On 21/05/20 02:44, Frederic Weisbecker wrote: > On Wed, May 20, 2020 at 08:47:10PM +0200, Juri Lelli wrote: > > On 20/05/20 19:02, Frederic Weisbecker wrote: > > > On Wed, May 20, 2020 at 06:49:25PM +0200, Juri Lelli wrote: > > > > On 20/05/20 18:24, Frederic Weisbecker wrote: > > > > > > > > Hummm, so I enabled 'timer:*', anything else you think I should be > > > > looking at? > > > > > > Are you sure you also enabled timer_expire_entry? > > > Because: > > > > I run with > > > > trace-cmd record -e sched_switch -e sched_wakeup -e sched_migrate_task \ > > -e 'timer:*' -e 'irq_vectors:*' -e 'irq:softirq*' \ > > -e 'irq:irq_handler*' taskset --cpu-list 4-35 ./sysjitter/sysjitter \ > > --runtime 10 200 > > > > And > > > > $ grep ' timer_expire_entry:' trace.txt | wc -l > > 1173 > > > > seems to confirm that the event was indeed enabled, e.g. > > > > ksoftirqd/11-100 [011] 159.270023: timer_expire_entry: timer=0xffff9807df8a5a60 function=delayed_work_timer_fn now=4294826418 baseclk=4294825984 > > That's interesting! I'll see if I can reproduce that with threaded irqs. If not we'll > have to investigate on your machine. We really don't want to trigger timer softirqs > when it's not necessary. > So, I managed to get a bit more info on this, please check the following (which I edited a bit to hopefully make it clearer and apologies for long lines, but I believe trimming them would mess things up) --- ksoftirqd/3-26 [003] 99.942485: timer_expire_entry: timer=0xffffffffa55a9d20 function=clocksource_watchdog now=4294759328 baseclk=4294759328 ksoftirqd/3-26 [003] 99.942489: timer_start: timer=0xffffffffa55a9d20 function=clocksource_watchdog expires=4294759822 [timeout=494] cpu=4 idx=114 flags=D|P|I ksoftirqd/3-26 [003] 99.942491: timer_expire_exit: timer=0xffffffffa55a9d20 sysjitter-2377 [004] 99.942491: call_function_single_entry: vector=251 sysjitter-2377 [004] 99.942492: call_function_single_exit: vector=251 ksoftirqd/3-26 [003] 99.942493: softirq_exit: vec=1 [action=TIMER] ksoftirqd/3-26 [003] 99.942494: softirq_entry: vec=7 [action=SCHED] sysjitter-2377 [004] 99.942494: hrtimer_start: hrtimer=0xffff92e9df91fd20 function=tick_sched_timer/0x0 expires=92630457385 softexpires=92630457385 --- So, my understanding is that clock gets reactivated on CPU 4 to run clocksource watchdog (indeed if I use tsc=nowatchdog this above doesn't happen, but the potential issue is still there I guess). --- sysjitter-2377 [004] 100.438455: local_timer_entry: vector=236 sysjitter-2377 [004] 100.438456: hrtimer_cancel: hrtimer=0xffff92e9df91fd20 sysjitter-2377 [004] 100.438458: hrtimer_expire_entry: hrtimer=0xffff92e9df91fd20 now=92630461337 function=tick_sched_timer/0x0 sysjitter-2377 [004] 100.438460: kernel_stack: <stack trace> => __ftrace_trace_stack (ffffffffa37da921) => __raise_softirq_irqoff (ffffffffa36daf50) => raise_softirq (ffffffffa36db0fe) => update_process_times (ffffffffa3768f9d) => tick_sched_handle (ffffffffa377aaa2) => tick_sched_timer (ffffffffa377ad53) => __hrtimer_run_queues (ffffffffa3769cf0) => hrtimer_interrupt (ffffffffa376a58a) => smp_apic_timer_interrupt (ffffffffa40028f8) => apic_timer_interrupt (ffffffffa4001b7f) sysjitter-2377 [004] 100.438461: softirq_raise: vec=1 [action=TIMER] sysjitter-2377 [004] 100.438464: kernel_stack: <stack trace> => __ftrace_trace_stack (ffffffffa37da921) => __raise_softirq_irqoff (ffffffffa36daf50) => raise_softirq (ffffffffa36db0fe) => trigger_load_balance (ffffffffa371cb9c) => update_process_times (ffffffffa3768fc7) => tick_sched_handle (ffffffffa377aaa2) => tick_sched_timer (ffffffffa377ad53) => __hrtimer_run_queues (ffffffffa3769cf0) => hrtimer_interrupt (ffffffffa376a58a) => smp_apic_timer_interrupt (ffffffffa40028f8) => apic_timer_interrupt (ffffffffa4001b7f) sysjitter-2377 [004] 100.438465: softirq_raise: vec=7 [action=SCHED] --- Tick fires, the