Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

Vincent Guittot
2024-09-05T13:33:52Z

On Thu, 5 Sept 2024 at 15:02, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> On 29/08/2024 17:42, Hongyan Xia wrote:
> > On 22/08/2024 15:58, Vincent Guittot wrote:
> >> On Thu, 22 Aug 2024 at 14:10, Vincent Guittot
> >> <vincent.guittot@linaro.org> wrote:
> >>>
> >>> On Thu, 22 Aug 2024 at 14:08, Luis Machado <luis.machado@arm.com> wrote:
> >>>>
> >>>> Vincent,
> >>>>
> >>>> On 8/22/24 11:28, Luis Machado wrote:
> >>>>> On 8/22/24 10:53, Vincent Guittot wrote:
> >>>>>> On Thu, 22 Aug 2024 at 11:22, Luis Machado <luis.machado@arm.com>
> >>>>>> wrote:
> >>>>>>>
> >>>>>>> On 8/22/24 09:19, Vincent Guittot wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> On Wed, 21 Aug 2024 at 15:34, Hongyan Xia <hongyan.xia2@arm.com>
> >>>>>>>> wrote:
> >>>>>>>>>
> >>>>>>>>> Hi Peter,
> >>>>>>>>>
> >>>>>>>>> Sorry for bombarding this thread in the last couple of days.
> >>>>>>>>> I'm seeing
> >>>>>>>>> several issues in the latest tip/sched/core after these patches
> >>>>>>>>> landed.
> >>>>>>>>>
> >>>>>>>>> What I'm now seeing seems to be an unbalanced call of util_est.
> >>>>>>>>> First, I applied
> >>>>>>>>
> >>>>>>>> I also see a remaining util_est for idle rq because of an unbalance
> >>>>>>>> call of util_est_enqueue|dequeue
> >>>>>>>>
> >>>>>>>
> >>>>>>> I can confirm issues with the utilization values and frequencies
> >>>>>>> being driven
> >>>>>>> seemingly incorrectly, in particular for little cores.
> >>>>>>>
> >>>>>>> What I'm seeing with the stock series is high utilization values
> >>>>>>> for some tasks
> >>>>>>> and little cores having their frequencies maxed out for extended
> >>>>>>> periods of
> >>>>>>> time. Sometimes for 5+ or 10+ seconds, which is excessive as the
> >>>>>>> cores are mostly
> >>>>>>> idle. But whenever certain tasks get scheduled there, they have a
> >>>>>>> very high util
> >>>>>>> level and so the frequency is kept at max.
> >>>>>>>
> >>>>>>> As a consequence this drives up power usage.
> >>>>>>>
> >>>>>>> I gave Hongyan's draft fix a try and observed a much more
> >>>>>>> reasonable behavior for
> >>>>>>> the util numbers and frequencies being used for the little cores.
> >>>>>>> With his fix,
> >>>>>>> I can also see lower energy use for my specific benchmark.
> >>>>>>
> >>>>>> The main problem is that the util_est of a delayed dequeued task
> >>>>>> remains on the rq and keeps the rq utilization high and as a result
> >>>>>> the frequency higher than needed.
> >>>>>>
> >>>>>> The below seems to works for me and keep sync the enqueue/dequeue of
> >>>>>> uti_test with the enqueue/dequeue of the task as if de dequeue was
> >>>>>> not
> >>>>>> delayed
> >>>>>>
> >>>>>> Another interest is that we will not try to migrate a delayed dequeue
> >>>>>> sleeping task that doesn't actually impact the current load of the
> >>>>>> cpu
> >>>>>> and as a result will not help in the load balance. I haven't yet
> >>>>>> fully
> >>>>>> checked what would happen with hotplug
> >>>>>
> >>>>> Thanks. Those are good points. Let me go and try your patch.
> >>>>
> >>>> I gave your fix a try, but it seems to make things worse. It is
> >>>> comparable
> >>>> to the behavior we had before Peter added the uclamp imbalance fix,
> >>>> so I
> >>>> believe there is something incorrect there.
> >>>
> >>> we need to filter case where task are enqueued/dequeued several
> >>> consecutive times. That's what I'm look now
> >>
> >> I just realize before that It's not only util_est but the h_nr_running
> >> that keeps delayed tasks as well so all stats of the rq are biased:
> >> h_nr_running, util_est, runnable avg and load_avg.
> >
> > After staring at the code even more, I think the situation is worse.
> >
> > First thing is that uclamp might also want to be part of these stats
> > (h_nr_running, util_est, runnable_avg, load_avg) that do not follow
> > delayed dequeue which needs to be specially handled in the same way. The
> > current way of handling uclamp in core.c misses the frequency update,
> > like I commented before.
> >
> > Second, there is also an out-of-sync issue in update_load_avg(). We only
> > update the task-level se in delayed dequeue and requeue, but we return
> > early and the upper levels are completely skipped, as if the delayed
> > task is still on rq. This de-sync is wrong.
>
> I had a look at the util_est issue.
>
> This keeps rq->cfs.avg.util_avg sane for me with
> SCHED_FEAT(DELAY_DEQUEUE, true):
>
> -->8--
>
> From 0d7e8d057f49a47e0f3f484ac7d41e047dccec38 Mon Sep 17 00:00:00 2001
> From: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Date: Thu, 5 Sep 2024 00:05:23 +0200
> Subject: [PATCH] kernel/sched: Fix util_est accounting for DELAY_DEQUEUE
>
> Remove delayed tasks from util_est even they are runnable.

Unfortunately, this is not only about util_est

cfs_rq's runnable_avg is also wrong because we normally have :
cfs_rq's runnable_avg == /Sum se's runnable_avg
but cfs_rq's runnable_avg uses cfs_rq's h_nr_running but delayed
entities are still accounted in h_nr_running

That also means that cfs_rq's h_nr_running is not accurate anymore
because it includes delayed dequeue

and cfs_rq load_avg is kept artificially high which biases
load_balance and cgroup's shares

>
> Exclude delayed task which are (a) migrating between rq's or (b) in a
> SAVE/RESTORE dequeue/enqueue.
>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
> kernel/sched/fair.c | 16 +++++++++-------
> 1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1e693ca8ebd6..5c32cc26d6c2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6948,18 +6948,19 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> int rq_h_nr_running = rq->cfs.h_nr_running;
> u64 slice = 0;
>
> - if (flags & ENQUEUE_DELAYED) {
> - requeue_delayed_entity(se);
> - return;
> - }
> -
> /*
> * The code below (indirectly) updates schedutil which looks at
> * the cfs_rq utilization to select a frequency.
> * Let's add the task's estimated utilization to the cfs_rq's
> * estimated utilization, before we update schedutil.
> */
> - util_est_enqueue(&rq->cfs, p);
> + if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & ENQUEUE_RESTORE))))
> + util_est_enqueue(&rq->cfs, p);
> +
> + if (flags & ENQUEUE_DELAYED) {
> + requeue_delayed_entity(se);
> + return;
> + }
>
> /*
> * If in_iowait is set, the code below may not trigger any cpufreq
> @@ -7177,7 +7178,8 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
> */
> static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> {
> - util_est_dequeue(&rq->cfs, p);
> + if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & DEQUEUE_SAVE))))
> + util_est_dequeue(&rq->cfs, p);
>
> if (dequeue_entities(rq, &p->se, flags) < 0) {
> if (!rq->cfs.h_nr_running)
> --
> 2.34.1
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