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SubjectRe: ftrace performance (sched events): cyclictest shows 25% more latency
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On 07/09/2018 12:06 PM, Claudio wrote:
> Hello Steve,
>
> thank you for your response,
>
> On 07/06/2018 11:24 PM, Steven Rostedt wrote:
>> On Fri, 6 Jul 2018 08:22:01 +0200
>> Claudio <claudio.fontana@gliwa.com> wrote:
>>
>>> Hello all,
>>>
>>> I have been experimenting with the idea of leaving ftrace enabled, with sched events,
>>> on production systems.
>>>
>>> The main concern that I am having at the moment is about the impact on the system.
>>> Enabling the sched events that I currently need for the tracing application
>>> seems to slow down context-switches considerably, and make the system less responsive.
>>>
>>> I have tested with cyclictest on the mainline kernel, and noticed an increase of min, avg latencies of around 25%.
>>>
>>> Is this expected?
>>
>> No it is not. And this is weird. I'm seeing it too.
>>
>>>
>>> Some initial investigation into ftrace seems to point at the reservation and commit of the events into the ring buffer
>>> as the highest sources of overhead, while event parameters copying, including COMM, does not seem to have any noticeable effect
>>> relative to those costs.
>>
>> Did you use perf to see where the issues were? If so what perf command
>> did you use?
>
> I did not use perf, I will now do it and see what I get.
> You can discard my comment regarding reservation and commit, it was based on a wrong premise.
>
> Regarding parameter copying, I just brutally modified the code to not fill fields (TP_fast_assign),
> and I just noticed that it did not change the result significantly.
>
>>
>> When running just:
>> perf record ./cyclictest --smp -p95 -m -s -N -l 1000000 -q
>>
>> I get this:
>>
>> 6.08% cyclictest [kernel.vmlinux] [k] syscall_return_via_sysret
>> 4.03% cyclictest cyclictest [.] timerthread
>> 3.97% cyclictest [kernel.vmlinux] [k] __schedule
>> 3.81% cyclictest [kernel.vmlinux] [k] native_write_msr
>> 3.13% cyclictest [kernel.vmlinux] [k] trace_save_cmdline
>> 2.71% cyclictest [kernel.vmlinux] [k] do_nanosleep
>> 2.48% cyclictest [kernel.vmlinux] [k] do_syscall_64
>> 2.11% cyclictest [kernel.vmlinux] [k] pick_next_task_fair
>> 2.03% cyclictest [kernel.vmlinux] [k] native_sched_clock
>> 1.99% cyclictest [kernel.vmlinux] [k] cpuacct_charge
>> 1.93% cyclictest [vdso] [.] __vdso_clock_gettime
>> 1.86% cyclictest [kernel.vmlinux] [k] hrtimer_nanosleep
>> 1.84% cyclictest [kernel.vmlinux] [k] cpupri_set
>> 1.66% cyclictest [kernel.vmlinux] [k] update_curr_rt
>> 1.65% cyclictest [kernel.vmlinux] [k] __perf_event_task_sched_out
>> 1.59% cyclictest [kernel.vmlinux] [k] dequeue_rt_stack
>> 1.55% cyclictest [kernel.vmlinux] [k] __rb_reserve_next
>>
>> Which shows there's a lot being done before we even get to the ring
>> buffer reserve.
>
> I get (CONFIG_PREEMPT_VOLUNTARY):
>
> SCHED EVENTS DISABLED:
>
> 8.57% cyclictest [kernel.vmlinux] [k] syscall_return_via_sysret
> 7.04% cyclictest [kernel.vmlinux] [k] native_write_msr
> 6.93% cyclictest [kernel.vmlinux] [k] entry_SYSCALL_64_trampoline
> 4.42% cyclictest cyclictest [.] timerthread
> 4.14% cyclictest [kernel.vmlinux] [k] __schedule
> 3.52% cyclictest [kernel.vmlinux] [k] cpupri_set
> 2.90% cyclictest [kernel.vmlinux] [k] finish_task_switch
> 2.42% cyclictest [kernel.vmlinux] [k] update_curr_rt
> 2.34% cyclictest [kernel.vmlinux] [k] _raw_spin_lock
> 2.28% cyclictest [kernel.vmlinux] [k] ktime_get
