Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 22 Dec 2021 09:20:17 +0100 | Subject | Re: [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs |
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On Mon, 20 Dec 2021 at 12:43, Vincent Donnefort <vincent.donnefort@arm.com> wrote: > > During a migration, the lock for the previous runqueue is not taken and > hence, the task contribution isn't directly removed from that runqueue > utilization but instead temporarily saved, until the next PELT signals > update where it would be accounted. There is then a window in which a > CPU can ben idle be nonetheless overutilized. > > The load balancer wouldn't be able to do anything to help a sleeping CPU, > it brings then no gain to raise overutilized there, only the risk of > spuriously doing it.
But how do you make the difference between a very short idle time of an overutilized CPU and a idle cpu with outdated utilization
Being idle is not a good reason for not being overutilized (ie ~80% of average utilisation)
> > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 51f6f55abb37..37f737c5f0b8 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8641,26 +8641,28 @@ static inline void update_sg_lb_stats(struct lb_env *env, > > nr_running = rq->nr_running; > sgs->sum_nr_running += nr_running; > - > - if (nr_running > 1) > - *sg_status |= SG_OVERLOAD; > - > - if (cpu_overutilized(i)) > - *sg_status |= SG_OVERUTILIZED; > - > #ifdef CONFIG_NUMA_BALANCING > sgs->nr_numa_running += rq->nr_numa_running; > sgs->nr_preferred_running += rq->nr_preferred_running; > #endif > + if (nr_running > 1) > + *sg_status |= SG_OVERLOAD;
Why do you move this code related to overload ?
> + > /* > * No need to call idle_cpu() if nr_running is not 0 > */ > if (!nr_running && idle_cpu(i)) { > sgs->idle_cpus++; > - /* Idle cpu can't have misfit task */ > + /* > + * Idle cpu can neither be overutilized nor have a > + * misfit task. > + */ > continue; > } > > + if (cpu_overutilized(i)) > + *sg_status |= SG_OVERUTILIZED; > + > if (local_group) > continue; > > -- > 2.25.1 >
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