linux-kernel - Re: [PATCH] sched/fair: Fix pelt lost idle time detection

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Message-ID: <CAKfTPtBRuiS4Rr6FotE2Q_sB+Ab-1tKGsagRYFCQg-=CQ8e9iA@mail.gmail.com>
Date: Tue, 20 Jan 2026 09:36:00 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Alex Hoh (賀振坤) <Alex.Hoh@...iatek.com>
Cc: "bsegall@...gle.com" <bsegall@...gle.com>, "vschneid@...hat.com" <vschneid@...hat.com>, 
	"dietmar.eggemann@....com" <dietmar.eggemann@....com>, "rostedt@...dmis.org" <rostedt@...dmis.org>, 
	"peterz@...radead.org" <peterz@...radead.org>, "mingo@...hat.com" <mingo@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "mgorman@...e.de" <mgorman@...e.de>, 
	"wusamuel@...gle.com" <wusamuel@...gle.com>, "juri.lelli@...hat.com" <juri.lelli@...hat.com>, 
	"kernel-team@...roid.com" <kernel-team@...roid.com>
Subject: Re: [PATCH] sched/fair: Fix pelt lost idle time detection

On Tue, 20 Jan 2026 at 06:46, Alex Hoh (賀振坤) <Alex.Hoh@...iatek.com> wrote:
>
> On Fri, 2026-01-16 at 17:13 +0100, Vincent Guittot wrote:
> > Le vendredi 16 janv. 2026 à 15:21:35 (+0100), Vincent Guittot a écrit
> > :
> > > Hi Alex,

[..]

>
> This fix looks good to me, and in my local testing,
> RT utilization dropped significantly from 72 to 20.

Thanks for your tests. Let see if this fixes Samuel's regression too
and I will prepare a patch

>
> > > >
> > > > The RT util update actually occurs later in
> > > > put_prev_set_next_task(),
> > > > and it relies on the original value of rq_clock_pelt as input.
> > > > Since
> > > > rq_clock_pelt has already been overwritten by the time the RT
> > > > util
> > > > update takes place, the original timestamp is lost.
> > > >
> > > > As a result, the intended CPU/frequency capacity scaling behavior
> > > > is
> > > > disrupted, causing RT util to increase more rapidly than
> > > > expected. This
> > > > appears to be an unintended consequence introduced by the patch.
> > > >
> > > > >
> > > > > >
> > > > > > - with patch:
> > > > > > https://urldefense.com/v3/__https://ui.perfetto.dev/*!/?s=964594d07a5a5ba51a159ba6c90bb7ab48e09326__;Iw!!CTRNKA9wMg0ARbw!hSdr5foMTuMuY9OK82IumAY3LPDt5HCKhkduyW-8UsLaQvcW8F-kwwsgcX0crwnkzU29xvN24l5bpsa55oqTy5MwyJxI$
> > > > > >
> > > > > > - without patch:
> > > > > > https://urldefense.com/v3/__https://ui.perfetto.dev/*!/?s=6ff6854c87ea187e4ca488acd2e6501b90ec9f6f__;Iw!!CTRNKA9wMg0ARbw!hSdr5foMTuMuY9OK82IumAY3LPDt5HCKhkduyW-8UsLaQvcW8F-kwwsgcX0crwnkzU29xvN24l5bpsa55oqTyzycmnIs$
> > > > > >
>
>
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