> 2.14% cyclictest [kernel.vmlinux] [k] timerqueue_add
> 2.06% cyclictest [kernel.vmlinux] [k] read_tsc
> 2.06% cyclictest [kernel.vmlinux] [k] native_sched_clock
> 2.01% cyclictest [vdso] [.] __vdso_clock_gettime
> 1.95% cyclictest [kernel.vmlinux] [k] do_nanosleep
> 1.93% cyclictest [kernel.vmlinux] [k] pick_next_task_fair
> 1.89% cyclictest [vdso] [.] 0x0000000000000977
> 1.75% cyclictest [kernel.vmlinux] [k] dequeue_task_rt
> 1.64% cyclictest [kernel.vmlinux] [k] x86_pmu_disable
> 1.57% cyclictest [kernel.vmlinux] [k] dequeue_rt_stack
> 1.53% cyclictest [kernel.vmlinux] [k] __x64_sys_nanosleep
> 1.51% cyclictest [kernel.vmlinux] [k] do_syscall_64
> 1.41% cyclictest [kernel.vmlinux] [k] put_prev_task_rt
> 1.34% cyclictest [kernel.vmlinux] [k] pick_next_task_dl
> 1.28% cyclictest [kernel.vmlinux] [k] pick_next_task_rt
> 1.25% cyclictest [kernel.vmlinux] [k] cpuacct_charge
> 1.25% cyclictest [kernel.vmlinux] [k] hrtimer_start_range_ns
> 1.24% cyclictest [kernel.vmlinux] [k] __x86_indirect_thunk_rax
> 1.07% cyclictest libpthread-2.23.so [.] 0x0000000000010c1b
> 0.94% cyclictest [kernel.vmlinux] [k] pull_rt_task
> 0.93% cyclictest [kernel.vmlinux] [k] entry_SYSCALL_64_stage2
> 0.89% cyclictest [kernel.vmlinux] [k] hrtimer_nanosleep
> 0.89% cyclictest libpthread-2.23.so [.] 0x0000000000010c1d
> 0.88% cyclictest [kernel.vmlinux] [k] entry_SYSCALL_64
> 0.79% cyclictest [kernel.vmlinux] [k] _raw_spin_lock_irqsave
> 0.79% cyclictest [kernel.vmlinux] [k] ctx_sched_out
> 0.78% cyclictest [kernel.vmlinux] [k] copy_user_generic_unrolled
> 0.77% cyclictest libpthread-2.23.so [.] __pthread_enable_asynccancel
> 0.71% cyclictest [kernel.vmlinux] [k] __perf_event_task_sched_out
> 0.71% cyclictest [kernel.vmlinux] [k] pick_next_task_idle
> 0.70% cyclictest libpthread-2.23.so [.] __pthread_disable_asynccancel
> 0.70% cyclictest [kernel.vmlinux] [k] __hrtimer_init
> 0.68% cyclictest [kernel.vmlinux] [k] clear_page_erms
> 0.64% cyclictest [kernel.vmlinux] [k] intel_pmu_disable_all
> 0.58% cyclictest [kernel.vmlinux] [k] copy_user_enhanced_fast_string
> 0.58% cyclictest [kernel.vmlinux] [k] _copy_from_user
> 0.58% cyclictest [kernel.vmlinux] [k] _raw_spin_unlock_irqrestore
> 0.54% cyclictest [kernel.vmlinux] [k] get_nohz_timer_target
> 0.54% cyclictest [kernel.vmlinux] [k] pick_next_task_stop
[...]
>
> SCHED EVENTS ENABLED:
>
> 7.33% cyclictest [kernel.vmlinux] [k] syscall_return_via_sysret
> 6.51% cyclictest [kernel.vmlinux] [k] entry_SYSCALL_64_trampoline
> 5.84% cyclictest [kernel.vmlinux] [k] native_write_msr
> 4.00% cyclictest cyclictest [.] timerthread
> 3.62% cyclictest [kernel.vmlinux] [k] __schedule
> 2.71% cyclictest [kernel.vmlinux] [k] cpupri_set
> 2.57% cyclictest [kernel.vmlinux] [k] finish_task_switch
> 2.46% cyclictest [kernel.vmlinux] [k] native_sched_clock
> 2.19% cyclictest [kernel.vmlinux] [k] __x86_indirect_thunk_rax
> 2.03% cyclictest [kernel.vmlinux] [k] ktime_get
> 2.01% cyclictest [kernel.vmlinux] [k] pick_next_task_fair
> 1.93% cyclictest [kernel.vmlinux] [k] _raw_spin_lock
> 1.90% cyclictest [kernel.vmlinux] [k] timerqueue_add
> 1.70% cyclictest [kernel.vmlinux] [k] do_nanosleep
> 1.67% cyclictest [kernel.vmlinux] [k] read_tsc
> 1.65% cyclictest [kernel.vmlinux] [k] cpuacct_charge
> 1.60% cyclictest [kernel.vmlinux] [k] update_curr_rt
> 1.49% cyclictest [kernel.vmlinux] [k] probe_sched_switch
> 1.40% cyclictest [kernel.vmlinux] [k] __x64_sys_nanosleep
> 1.34% cyclictest [kernel.vmlinux] [k] dequeue_rt_stack
