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SubjectRe: ftrace performance (sched events): cyclictest shows 25% more latency
Date
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
0.48% cyclictest [kernel.vmlinux] [k] schedule
0.47% cyclictest [kernel.vmlinux] [k] lapic_next_deadline
0.43% cyclictest [kernel.vmlinux] [k] sched_avg_update
0.42% cyclictest [kernel.vmlinux] [k] __perf_event_task_sched_in
0.37% cyclictest libc-2.23.so [.] __clock_gettime
0.37% cyclictest [vdso] [.] 0x0000000000000974
0.35% cyclictest [kernel.vmlinux] [k] get_timespec64
0.32% cyclictest [kernel.vmlinux] [k] dequeue_top_rt_rq
0.31% cyclictest [kernel.vmlinux] [k] try_to_unmap_one
0.27% cyclictest [kernel.vmlinux] [k] update_rq_clock
0.24% cyclictest [kernel.vmlinux] [k] rb_insert_color
0.23% cyclictest [kernel.vmlinux] [k] deactivate_task
0.22% cyclictest [kernel.vmlinux] [k] follow_page_pte
0.21% cyclictest [kernel.vmlinux] [k] page_not_mapped
0.20% cyclictest [kernel.vmlinux] [k] sched_clock_cpu
0.18% cyclictest cyclictest [.] main
0.17% cyclictest [kernel.vmlinux] [k] clockevents_program_event
0.17% cyclictest [kernel.vmlinux] [k] hrtimer_reprogram
0.16% cyclictest [kernel.vmlinux] [k] find_busiest_group
0.15% cyclictest [kernel.vmlinux] [k] enqueue_hrtimer
0.15% cyclictest libc-2.23.so [.] __nanosleep
0.14% cyclictest cyclictest [.] clock_gettime@plt
0.14% cyclictest [kernel.vmlinux] [k] rcu_note_context_switch
0.14% cyclictest [kernel.vmlinux] [k] __intel_pmu_enable_all
0.13% cyclictest [kernel.vmlinux] [k] hrtimer_active
0.13% cyclictest libpthread-2.23.so [.] __nanosleep
0.13% cyclictest [kernel.vmlinux] [k] __msecs_to_jiffies
0.13% cyclictest [kernel.vmlinux] [k] rcu_all_qs
0.13% cyclictest libpthread-2.23.so [.] 0x0000000000010c24
0.13% cyclictest [kernel.vmlinux] [k] hrtimer_try_to_cancel
0.13% cyclictest cyclictest [.] nanosleep@plt
0.12% cyclictest [kernel.vmlinux] [k] __intel_pmu_disable_all
0.12% cyclictest [kernel.vmlinux] [k] _cond_resched
0.10% cyclictest cyclictest [.] print_stat
0.10% cyclictest [kernel.vmlinux] [k] perf_ctx_unlock
0.10% cyclictest [kernel.vmlinux] [k] sched_clock
0.10% cyclictest [kernel.vmlinux] [k] free_unref_page_list
0.09% cyclictest [vdso] [.] 0x0000000000000987
0.09% cyclictest [kernel.vmlinux] [k] update_cfs_group
0.08% cyclictest [kernel.vmlinux] [k] copy_semundo
0.08% cyclictest [kernel.vmlinux] [k] update_blocked_averages
0.07% cyclictest [kernel.vmlinux] [k] tick_program_event
0.07% cyclictest [kernel.vmlinux] [k] dequeue_task_fair
0.07% cyclictest [kernel.vmlinux] [k] __handle_mm_fault
0.06% cyclictest [kernel.vmlinux] [k] plist_del
0.06% cyclictest libpthread-2.23.so [.] 0x0000000000010c0d
0.06% cyclictest [kernel.vmlinux] [k] put_prev_task_fair
0.06% cyclictest [vdso] [.] 0x0000000000000979
0.06% cyclictest [kernel.vmlinux] [k] munlock_vma_pages_range
0.05% cyclictest [vdso] [.] 0x000000000000097d
0.05% cyclictest [kernel.vmlinux] [k] __lru_cache_add
0.05% cyclictest [kernel.vmlinux] [k] update_load_avg
