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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 7 May 2021 08:53:08 +0200
Message-ID: <CAKfTPtDUEwvLXOVcGztGALrUx0dqi=KtT2vAcxK15yQJswyHfA@mail.gmail.com>
Subject: Re: [PATCH] sched/pelt: Add UTIL_AVG_UNCHANGED flag for last_enqueued_diff
To:     Xuewen Yan <xuewen.yan94@gmail.com>
Cc:     Vincent Donnefort <vincent.donnefort@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        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>,
        Chunyan Zhang <zhang.lyra@gmail.com>,
        Ryan Y <xuewyan@foxmail.com>
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On Fri, 7 May 2021 at 03:36, Xuewen Yan <xuewen.yan94@gmail.com> wrote:
>
> On Fri, May 7, 2021 at 12:26 AM Vincent Donnefort
> <vincent.donnefort@arm.com> wrote:
> >
> > On Thu, May 06, 2021 at 08:46:08PM +0800, Xuewen Yan wrote:
> > > Hi
> > > On Thu, May 6, 2021 at 8:28 PM Vincent Donnefort
> > > <vincent.donnefort@arm.com> wrote:
> > > >
> > > > On Thu, May 06, 2021 at 07:09:36PM +0800, Xuewen Yan wrote:
> > > > > From: Xuewen Yan <xuewen.yan@unisoc.com>
> > > > >
> > > > > The UTIL_AVG_UNCHANGED flag had been cleared when the task util c=
hanged.
> > > > > And the enqueued is equal to task_util with the flag, so it is be=
tter
> > > > > to add the UTIL_AVG_UNCHANGED flag for last_enqueued_diff.
> >
> > Could we change the description here a bit? I don't think this is accur=
ately
> > explaning the issue. Would probably be interesting to mention that by n=
ot
> > setting the flag, which is the LSB, we add +1 to the diff. This is ther=
efore
> > reducing slightly UTIL_EST_MARGIN.
>
> ok, If you agree with this patch, I'll change it in V2.

Although the impact is not significant , it's worth having an accurate
computation. So the patch makes sense to me. Please submit a v2

> >
> > > > >
> > > > > Fixes: b89997aa88f0b sched/pelt: Fix task util_est update filteri=
ng
> > > > >
> > > > > Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
> > > > > ---
> > > > >  kernel/sched/fair.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > > index e5e457fa9dc8..94d77b4fa601 100644
> > > > > --- a/kernel/sched/fair.c
> > > > > +++ b/kernel/sched/fair.c
> > > > > @@ -3996,7 +3996,7 @@ static inline void util_est_update(struct c=
fs_rq *cfs_rq,
> > > > >       if (ue.enqueued & UTIL_AVG_UNCHANGED)
> > > > >               return;
> > > > >
> > > > > -     last_enqueued_diff =3D ue.enqueued;
> > > > > +     last_enqueued_diff =3D (ue.enqueued | UTIL_AVG_UNCHANGED);
> > > > >
> > > > >       /*
> > > > >        * Reset EWMA on utilization increases, the moving average =
is used only
> > > > > --
> > > > > 2.29.0
> > > > >
> > > >
> > > > Hi,
> > > >
> > > > We do indeed for the diff use the flag for the value updated and no=
 flag for the
> > > > value before the update. However, last_enqueued_diff is only used f=
or the margin
> > > > check which is an heuristic and is not an accurate value (~1%) and =
as we know
> > > The last_enqueued_diff is compared with the UTIL_EST_MARGIN which is
> > > "1024/100 =3D 10",
> > > and The LSB may cause ~10% error.
> >
> > I meant ~1% being the original margin. With the bit set, we would use 0=
.87% instead
> > of 0.97%.
>
> Because the within_margin() does not contain =E2=80=9C=3D=E2=80=9D=EF=BC=
=8C if the enqueued
> without the flag, the margin may be 0.97%(10/1024),
> with the flag, be 1.07%(11/1024) instead of 0.87% I think.
> >
> > > > we already loose the LSB in util_est, I'm not sure this is really n=
ecessary.
> > > I'm also not very sure, maybe the calculation will be more rigorous
> > > with the flag?
> > > >
> > > > --
> > > > Vincent
> > > >