> 1.28% cyclictest [kernel.vmlinux] [k] do_syscall_64
> 1.27% cyclictest [kernel.vmlinux] [k] x86_pmu_disable
> 1.24% cyclictest [kernel.vmlinux] [k] __rb_reserve_next
> 1.23% cyclictest [vdso] [.] 0x0000000000000977
> 1.21% cyclictest [vdso] [.] __vdso_clock_gettime
> 1.06% cyclictest libpthread-2.23.so [.] 0x0000000000010c1d
> 1.06% cyclictest [kernel.vmlinux] [k] entry_SYSCALL_64_stage2
> 1.03% cyclictest [kernel.vmlinux] [k] pick_next_task_dl
> 1.00% cyclictest [kernel.vmlinux] [k] put_prev_task_rt
> 1.00% cyclictest [kernel.vmlinux] [k] pick_next_task_rt
> 0.98% cyclictest [kernel.vmlinux] [k] trace_event_raw_event_sched_swit
> 0.97% cyclictest libpthread-2.23.so [.] 0x0000000000010c1b
> 0.95% cyclictest [kernel.vmlinux] [k] pick_next_task_idle
> 0.91% cyclictest [kernel.vmlinux] [k] trace_save_cmdline
> 0.90% cyclictest [kernel.vmlinux] [k] copy_user_generic_unrolled
> 0.90% cyclictest [kernel.vmlinux] [k] ctx_sched_out
> 0.90% cyclictest [kernel.vmlinux] [k] lapic_next_deadline
> 0.86% cyclictest [kernel.vmlinux] [k] intel_pmu_disable_all
> 0.85% cyclictest [kernel.vmlinux] [k] rb_commit
> 0.84% cyclictest [kernel.vmlinux] [k] entry_SYSCALL_64
> 0.82% cyclictest [kernel.vmlinux] [k] _raw_spin_lock_irqsave
> 0.80% cyclictest [kernel.vmlinux] [k] hrtimer_nanosleep
> 0.76% cyclictest [kernel.vmlinux] [k] _copy_from_user
> 0.75% cyclictest [kernel.vmlinux] [k] __hrtimer_init
> 0.72% cyclictest [kernel.vmlinux] [k] ring_buffer_lock_reserve
> 0.69% cyclictest [kernel.vmlinux] [k] __perf_event_task_sched_out
> 0.66% cyclictest [kernel.vmlinux] [k] hrtimer_start_range_ns
> 0.63% cyclictest [kernel.vmlinux] [k] get_nohz_timer_target
> 0.62% cyclictest [kernel.vmlinux] [k] clear_page_erms
> 0.61% cyclictest libpthread-2.23.so [.] __pthread_disable_asynccancel
> 0.59% cyclictest [kernel.vmlinux] [k] dequeue_task_rt
> 0.58% cyclictest [kernel.vmlinux] [k] schedule
> 0.56% cyclictest [kernel.vmlinux] [k] pull_rt_task
> 0.55% cyclictest [kernel.vmlinux] [k] _raw_spin_unlock_irqrestore
> 0.54% cyclictest [kernel.vmlinux] [k] rb_insert_color
> 0.52% cyclictest [kernel.vmlinux] [k] trace_event_buffer_reserve
> 0.51% cyclictest [kernel.vmlinux] [k] trace_clock_local
> 0.50% cyclictest [kernel.vmlinux] [k] pick_next_task_stop
[...]
>
>> One thing that I notice too is that we have trace_save_cmdline that
>> also gets activated when enabling trace events. Perhaps I should try to
>> optimize that more.
>>>
>>> I have been running 20 times the following test, and thrown away the first results:
>>>
>>> $ sudo ./cyclictest --smp -p95 -m -s -N -l 100000 -q
>>>
>>> $ uname -a
>>> Linux claudio-HP-ProBook-470-G5 4.18.0-rc3+ #3 SMP Tue Jul 3 15:50:30 CEST 2018 x86_64 x86_64 x86_64 GNU/Linux
>>>
>>> For brevity, this is a comparison of one test's results. All other test results show the same ~25% increase.
>>>
>>> On the left side, the run without ftrace sched events, on the right side with ftrace sched events enabled.
>>>
>>> CPU Count Min Act Avg Max Count Min-ftrace Act-ftrace Avg-ftrace Max-ftrace
>>> 0 100000 2339 2936 2841 139478 100000 2900 3182 3566 93056
>>> 1 66742 2365 3386 2874 93639 66750 2959 3786 3646 154074
>>> 2 50080 2376 3058 2910 196221 50097 2997 4209 3655 18707
>>> 3 40076 2394 3461 2931 17914 40091 3006 4417 3750 17159
>>> 4 33404 2371 3612 2834 15336 33419 2997 3836 3594 23172
>>> 5 28635 2387 3313 2885 25863 28649 2995 3795 3647 9956
>>> 6 25058 2384 3428 2968 12162 25071 3051 4366 3719 18151
>>> 7 22275 2381 2859 2982 10706 22287 3046 5078 3825 10781
>>>