0.05% cyclictest [kernel.vmlinux] [k] pick_next_entity
0.04% cyclictest [kernel.vmlinux] [k] update_rt_migration
0.04% cyclictest [kernel.vmlinux] [k] pagevec_lru_move_fn
0.04% cyclictest [kernel.vmlinux] [k] reusable_anon_vma
0.04% cyclictest [kernel.vmlinux] [k] pmd_huge
0.04% cyclictest libpthread-2.23.so [.] 0x0000000000010c21
0.04% cyclictest ld-2.23.so [.] strcmp
0.04% cyclictest libc-2.23.so [.] __call_tls_dtors
0.04% cyclictest [kernel.vmlinux] [k] free_unref_page_commit
0.04% cyclictest [kernel.vmlinux] [k] unmap_page_range
0.04% cyclictest [kernel.vmlinux] [k] PageHeadHuge
0.04% cyclictest [kernel.vmlinux] [k] vma_interval_tree_iter_next
0.04% cyclictest [kernel.vmlinux] [k] rmap_walk_file
0.04% cyclictest libpthread-2.23.so [.] 0x0000000000010c12
0.04% cyclictest [kernel.vmlinux] [k] __update_load_avg_se.isra.41
0.03% cyclictest [kernel.vmlinux] [k] __munlock_pagevec
0.03% cyclictest libc-2.23.so [.] _itoa_word
0.03% cyclictest [kernel.vmlinux] [k] __pagevec_lru_add_fn
0.03% cyclictest [kernel.vmlinux] [k] ext4_file_mmap
0.03% cyclictest [kernel.vmlinux] [k] free_pcppages_bulk
0.02% cyclictest [kernel.vmlinux] [k] lock_hrtimer_base.isra.19
0.02% cyclictest [kernel.vmlinux] [k] __mod_node_page_state
0.02% cyclictest [kernel.vmlinux] [k] rb_erase
0.02% cyclictest [kernel.vmlinux] [k] release_pages
0.02% cyclictest [kernel.vmlinux] [k] clear_buddies
0.02% cyclictest [kernel.vmlinux] [k] __hrtimer_next_event_base
0.02% cyclictest [kernel.vmlinux] [k] swapgs_restore_regs_and_return_t
0.02% cyclictest [kernel.vmlinux] [k] reweight_entity
0.02% cyclictest [kernel.vmlinux] [k] update_curr
0.02% cyclictest libpthread-2.23.so [.] 0x0000000000010c29
0.02% cyclictest libpthread-2.23.so [.] start_thread
0.02% cyclictest [kernel.vmlinux] [k] xhci_queue_intr_tx
0.02% cyclictest [kernel.vmlinux] [k] __acct_update_integrals
0.02% cyclictest [vdso] [.] 0x0000000000000971
0.02% cyclictest [kernel.vmlinux] [k] xhci_check_args
0.01% cyclictest [kernel.vmlinux] [k] idle_cpu
0.01% cyclictest [kernel.vmlinux] [k] scheduler_tick
0.01% cyclictest [kernel.vmlinux] [k] __delayacct_add_tsk
0.01% cyclictest [kernel.vmlinux] [k] perf_pmu_disable.part.98
0.01% cyclictest [kernel.vmlinux] [k] end_repeat_nmi
0.00% cyclictest [kernel.vmlinux] [k] native_irq_return_iret
0.00% cyclictest [kernel.vmlinux] [k] nmi_restore
0.00% cyclictest [kernel.vmlinux] [k] do_nmi
0.00% cyclictest [kernel.vmlinux] [k] nmi_handle
0.00% perf [kernel.vmlinux] [k] perf_event_addr_filters_exec
0.00% cyclictest [kernel.vmlinux] [k] perf_event_nmi_handler
0.00% perf [kernel.vmlinux] [k] native_write_msr
0.00% cyclictest [unknown] [.] 0000000000000000

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
0.48% cyclictest [kernel.vmlinux] [k] __perf_event_task_sched_in
0.43% cyclictest [kernel.vmlinux] [k] rcu_note_context_switch
0.41% cyclictest [kernel.vmlinux] [k] trace_event_buffer_commit
0.41% cyclictest [kernel.vmlinux] [k] deactivate_task
