Messages in this thread | | | Date | Thu, 23 Nov 2023 13:13:27 +0000 | Subject | Re: [PATCH] sched/pelt: avoid underestimate of task utilization | From | Lukasz Luba <> |
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On 11/22/23 14:01, Vincent Guittot wrote: > It has been reported that thread's util_est can significantly decrease as > a result of sharing the CPU with other threads. The use case can be easily > reproduced with a periodic task TA that runs 1ms and sleeps 100us. > When the task is alone on the CPU, its max utilization and its util_est is > around 888. If another similar task starts to run on the same CPU, TA will > have to share the CPU runtime and its maximum utilization will decrease > around half the CPU capacity (512) then TA's util_est will follow this new > maximum trend which is only the result of sharing the CPU with others > tasks. Such situation can be detected with runnable_avg wich is close or > equal to util_avg when TA is alone but increases above util_avg when TA > shares the CPU with other threads and wait on the runqueue. > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > > This patch implements what I mentioned in [1]. I have been able to > reproduce such pattern with rt-app. > > [1] https://lore.kernel.org/lkml/CAKfTPtDd-HhF-YiNTtL9i5k0PfJbF819Yxu4YquzfXgwi7voyw@mail.gmail.com/#t
Thanks Vincet for looking at it! I didn't have to to come back to this issue.
> > kernel/sched/fair.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 07f555857698..eeb505d28905 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4774,6 +4774,11 @@ static inline unsigned long task_util(struct task_struct *p) > return READ_ONCE(p->se.avg.util_avg); > } > > +static inline unsigned long task_runnable(struct task_struct *p) > +{ > + return READ_ONCE(p->se.avg.runnable_avg); > +} > + > static inline unsigned long _task_util_est(struct task_struct *p) > { > struct util_est ue = READ_ONCE(p->se.avg.util_est); > @@ -4892,6 +4897,14 @@ static inline void util_est_update(struct cfs_rq *cfs_rq, > if (task_util(p) > arch_scale_cpu_capacity(cpu_of(rq_of(cfs_rq)))) > return; > > + /* > + * To avoid underestimate of task utilization, skip updates of ewma if > + * we cannot grant that thread got all CPU time it wanted. > + */ > + if ((ue.enqueued + UTIL_EST_MARGIN) < task_runnable(p)) > + goto done; > + > + > /* > * Update Task's estimated utilization > *
That looks reasonable to do. I cannot test it right now on my pixel6. It should do the trick to the task util that we need in bigger apps than rt-app as well.
Reviewed-by: Lukasz luba <lukasz.luba@arm.com>
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