>>> I would be thankful for any advice or comments on this,
>>> especially with the goal in mind to lower as much as possible the runtime impact on the system.
>>
>> But still, we use all sorts of trace events when dealing with real
>> time, and trace events hardly shows anything in the PREEMPT_RT kernel.
>> Where cyclictest shows 20 microseconds, and sched events hardly touches
>> that. But I see this weird side effect on a distro kernel too. Maybe
>> there's something weird going on when we have CONFIG_VOLUNTARY_PREEMPT
>> set :-/
>
> If it helps, I tested also CONFIG_PREEMPT_NONE=y and CONFIG_PREEMPT=y,
> with the same relative result (~25% increase).
>
>>
>> I'll investigate further.
>
> If stress-ng --switch-ops is of any help, I have some results for that as well.
>
> This is with CONFIG_PREEMPT_NONE:
>
> SCHED EVENTS DISABLED
>
> $ stress-ng --times -s 8 --switch-ops 20000000
>
> Test Runtime(s) User(%) Sys(%) Total(%) bogo ops/s
> 1 22.56 6.33 92.96 99.29 886525
> 2 23.09 6.34 92.34 98.68 866176
> 3 23.54 6.35 89.02 95.37 849618
> 4 22.19 6.56 92.73 99.29 901306
> 5 22.69 6.41 92.77 99.18 881445
>
> SCHED EVENTS ENABLED
>
> $ stress-ng --times -s 8 --switch-ops 20000000
>
> Test Runtime(s) User(%) Sys(%) Total(%) bogo ops/s
> 1 36.28 4.12 95.31 99.43 551268
> 2 37.61 4.07 95.18 99.25 531773
> 3 37.96 4.09 94.94 99.03 526870
> 4 38.17 4.01 94.97 98.99 523972
> 5 37.47 4.11 94.92 99.03 533760
>
> This is with CONFIG_PREEMPT:
>
> SCHED EVENTS DISABLED
>
> $ stress-ng --times -s 8 --switch-ops 20000000
>
> Test Runtime(s) User(%) Sys(%) Total(%) bogo ops/s
> 1 25.73 5.65 93.28 98.93 777303
> 2 25.74 5.68 93.14 98.82 777001
> 3 24.84 5.88 92.95 98.83 805153
> 4 25.12 5.69 93.06 98.75 796178
> 5 26.04 5.58 92.79 98.37 768049
>
> SCHED EVENTS ENABLED
>
> Test Runtime(s) User(%) Sys(%) Total(%) bogo ops/s
> 1 41.70 3.72 95.90 99.62 479616
> 2 42.91 3.59 95.52 99.11 466092
> 3 43.24 3.48 95.57 99.05 462535
> 4 43.04 3.46 95.83 99.30 464684
> 5 42.62 3.60 95.75 99.34 469263
>
> This is with CONFIG_VOLUNTARY_PREEMPT:
>
> SCHED EVENTS DISABLED
>
> $ stress-ng --times -s 8 --switch-ops 20000000
>
> Test Runtime(s) User(%) Sys(%) Total(%) bogo ops/s
> 1 23.27 6.24 91.41 97.64 859476
> 2 23.51 6.16 92.70 98.86 850702
> 3 23.47 6.42 92.62 99.04 852152
> 4 23.92 6.34 92.47 98.81 836120
> 5 23.63 6.28 92.77 99.05 846382
>
> SCHED EVENTS ENABLED
>
> Test Runtime(s) User(%) Sys(%) Total(%) bogo ops/s
> 1 38.65 3.82 95.12 98.94 517464
> 2 38.77 4.27 94.77 99.03 515863
> 3 38.70 3.96 95.29 99.25 516796
> 4 38.52 4.06 95.32 99.38 519211
> 5 38.58 4.24 94.71 98.95 518403
>
>> Thanks for the report.
>>
>> -- Steve
>>
>
> Thank you,
>
> I will try to get more familiar with the internals
> and then investigate as well.
>
> Claudio
>

One additional data point, based on brute force again:

I applied this change, in order to understand if it was the

trace_event_raw_event_* (I suppose primarily trace_event_raw_event_switch)

that contained the latency "offenders":

diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 4ecdfe2..969467d 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -704,6 +704,8 @@ trace_event_raw_event_##call(void *__data, proto)
struct trace_event_raw_##call *entry; \
int __data_size; \
\
+ return; \
+ \
if (trace_trigger_soft_disabled(trace_file)) \
return; \
\

This reduces the latency overhead to 6% down from 25%.

Maybe obvious? Wanted to share in case it helps, and will dig further.

Ciao,

Claudio

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