Messages in this thread Patch in this message | | | Date | Wed, 26 Jul 2023 10:07:30 -0400 | Subject | Re: [RFC PATCH 1/1] sched: Extend cpu idle state for 1ms | From | Mathieu Desnoyers <> |
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On 7/26/23 04:04, Shrikanth Hegde wrote: > > > On 7/26/23 1:00 AM, Mathieu Desnoyers wrote: >> Allow select_task_rq to consider a cpu as idle for 1ms after that cpu >> has exited the idle loop. >> >> This speeds up the following hackbench workload on a 192 cores AMD EPYC >> 9654 96-Core Processor (over 2 sockets): >> >> hackbench -g 32 -f 20 --threads --pipe -l 480000 -s 100 >> >> from 49s to 34s. (30% speedup) >> >> My working hypothesis for why this helps is: queuing more than a single >> task on the runqueue of a cpu which just exited idle rather than >> spreading work over other idle cpus helps power efficiency on systems >> with large number of cores. >> >> This was developed as part of the investigation into a weird regression >> reported by AMD where adding a raw spinlock in the scheduler context >> switch accelerated hackbench. >> >> It turned out that changing this raw spinlock for a loop of 10000x >> cpu_relax within do_idle() had similar benefits. >> >> This patch achieve a similar effect without the busy-waiting by >> introducing a runqueue state sampling the sched_clock() when exiting >> idle, which allows select_task_rq to consider "as idle" a cpu which has >> recently exited idle. >> >> This patch should be considered "food for thoughts", and I would be glad >> to hear feedback on whether it causes regressions on _other_ workloads, >> and whether it helps with the hackbench workload on large Intel system >> as well. >> >> Link: https://lore.kernel.org/r/09e0f469-a3f7-62ef-75a1-e64cec2dcfc5@amd.com >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> >> Cc: Ingo Molnar <mingo@redhat.com> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Valentin Schneider <vschneid@redhat.com> >> Cc: Steven Rostedt <rostedt@goodmis.org> >> Cc: Ben Segall <bsegall@google.com> >> Cc: Mel Gorman <mgorman@suse.de> >> Cc: Daniel Bristot de Oliveira <bristot@redhat.com> >> Cc: Vincent Guittot <vincent.guittot@linaro.org> >> Cc: Juri Lelli <juri.lelli@redhat.com> >> Cc: Swapnil Sapkal <Swapnil.Sapkal@amd.com> >> Cc: Aaron Lu <aaron.lu@intel.com> >> Cc: x86@kernel.org >> --- >> kernel/sched/core.c | 4 ++++ >> kernel/sched/sched.h | 3 +++ >> 2 files changed, 7 insertions(+) >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index a68d1276bab0..d40e3a0a5ced 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -6769,6 +6769,7 @@ void __sched schedule_idle(void) >> * TASK_RUNNING state. >> */ >> WARN_ON_ONCE(current->__state); >> + WRITE_ONCE(this_rq()->idle_end_time, sched_clock()); >> do { >> __schedule(SM_NONE); >> } while (need_resched()); >> @@ -7300,6 +7301,9 @@ int idle_cpu(int cpu) >> { >> struct rq *rq = cpu_rq(cpu); >> >> + if (sched_clock() < READ_ONCE(rq->idle_end_time) + IDLE_CPU_DELAY_NS) > > > Wouldn't this hurt the latency badly? Specially on a loaded system with > a workload that does a lot of wakeup.
Good point !
Can you try your benchmark replacing the if () statement above by:
+ if (sched_clock() < READ_ONCE(rq->idle_end_time) + IDLE_CPU_DELAY_NS && + READ_ONCE(rq->nr_running) <= 4) + return 1;
It speeds up the hackbench test-case even more here. It's now 30s, and it should improve tail latency.
Thanks,
Mathieu
> > ran schbench on a 50% loaded system with stress-ng. (there could be a better benchmark to measure latency) > I see that latency takes a hit. specially tail latencies.full log below with different schbench groups. > > 6.5-rc3 6.5-rc3+this patch > > Groups: 1 > 50.0th: 14.0 13.0 > 75.0th: 16.0 16.0 > 90.0th: 19.5 20.0 > 95.0th: 53.0 226.0 > 99.0th: 1969.0 2165.0 > 99.5th: 2912.0 2648.0 > 99.9th: 4680.0 4142.0 > > Groups: 2 > 50.0th: 15.5 15.5 > 75.0th: 18.0 19.5 > 90.0th: 25.5 497.0 > 95.0th: 323.0 1384.0 > 99.0th: 2055.0 3144.0 > 99.5th: 2972.0 4014.0 > 99.9th: 6026.0 6560.0 > > Groups: 4 > 50.0th: 18.0 18.5 > 75.0th: 21.5 26.0 > 90.0th: 56.0 940.5 > 95.0th: 678.0 1896.0 > 99.0th: 2484.0 3756.0 > 99.5th: 3224.0 4616.0 > 99.9th: 4960.0 6824.0 > > Groups: 8 > 50.0th: 23.5 25.5 > 75.0th: 30.5 421.5 > 90.0th: 443.5 1722.0 > 95.0th: 1410.0 2736.0 > 99.0th: 3942.0 5496.0 > 99.5th: 5232.0 7016.0 > 99.9th: 7996.0 8896.0 > > Groups: 16 > 50.0th: 33.5 41.5 > 75.0th: 49.0 752.0 > 90.0th: 1067.5 2332.0 > 95.0th: 2093.0 3468.0 > 99.0th: 5048.0 6728.0 > 99.5th: 6760.0 7624.0 > 99.9th: 8592.0 9504.0 > > Groups: 32 > 50.0th: 60.0 79.0 > 75.0th: 456.5 1712.0 > 90.0th: 2788.0 3996.0 > 95.0th: 4544.0 5768.0 > 99.0th: 8444.0 9104.0 > 99.5th: 9168.0 9808.0 > 99.9th: 11984.0 12448.0 > > >> + return 1; >> + >> if (rq->curr != rq->idle) >> return 0; >> >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >> index 81ac605b9cd5..8932e198a33a 100644 >> --- a/kernel/sched/sched.h >> +++ b/kernel/sched/sched.h >> @@ -97,6 +97,8 @@ >> # define SCHED_WARN_ON(x) ({ (void)(x), 0; }) >> #endif >> >> +#define IDLE_CPU_DELAY_NS 1000000 /* 1ms */ >> + >> struct rq; >> struct cpuidle_state; >> >> @@ -1010,6 +1012,7 @@ struct rq { >> >> struct task_struct __rcu *curr; >> struct task_struct *idle; >> + u64 idle_end_time; >> struct task_struct *stop; >> unsigned long next_balance; >> struct mm_struct *prev_mm;
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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