0.39% cyclictest [kernel.vmlinux] [k] hrtimer_reprogram
0.36% cyclictest libpthread-2.23.so [.] __pthread_enable_asynccancel
0.36% cyclictest libc-2.23.so [.] __clock_gettime
0.34% cyclictest [kernel.vmlinux] [k] tracing_generic_entry_update
0.34% cyclictest [kernel.vmlinux] [k] enqueue_hrtimer
0.29% cyclictest [kernel.vmlinux] [k] dequeue_top_rt_rq
0.29% cyclictest [kernel.vmlinux] [k] perf_ctx_unlock
0.28% cyclictest [kernel.vmlinux] [k] copy_user_enhanced_fast_string
0.27% cyclictest libpthread-2.23.so [.] __nanosleep
0.26% cyclictest [kernel.vmlinux] [k] ring_buffer_unlock_commit
0.25% cyclictest [kernel.vmlinux] [k] trace_event_buffer_lock_reserve
0.24% cyclictest [kernel.vmlinux] [k] get_timespec64
0.23% cyclictest [kernel.vmlinux] [k] find_busiest_group
0.22% cyclictest cyclictest [.] print_stat
0.21% cyclictest [kernel.vmlinux] [k] free_unref_page_list
0.21% cyclictest [kernel.vmlinux] [k] __intel_pmu_enable_all
0.21% cyclictest [kernel.vmlinux] [k] rb_move_tail
0.20% cyclictest [kernel.vmlinux] [k] ring_buffer_event_data
0.20% cyclictest [kernel.vmlinux] [k] vma_interval_tree_subtree_search
0.20% cyclictest [kernel.vmlinux] [k] sched_avg_update
0.19% cyclictest [kernel.vmlinux] [k] tick_program_event
0.19% cyclictest [kernel.vmlinux] [k] follow_page_mask
0.19% cyclictest [kernel.vmlinux] [k] sched_clock_cpu
0.19% cyclictest [kernel.vmlinux] [k] tracing_record_taskinfo_sched_sw
0.16% cyclictest [kernel.vmlinux] [k] get_page_from_freelist
0.16% cyclictest [kernel.vmlinux] [k] clockevents_program_event
0.15% cyclictest [vdso] [.] 0x0000000000000974
0.15% cyclictest [kernel.vmlinux] [k] hrtimer_try_to_cancel
0.14% cyclictest [kernel.vmlinux] [k] __alloc_pages_nodemask
0.13% cyclictest cyclictest [.] main
0.13% cyclictest ld-2.23.so [.] _dl_sysdep_start
0.13% cyclictest [kernel.vmlinux] [k] rmap_walk_file
0.12% cyclictest [kernel.vmlinux] [k] update_rq_clock
0.12% cyclictest libc-2.23.so [.] __libc_disable_asynccancel
0.12% cyclictest [vdso] [.] 0x0000000000000970
0.12% cyclictest [kernel.vmlinux] [k] hrtimer_active
0.11% cyclictest [kernel.vmlinux] [k] page_not_mapped
0.11% cyclictest [kernel.vmlinux] [k] sched_clock
0.10% cyclictest [kernel.vmlinux] [k] __mod_zone_page_state
0.10% cyclictest [kernel.vmlinux] [k] account_entity_dequeue
0.09% cyclictest libc-2.23.so [.] __nanosleep
0.09% cyclictest [kernel.vmlinux] [k] rcu_all_qs
0.09% cyclictest [kernel.vmlinux] [k] _cond_resched
0.08% cyclictest [kernel.vmlinux] [k] ring_buffer_record_is_on
0.08% cyclictest [kernel.vmlinux] [k] dequeue_entity
0.08% cyclictest [kernel.vmlinux] [k] lock_hrtimer_base.isra.19
0.08% cyclictest [kernel.vmlinux] [k] update_blocked_averages
0.08% cyclictest [kernel.vmlinux] [k] ftrace_trace_userstack
0.07% cyclictest [kernel.vmlinux] [k] try_to_unmap_one
0.07% cyclictest [kernel.vmlinux] [k] tracing_record_taskinfo_skip
0.07% cyclictest libpthread-2.23.so [.] 0x0000000000010c38
0.07% cyclictest [kernel.vmlinux] [k] __intel_pmu_disable_all