two softirq_raise (which you wondered about) seems to come from 1. update_process_times::run_local_timers (TIMER_SOFTIRQ) 2. update_process_times::scheduler_tick::trigger_load_balance (SCHED_SOFTIRQ) --- sysjitter-2377 [004] 100.438468: hrtimer_expire_exit: hrtimer=0xffff92e9df91fd20 sysjitter-2377 [004] 100.438470: local_timer_exit: vector=236 sysjitter-2377 [004] 100.438473: sched_wakeup: ksoftirqd/4:31 [120] success=1 CPU:004 sysjitter-2377 [004] 100.438474: tick_stop: success=0 dependency=SCHED sysjitter-2377 [004] 100.438476: hrtimer_start: hrtimer=0xffff92e9df91fd20 function=tick_sched_timer/0x0 expires=92630512937 softexpires=92630512937 sysjitter-2377 [004] 100.438484: irq_work_entry: vector=246 sysjitter-2377 [004] 100.438491: irq_work_exit: vector=246 sysjitter-2377 [004] 100.438492: tick_stop: success=0 dependency=SCHED sysjitter-2377 [004] 100.438495: sched_switch: sysjitter:2377 [120] R ==> ksoftirqd/4:31 [120] ksoftirqd/4-31 [004] 100.438497: softirq_entry: vec=1 [action=TIMER] ksoftirqd/4-31 [004] 100.438499: timer_cancel: timer=0xffffffffa55a9d20 ksoftirqd/4-31 [004] 100.438501: timer_expire_entry: timer=0xffffffffa55a9d20 function=clocksource_watchdog now=4294759824 baseclk=4294759824 ksoftirqd/4-31 [004] 100.438504: timer_start: timer=0xffffffffa55a9d20 function=clocksource_watchdog expires=4294760322 [timeout=498] cpu=5 idx=113 flags=D|P|I --- Queueing clocksource watchdog on CPU5. --- ksoftirqd/4-31 [004] 100.438506: timer_expire_exit: timer=0xffffffffa55a9d20 ksoftirqd/4-31 [004] 100.438507: local_timer_entry: vector=236 --- Back to back timer? --- ksoftirqd/4-31 [004] 100.438509: hrtimer_cancel: hrtimer=0xffff92e9df91fd20 ksoftirqd/4-31 [004] 100.438511: hrtimer_expire_entry: hrtimer=0xffff92e9df91fd20 now=92630515022 function=tick_sched_timer/0x0 ksoftirqd/4-31 [004] 100.438515: kernel_stack: <stack trace> => __ftrace_trace_stack (ffffffffa37da921) => __raise_softirq_irqoff (ffffffffa36daf50) => raise_softirq (ffffffffa36db0fe) => rcu_sched_clock_irq (ffffffffa375af4a) => update_process_times (ffffffffa3768fa4) => tick_sched_handle (ffffffffa377aaa2) => tick_sched_timer (ffffffffa377ad53) => __hrtimer_run_queues (ffffffffa3769cf0) => hrtimer_interrupt (ffffffffa376a58a) => smp_apic_timer_interrupt (ffffffffa40028f8) => apic_timer_interrupt (ffffffffa4001b7f) => filter_pred_32 (ffffffffa37f3357) => filter_match_preds (ffffffffa37f3510) => trace_event_buffer_commit (ffffffffa37dc7eb) => trace_event_raw_event_softirq (ffffffffa36dab77) => __do_softirq (ffffffffa420025a) => run_ksoftirqd (ffffffffa36dadc6) => smpboot_thread_fn (ffffffffa36ffdb8) => kthread (ffffffffa36f9fb7) => ret_from_fork (ffffffffa4000215) ksoftirqd/4-31 [004] 100.438516: softirq_raise: vec=9 [action=RCU] ksoftirqd/4-31 [004] 100.438520: kernel_stack: <stack trace> => __ftrace_trace_stack (ffffffffa37da921) => __raise_softirq_irqoff (ffffffffa36daf50) => raise_softirq (ffffffffa36db0fe) => trigger_load_balance (ffffffffa371cb9c) => update_process_times (ffffffffa3768fc7) => tick_sched_handle (ffffffffa377aaa2) => tick_sched_timer (ffffffffa377ad53) => __hrtimer_run_queues (ffffffffa3769cf0) => hrtimer_interrupt (ffffffffa376a58a) => smp_apic_timer_interrupt (ffffffffa40028f8) => apic_timer_interrupt (ffffffffa4001b7f) => filter_pred_32 (ffffffffa37f3357) => filter_match_preds (ffffffffa37f3510) => trace_event_buffer_commit (ffffffffa37dc7eb) => trace_event_raw_event_softirq (ffffffffa36dab77) => __do_softirq (ffffffffa420025a) => run_ksoftirqd (ffffffffa36dadc6) => smpboot_thread_fn (ffffffffa36ffdb8) => kthread (ffffffffa36f9fb7) => ret_from_fork (ffffffffa4000215) ksoftirqd/4-31 [004] 100.438520: softirq_raise: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.438521: hrtimer_expire_exit: hrtimer=0xffff92e9df91fd20 ksoftirqd/4-31 [004] 100.438523: hrtimer_start: hrtimer=0xffff92e9df91fd20 