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

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Message-ID: <CAKfTPtDO3n-4mcr2Sk-uu0ZS5xQnagdicQmaBh-CyrndPLM8eQ@mail.gmail.com>
Date: Thu, 22 Aug 2024 16:58:01 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Luis Machado <luis.machado@....com>
Cc: Hongyan Xia <hongyan.xia2@....com>, Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, kprateek.nayak@....com, 
	wuyun.abel@...edance.com, youssefesmat@...omium.org, tglx@...utronix.de, 
	efault@....de
Subject: Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

On Thu, 22 Aug 2024 at 14:10, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Thu, 22 Aug 2024 at 14:08, Luis Machado <luis.machado@....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@....com> wrote:
> > >>>
> > >>> On 8/22/24 09:19, Vincent Guittot wrote:
> > >>>> Hi,
> > >>>>
> > >>>> On Wed, 21 Aug 2024 at 15:34, Hongyan Xia <hongyan.xia2@....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.