0.07% cyclictest [kernel.vmlinux] [k] free_pcppages_bulk
0.07% cyclictest [kernel.vmlinux] [k] task_ctx_sched_out
0.07% cyclictest [kernel.vmlinux] [k] __msecs_to_jiffies
0.07% cyclictest [vdso] [.] 0x0000000000000987
0.07% cyclictest [kernel.vmlinux] [k] update_rt_migration
0.06% cyclictest [kernel.vmlinux] [k] reweight_entity
0.06% cyclictest [kernel.vmlinux] [k] dequeue_task_fair
0.06% cyclictest [kernel.vmlinux] [k] update_load_avg
0.06% cyclictest [kernel.vmlinux] [k] trace_buffer_unlock_commit_regs
0.06% cyclictest [kernel.vmlinux] [k] trace_event_raw_event_sched_stat
0.05% cyclictest [kernel.vmlinux] [k] update_curr
0.05% cyclictest cyclictest [.] clock_gettime@plt
0.04% cyclictest [vdso] [.] 0x000000000000097d
0.04% cyclictest [kernel.vmlinux] [k] rcu_check_callbacks
0.04% cyclictest [kernel.vmlinux] [k] load_balance
0.03% cyclictest [kernel.vmlinux] [k] perf_pmu_disable.part.98
0.03% cyclictest [kernel.vmlinux] [k] __update_load_avg_se.isra.41
0.03% cyclictest [kernel.vmlinux] [k] __calc_delta
0.03% cyclictest [kernel.vmlinux] [k] futex_wake
0.03% cyclictest libpthread-2.23.so [.] 0x0000000000010c0d
0.03% cyclictest cyclictest [.] nanosleep@plt
0.02% cyclictest libpthread-2.23.so [.] 0x0000000000010c09
0.02% cyclictest [kernel.vmlinux] [k] set_next_entity
0.02% cyclictest [kernel.vmlinux] [k] switch_mm_irqs_off
0.02% cyclictest [kernel.vmlinux] [k] balance_runtime
0.02% cyclictest [kernel.vmlinux] [k] put_prev_task_fair
0.02% cyclictest [kernel.vmlinux] [k] find_task_by_vpid
0.02% cyclictest libpthread-2.23.so [.] 0x0000000000010c12
0.02% cyclictest [kernel.vmlinux] [k] kfree
0.02% cyclictest [kernel.vmlinux] [k] put_prev_entity
0.01% cyclictest [kernel.vmlinux] [k] rb_next
0.01% cyclictest [kernel.vmlinux] [k] ring_buffer_time_stamp_abs
0.01% cyclictest [kernel.vmlinux] [k] update_min_vruntime
0.01% cyclictest [kernel.vmlinux] [k] enqueue_top_rt_rq
0.01% cyclictest [kernel.vmlinux] [k] __kmalloc_node
0.01% cyclictest libpthread-2.23.so [.] 0x0000000000010c30
0.01% cyclictest [kernel.vmlinux] [k] ktime_get_update_offsets_now
0.01% cyclictest [kernel.vmlinux] [k] check_cfs_rq_runtime
0.01% cyclictest [kernel.vmlinux] [k] cpu_load_update
0.01% cyclictest [kernel.vmlinux] [k] flush_signal_handlers
0.01% cyclictest [kernel.vmlinux] [k] page_counter_uncharge
0.00% cyclictest [kernel.vmlinux] [k] end_repeat_nmi
0.00% cyclictest [kernel.vmlinux] [k] nmi_cpu_backtrace_handler
0.00% cyclictest [kernel.vmlinux] [k] nmi_restore
0.00% cyclictest [kernel.vmlinux] [k] nmi_cpu_backtrace
0.00% cyclictest [kernel.vmlinux] [k] nmi_handle
0.00% perf [kernel.vmlinux] [k] native_sched_clock
0.00% cyclictest [kernel.vmlinux] [k] intel_pmu_handle_irq
0.00% cyclictest [kernel.vmlinux] [k] perf_event_nmi_handler
0.00% cyclictest [kernel.vmlinux] [k] native_apic_msr_write
0.00% perf [kernel.vmlinux] [k] native_write_msr


> 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

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