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From:   Xuewen Yan <xuewen.yan94@gmail.com>
Date:   Fri, 2 Jul 2021 21:03:31 +0800
Message-ID: <CAB8ipk-+BRXg_2=-=NXWr_OEi0rAN4Eo8hHwfOjo_YbkQCvVmg@mail.gmail.com>
Subject: Re: [PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value
 when rq is idle
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Qais Yousef <qais.yousef@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Quentin Perret <qperret@google.com>
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On Fri, Jul 2, 2021 at 8:12 PM Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> On 02/07/21 12:54, Qais Yousef wrote:
> > sched/uclamp: Ignore max aggregation if rq is idle
> >
> > When a task wakes up on an idle rq, uclamp_rq_util_with() would max
> > aggregate with rq value. But since there is no task enqueued yet, the
> > values are stale based on the last task that was running. When the new
>
> Nit: those values are "intentionally stale" for UCLAMP_MAX, per
>
>   e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX")
>
> for UCLAMP_MIN we'll set uclamp_none(UCLAMP_MIN) == 0 upon dequeueing the
> last runnable task, which DTRT.
>
> > task actually wakes up and enqueued, then the rq uclamp values should
> > reflect that of the newly woken up task effective uclamp values.
> >
> > This is a problem particularly for uclamp_max because it default to
>                     ^^^^^^^^^^^^
> Per the above, it's "only" a problem for UCLAMP_MAX.
>
> > 1024. If a task p with uclamp_max = 512 wakes up, then max aggregation
> > would ignore the capping that should apply when this task is enqueued,
> > which is wrong.
> >
> > Fix that by ignoring max aggregation if the rq is idle since in that
> > case the effective uclamp value of the rq will be the ones of the task
> > that will wake up.
> >

Thanks!
xuewen