_wait: comm=rcuos/0 pid=9 delay=0 [ns] */
 1)               |  /* sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 ...
 ------------------------------------------
 1)    <idle>-0    =>   rcuos/0-9 
 ------------------------------------------

 

2) Latency Tracing

wakeup

# cat trace
# tracer: wakeup
#
# wakeup latency trace v1.1.5 on 4.0.5
# --------------------------------------------------------------------
# latency: 14 us, #75/75, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
#    -----------------
#    | task: watchdog/0-12 (uid:0 nice:0 policy:1 rt_prio:99)
#    -----------------
#
#                  _------=> CPU#
#                 / _-----=> irqs-off
#                | / _----=> need-resched
#                || / _---=> hardirq/softirq
#                ||| / _--=> preempt-depth
#                |||| /     delay
#  cmd     pid   ||||| time  |   caller
#     \   /      |||||  \    |   /
  <idle>-0       0dNh.    0us :      0:120:R   + [000]    12:  0:R watchdog/0
  <idle>-0       0dNh.    0us : ttwu_do_activate.constprop.92 <-try_to_wake_up
  <idle>-0       0dNh.    1us : task_woken_rt <-ttwu_do_wakeup
  <idle>-0       0dNh.    1us : _raw_spin_unlock <-try_to_wake_up
  <idle>-0       0dNh.    1us : ttwu_stat <-try_to_wake_up
  <idle>-0       0dNh.    1us : _raw_spin_unlock_irqrestore <-try_to_wake_up
  <idle>-0       0dNh.    1us : ktime_get <-watchdog_timer_fn
  <idle>-0       0dNh.    1us : hrtimer_forward <-watchdog_timer_fn
  <idle>-0       0dNh.    2us : _raw_spin_lock <-__run_hrtimer

 

wake_dl

# cat trace
# tracer: wakeup_dl
#
# wakeup_dl latency trace v1.1.5 on 4.0.5
# --------------------------------------------------------------------
# latency: 665155 us, #0/0, CPU#2 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
#    -----------------
#    | task: Xorg-840 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#
#                  _------=> CPU#
#                 / _-----=> irqs-off
#                | / _----=> need-resched
#                || / _---=> hardirq/softirq
#                ||| / _--=> preempt-depth
#                |||| /     delay
#  cmd     pid   ||||| time  |   caller
#     \   /      |||||  \    |   /

 

wakeup_rt

# cat trace
# tracer: wakeup_rt
#
# wakeup_rt latency trace v1.1.5 on 4.0.5
# --------------------------------------------------------------------
# latency: 18 us, #75/75, CPU#1 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
#    -----------------
#    | task: irq/51-iwlwifi-423 (uid:0 nice:0 policy:1 rt_prio:50)
#    -----------------
#
#                  _------=> CPU#
#                 / _-----=> irqs-off
#                | / _----=> need-resched
#                || / _---=> hardirq/softirq
#                ||| / _--=> preempt-depth
#                |||| /     delay
#  cmd     pid   ||||| time  |   caller
#     \   /      |||||  \    |   /
  <idle>-0       1dNh.    0us :      0:120:R   + [001]   423: 49:R irq/51-iwlwifi
  <idle>-0       1dNh.    0us : ttwu_do_activate.constprop.92 <-try_to_wake_up
  <idle>-0       1dNh.    1us : task_woken_rt <-ttwu_do_wakeup
  <idle>-0       1dNh.    1us : _raw_spin_unlock <-try_to_wake_up
  <idle>-0       1dNh.    1us : ttwu_stat <-try_to_wake_up
  <idle>-0       1dNh.    1us : _raw_spin_unlock_irqrestore <-try_to_wake_up
  <idle>-0       1dNh.    1us : add_interrupt_randomness <-handle_irq_event_percpu

 

3) Event Tracing

event 디렉토리

 

# ls events/
asoc              filemap       irq_vectors    mac80211_msg  random        sock      vsyscall
block             ftrace        iwlwifi        mce           ras           spi       workqueue
cfg80211          gpio          iwlwifi_data   migrate       raw_syscalls  swiotlb   writeback
compaction        hda           iwlwifi_io     module        rcu           syscalls  xen
context_tracking  hda_intel     iwlwifi_msg    napi          regmap        task      xhci-hcd
drm               header_event  iwlwifi_ucode  net           regulator     thermal
enable            header_page   jbd2           nmi           rpm           timer
exceptions        i2c           kmem           oom           sched         tlb
ext4              i915          kvm            pagemap       scsi          udp
fence             iommu         kvmmmu         power         signal        v4l2
filelock          irq           mac80211       printk        skb           vmscan