function=tick_sched_timer/0x0 expires=92631512937 softexpires=92631512937 ksoftirqd/4-31 [004] 100.438525: local_timer_exit: vector=236 ksoftirqd/4-31 [004] 100.438527: tick_stop: success=0 dependency=SCHED --- At this point (I think) ksoftirqd is already going to sleep, so only sysjitter will be eventually executing on CPU4, but tick_stop check still sees nr_running == 2. --- ksoftirqd/4-31 [004] 100.438530: softirq_exit: vec=1 [action=TIMER] ksoftirqd/4-31 [004] 100.438531: softirq_entry: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.438533: softirq_exit: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.438534: softirq_entry: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.438535: softirq_exit: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.438536: softirq_entry: vec=9 [action=RCU] ksoftirqd/4-31 [004] 100.438537: softirq_exit: vec=9 [action=RCU] ksoftirqd/4-31 [004] 100.438540: sched_switch: ksoftirqd/4:31 [120] S ==> sysjitter:2377 [120] --- And tick again fires below. --- sysjitter-2377 [004] 100.439509: local_timer_entry: vector=236 sysjitter-2377 [004] 100.439511: hrtimer_cancel: hrtimer=0xffff92e9df91fd20 sysjitter-2377 [004] 100.439512: hrtimer_expire_entry: hrtimer=0xffff92e9df91fd20 now=92631515408 function=tick_sched_timer/0x0 sysjitter-2377 [004] 100.439514: kernel_stack: <stack trace> => __ftrace_trace_stack (ffffffffa37da921) => __raise_softirq_irqoff (ffffffffa36daf50) => raise_softirq (ffffffffa36db0fe) => update_process_times (ffffffffa3768f9d) => tick_sched_handle (ffffffffa377aaa2) => tick_sched_timer (ffffffffa377ad53) => __hrtimer_run_queues (ffffffffa3769cf0) => hrtimer_interrupt (ffffffffa376a58a) => smp_apic_timer_interrupt (ffffffffa40028f8) => apic_timer_interrupt (ffffffffa4001b7f) sysjitter-2377 [004] 100.439515: softirq_raise: vec=1 [action=TIMER] sysjitter-2377 [004] 100.439517: kernel_stack: <stack trace> => __ftrace_trace_stack (ffffffffa37da921) => __raise_softirq_irqoff (ffffffffa36daf50) => raise_softirq (ffffffffa36db0fe) => trigger_load_balance (ffffffffa371cb9c) => update_process_times (ffffffffa3768fc7) => tick_sched_handle (ffffffffa377aaa2) => tick_sched_timer (ffffffffa377ad53) => __hrtimer_run_queues (ffffffffa3769cf0) => hrtimer_interrupt (ffffffffa376a58a) => smp_apic_timer_interrupt (ffffffffa40028f8) => apic_timer_interrupt (ffffffffa4001b7f) sysjitter-2377 [004] 100.439518: softirq_raise: vec=7 [action=SCHED] sysjitter-2377 [004] 100.439520: hrtimer_expire_exit: hrtimer=0xffff92e9df91fd20 sysjitter-2377 [004] 100.439521: hrtimer_start: hrtimer=0xffff92e9df91fd20 function=tick_sched_timer/0x0 expires=92632512937 softexpires=92632512937 sysjitter-2377 [004] 100.439522: local_timer_exit: vector=236 sysjitter-2377 [004] 100.439525: sched_wakeup: ksoftirqd/4:31 [120] success=1 CPU:004 sysjitter-2377 [004] 100.439527: tick_stop: success=0 dependency=SCHED sysjitter-2377 [004] 100.439529: irq_work_entry: vector=246 sysjitter-2377 [004] 100.439531: irq_work_exit: vector=246 sysjitter-2377 [004] 100.439532: tick_stop: success=0 dependency=SCHED sysjitter-2377 [004] 100.439534: sched_switch: sysjitter:2377 [120] R ==> ksoftirqd/4:31 [120] ksoftirqd/4-31 [004] 100.439536: softirq_entry: vec=1 [action=TIMER] ksoftirqd/4-31 [004] 100.439537: softirq_exit: vec=1 [action=TIMER] ksoftirqd/4-31 [004] 100.439538: softirq_entry: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.439540: softirq_exit: vec=7 [action=SCHED] ksoftirqd/4-31 [004] 100.439541: sched_switch: ksoftirqd/4:31 [120] S ==> sysjitter:2377 [120] --- History seems to repeat itself until ~2s later (this is one of the pathological runs) when an ipi fires on CPU4 and SCHED dependecy is seen as cleared (as only sysjitter thread was is running at that point). Does this make sense and help in any way? Suggestions for debugging this further? :-) Thanks! Juri
