Messages in this thread | | | Date | Wed, 27 Apr 2022 11:22:27 +0100 | Subject | Re: [PATCH v6 2/7] sched/fair: Decay task PELT values during wakeup migration | From | Vincent Donnefort <> |
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On 27/04/2022 10:25, Tao Zhou wrote: > On Tue, Apr 26, 2022 at 10:35:01AM +0100, Vincent Donnefort wrote: > >> Before being migrated to a new CPU, a task sees its PELT values >> synchronized with rq last_update_time. Once done, that same task will also >> have its sched_avg last_update_time reset. This means the time between >> the migration and the last clock update (B) will not be accounted for in >> util_avg and a discontinuity will appear. This issue is amplified by the >> PELT clock scaling. If the clock hasn't been updated while the CPU is >> idle, clock_pelt will not be aligned with clock_task and that time (A) >> will be also lost. >> >> ---------|----- A -----|-----------|------- B -----|> >> clock_pelt clock_task clock now >> >> This is especially problematic for asymmetric CPU capacity systems which >> need stable util_avg signals for task placement and energy estimation. >> >> Ideally, this problem would be solved by updating the runqueue clocks >> before the migration. But that would require taking the runqueue lock >> which is quite expensive [1]. Instead estimate the missing time and update >> the task util_avg with that value: >> >> A + B = clock_task - clock_pelt + sched_clock_cpu() - clock >> >> sched_clock_cpu() is a costly functinon. Limit the usage to the case where >> the source CPU is idle as we know this is when the clock is having the >> biggest risk of being outdated. >> >> Neither clock_task, clock_pelt nor clock can be accessed without the >> runqueue lock. We then need to store those values in a timestamp variable >> which can be accessed during the migration. rq's enter_idle will give the >> wall-clock time when the rq went idle. We have then: >> >> B = sched_clock_cpu() - rq->enter_idle. >> >> Then, to catch-up the PELT clock scaling (A), two cases: >> >> * !CFS_BANDWIDTH: We can simply use clock_task(). This value is stored >> in rq's clock_pelt_idle, before the rq enters idle. The estimated time >> is then: >> >> rq->clock_pelt_idle + sched_clock_cpu() - rq->enter_idle. >> >> * CFS_BANDWIDTH: We can't catch-up with clock_task because of the >> throttled_clock_task_time offset. cfs_rq's clock_pelt_idle is then >> giving the PELT clock when the cfs_rq becomes idle. This gives: >> >> A = rq->clock_pelt_idle - cfs_rq->clock_pelt_idle > > The code calulating A below is not consistent with this. The order is reversed. >
Good catch, but this comment is actually correct, the code is not. rq->clock_pelt_idle is updated _after_ cfs_rq->clock_pelt_idle. (see previous email to Vincent)
Thanks,
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