From: Vincent Guittot <vincent.guittot@linaro.org>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Morten Rasmussen <Morten.Rasmussen@arm.com>,
Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
Thara Gopinath <thara.gopinath@linaro.org>,
pkondeti@codeaurora.org, Quentin Perret <quentin.perret@arm.com>,
Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Subject: Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT
Date: Wed, 28 Nov 2018 14:33:17 +0100 [thread overview]
Message-ID: <CAKfTPtBsKc7v5gc=XUrzO-_4kahGfdNteo=t9W5fLv0Ee8co_w@mail.gmail.com> (raw)
In-Reply-To: <20181128115336.GB23094@e110439-lin>
On Wed, 28 Nov 2018 at 12:53, Patrick Bellasi <patrick.bellasi@arm.com> wrote:
>
> On 28-Nov 11:02, Peter Zijlstra wrote:
> > On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote:
> >
> > > Is there anything else that I should do for these patches ?
> >
> > IIRC, Morten mention they break util_est; Patrick was going to explain.
>
> I guess the problem is that, once we cross the current capacity,
> strictly speaking util_avg does not represent anymore a utilization.
>
> With the new signal this could happen and we end up storing estimated
> utilization samples which will overestimate the task requirements.
>
> We will have a spike in estimated utilization at next wakeup, since we
> use MAX(util_avg@dequeue_time, ewma). Potentially we also inflate the EWMA in
> case we collect multiple samples above the current capacity.
TBH I don't see how it's different from current implementation with a
task that was scheduled on big core and now wakes up on little core.
The util_est is overestimated as well.
But I'm fine with adding your proposal on to on the patchset
>
> So, a possible fix could be to avoid storing util_est samples if we
> end up with a utilization above the current capacity.
>
> Something like:
>
> ----8<---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ac855b2f4774..93e0cf5d8a76 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3661,6 +3661,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
> if (!task_sleep)
> return;
>
> + /* Skip samples which do not represent an actual utilization */
> + if (unlikely(task_util(p) > capacity_of(task_cpu(p))))
> + return;
> +
> /*
> * If the PELT values haven't changed since enqueue time,
> * skip the util_est update.
> ---8<---
>
> Could that work ?
>
> Maybe using a new utility function to wrap the new check.
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi
next prev parent reply other threads:[~2018-11-28 13:33 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-20 10:55 [PATCH v7 0/2] sched/fair: update scale invariance of PELT Vincent Guittot
2018-11-20 10:55 ` [PATCH v7 1/2] sched/fair: move rq_of helper function Vincent Guittot
2018-11-20 10:55 ` [PATCH v7 2/2] sched/fair: update scale invariance of PELT Vincent Guittot
2018-11-28 9:54 ` Vincent Guittot
2018-11-28 10:02 ` Peter Zijlstra
2018-11-28 11:53 ` Patrick Bellasi
2018-11-28 13:33 ` Vincent Guittot [this message]
2018-11-28 13:35 ` Vincent Guittot
2018-11-28 14:40 ` Patrick Bellasi
2018-11-28 14:55 ` Vincent Guittot
2018-11-28 15:21 ` Patrick Bellasi
2018-11-28 15:42 ` Vincent Guittot
2018-11-28 16:35 ` Patrick Bellasi
2018-11-29 10:43 ` Vincent Guittot
2018-11-29 15:00 ` Patrick Bellasi
2018-11-29 16:19 ` Vincent Guittot
2019-01-10 15:30 ` Patrick Bellasi
2019-01-11 14:29 ` Vincent Guittot
2018-11-29 12:53 ` Peter Zijlstra
2018-11-29 15:13 ` Patrick Bellasi
2019-01-24 9:07 ` Peter Zijlstra
2019-01-24 14:04 ` Patrick Bellasi
2019-01-29 19:42 ` Peter Zijlstra
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