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From: Vincent Guittot <vincent.guittot@linaro.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Aubrey Li <aubrey.li@linux.intel.com>,
	Barry Song <song.bao.hua@hisilicon.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Ziljstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Linux-ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched
Date: Fri, 4 Dec 2020 14:13:08 +0100	[thread overview]
Message-ID: <CAKfTPtDRqwUoX51rU0Xd3H9Dwqf8bLAFBxhoeMF1brKYmAZDJg@mail.gmail.com> (raw)
In-Reply-To: <20201204113030.GZ3371@techsingularity.net>

On Fri, 4 Dec 2020 at 12:30, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote:
> > > The intent was that the sibling might still be an idle candidate. In
> > > the current draft of the series, I do not even clear this so that the
> > > SMT sibling is considered as an idle candidate. The reasoning is that if
> > > there are no idle cores then an SMT sibling of the target is as good an
> > > idle CPU to select as any.
> >
> > Isn't the purpose of select_idle_smt ?
> >
>
> Only in part.
>
> > select_idle_core() looks for an idle core and opportunistically saves
> > an idle CPU candidate to skip select_idle_cpu. In this case this is
> > useless loops for select_idle_core() because we are sure that the core
> > is not idle
> >
>
> If select_idle_core() finds an idle candidate other than the sibling,
> it'll use it if there is no idle core -- it picks a busy sibling based
> on a linear walk of the cpumask. Similarly, select_idle_cpu() is not

My point is that it's a waste of time to loop the sibling cpus of
target in select_idle_core because it will not help to find an idle
core. The sibling  cpus will then be check either by select_idle_cpu
of select_idle_smt

> guaranteed to scan the sibling first (ordering) or even reach the sibling
> (throttling). select_idle_smt() is a last-ditch effort.
>
> --
> Mel Gorman
> SUSE Labs

  reply	other threads:[~2020-12-04 13:14 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-03 14:11 [RFC PATCH 00/10] Reduce time complexity of select_idle_sibling Mel Gorman
2020-12-03 14:11 ` [PATCH 01/10] sched/fair: Track efficiency " Mel Gorman
2020-12-03 14:11 ` [PATCH 02/10] sched/fair: Track efficiency of task recent_used_cpu Mel Gorman
2020-12-03 14:11 ` [PATCH 03/10] sched/fair: Remove SIS_AVG_CPU Mel Gorman
2020-12-03 14:11 ` [PATCH 04/10] sched/fair: Return an idle cpu if one is found after a failed search for an idle core Mel Gorman
2020-12-03 16:35   ` Vincent Guittot
2020-12-03 17:50     ` Mel Gorman
2020-12-03 14:11 ` [PATCH 05/10] sched/fair: Do not replace recent_used_cpu with the new target Mel Gorman
2020-12-03 14:11 ` [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched Mel Gorman
2020-12-03 16:38   ` Vincent Guittot
2020-12-03 17:52     ` Mel Gorman
2020-12-04 10:56       ` Vincent Guittot
2020-12-04 11:30         ` Mel Gorman
2020-12-04 13:13           ` Vincent Guittot [this message]
2020-12-04 13:17             ` Vincent Guittot
2020-12-04 13:40               ` Li, Aubrey
2020-12-04 13:47                 ` Li, Aubrey
2020-12-04 13:47                 ` Vincent Guittot
2020-12-04 14:07                   ` Li, Aubrey
2020-12-04 14:31                   ` Mel Gorman
2020-12-04 15:23                     ` Vincent Guittot
2020-12-04 15:40                       ` Mel Gorman
2020-12-04 15:43                         ` Vincent Guittot
2020-12-04 18:41                           ` Mel Gorman
2020-12-04 14:27               ` Mel Gorman
2020-12-03 14:11 ` [PATCH 07/10] sched/fair: Account for the idle cpu/smt search cost Mel Gorman
2020-12-03 14:11 ` [PATCH 08/10] sched/fair: Reintroduce SIS_AVG_CPU but in the context of SIS_PROP to reduce search depth Mel Gorman
2020-12-03 14:11 ` [PATCH 09/10] sched/fair: Limit the search for an idle core Mel Gorman
2020-12-03 14:19 ` Mel Gorman
2020-12-03 14:20 ` [PATCH 10/10] sched/fair: Avoid revisiting CPUs multiple times during select_idle_sibling Mel Gorman

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