# ls events/sched/
enable                  sched_process_exec  sched_stat_iowait   sched_wait_task
filter                  sched_process_exit  sched_stat_runtime  sched_wake_idle_without_ipi
sched_kthread_stop      sched_process_fork  sched_stat_sleep    sched_wakeup
sched_kthread_stop_ret  sched_process_free  sched_stat_wait     sched_wakeup_new
sched_migrate_task      sched_process_hang  sched_stick_numa
sched_move_numa         sched_process_wait  sched_swap_numa
sched_pi_setprio        sched_stat_blocked  sched_switch

# ls events/sched/sched_wakeup
enable  filter  format  id  trigger

 

event 목록

# cat available_events | head -20
mac80211:drv_return_void
mac80211:drv_return_int
mac80211:drv_return_bool
mac80211:drv_return_u32
mac80211:drv_return_u64
mac80211:drv_start
mac80211:drv_get_et_strings
mac80211:drv_get_et_sset_count
mac80211:drv_get_et_stats
mac80211:drv_suspend
mac80211:drv_resume
mac80211:drv_set_wakeup
mac80211:drv_stop
mac80211:drv_add_interface
mac80211:drv_change_interface
mac80211:drv_remove_interface
mac80211:drv_config
mac80211:drv_bss_info_changed
mac80211:drv_prepare_multicast
mac80211:drv_configure_filter

 

4) Stack Tracing

# echo 1 > /proc/sys/kernel/stack_tracer_enabled
# cat stack_trace
        Depth    Size   Location    (16 entries)
        -----    ----   --------
  0)     2824      16   update_group_capacity+0x2c/0x270
  1)     2808       8   msecs_to_jiffies+0x5/0x30
  2)     2800      64   update_group_capacity+0x2c/0x270
  3)     2736     384   find_busiest_group+0xfb/0x940
  4)     2352     288   load_balance+0x1ac/0x8f0
  5)     2064     128   pick_next_task_fair+0x488/0x8a0
  6)     1936      80   __schedule+0x116/0xa10
  7)     1856       8   schedule+0x37/0x90
  8)     1848     184   schedule_hrtimeout_range_clock.part.26+0xfc/0x120
  9)     1664      16   schedule_hrtimeout_range_clock+0x19/0x50
 10)     1648      16   schedule_hrtimeout_range+0x13/0x20
 11)     1632      24   poll_schedule_timeout+0x49/0x70
 12)     1608     952   do_select+0x57c/0x7d0
 13)      656     416   core_sys_select+0x1c7/0x2f0
 14)      240     112   SyS_select+0xab/0x100
 15)      128     128   system_call_fastpath+0x16/0x1b

 

5) Function profiling

# echo 1 > function_profile_enabled
# cat trace_stat/function0 | head -15
  Function                               Hit    Time            Avg             s^2
  --------                               ---    ----            ---             ---
  __schedule                           10960    647848540 us     59110.26 us     2099595 us
  schedule                              6499    647692443 us     99660.32 us     4087623 us
  schedule_hrtimeout_range              1021    461369396 us     451879.9 us     5813184 us
  schedule_hrtimeout_range_clock        1021    461369211 us     451879.7 us     5763201 us
  poll_schedule_timeout                 1016    454877997 us     447714.5 us     8181934 us
  SyS_poll                               340    332641192 us     978356.4 us     105339792 us
  do_sys_poll                            340    332640884 us     978355.5 us     105204044 us
  schedule_hrtimeout_range_clock         138    188387417 us     1365126 us     582984815 us
  SyS_select                            1720    122266989 us     71085.45 us     782863.5 us
  core_sys_select                       1720    122266182 us     71084.98 us     653943.4 us
  do_select                             1720    122265404 us     71084.53 us     556184.2 us
  schedule_timeout                        72    51148556 us     710396.6 us     1317174442 us
  SyS_read                              1249    31128240 us     24922.53 us     2673959 us

* 결과 시간에서 sleep time을 제거하고 출력하려면 “echo 0 > options/sleep-time”을 사용한다.

 

참고

arm에서 kprobes 사용하기(1)
arm에서 kprobes 사용하기(2)
Function Tracer
FTrace

답글 남기기

이메일은 공개되지 않습니다. 필수 입력창은 * 로 표시되어 있습니다.