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From: Mel Gorman <mgorman@techsingularity.net>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Barry Song <song.bao.hua@hisilicon.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Peter Ziljstra <peterz@infradead.org>,
	"Li, Aubrey" <aubrey.li@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@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 15:40:29 +0000	[thread overview]
Message-ID: <20201204154029.GC3371@techsingularity.net> (raw)
In-Reply-To: <CAKfTPtBXUK7NnhN1Rxeq-o5t-G+csN6Tj0yP=BSV_sdD1uZS0g@mail.gmail.com>

On Fri, Dec 04, 2020 at 04:23:48PM +0100, Vincent Guittot wrote:
> On Fri, 4 Dec 2020 at 15:31, Mel Gorman <mgorman@techsingularity.net> wrote:
> >
> > On Fri, Dec 04, 2020 at 02:47:48PM +0100, Vincent Guittot wrote:
> > > > IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
> > > > If the target's sibling is removed from select_idle_mask from select_idle_core(),
> > > > select_idle_cpu() will lose the chance to pick it up?
> > >
> > > This is only relevant for patch 10 which is not to be included IIUC
> > > what mel said in cover letter : "Patches 9 and 10 are stupid in the
> > > context of this series."
> > >
> >
> > Patch 10 was stupid in the context of the prototype because
> > select_idle_core always returned a CPU. A variation ended up being
> > reintroduced at the end of the Series Yet To Be Posted so that SMT siblings
> > are cleared during select_idle_core() but select_idle_cpu() still has a
> > mask with unvisited CPUs to consider if no idle cores are found.
> >
> > As far as I know, this would still be compatible with Aubrey's idle
> > cpu mask as long as it's visited and cleared between select_idle_core
> > and select_idle_cpu. It relaxes the contraints on Aubrey to some extent
> > because the idle cpu mask would be a hint so if the information is out
> > of date, an idle cpu may still be found the normal way.
> 
> But even without patch 10, just replacing sched_domain_span(sd) by
> sds_idle_cpus(sd->shared) will ensure that sis loops only on cpus that
> get a chance to be idle so select_idle_core is likely to return an
> idle_candidate
> 

Yes but if the idle mask is out of date for any reason then idle CPUs might
be missed -- hence the intent to maintain a mask of CPUs visited and use
the idle cpu mask as a hint to prioritise CPUs that are likely idle but
fall back to a normal scan if none of the "idle cpu mask" CPUs are
actually idle.

-- 
Mel Gorman
SUSE Labs

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WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mgorman@techsingularity.net>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: "Li, Aubrey" <aubrey.li@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	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 15:40:29 +0000	[thread overview]
Message-ID: <20201204154029.GC3371@techsingularity.net> (raw)
In-Reply-To: <CAKfTPtBXUK7NnhN1Rxeq-o5t-G+csN6Tj0yP=BSV_sdD1uZS0g@mail.gmail.com>

On Fri, Dec 04, 2020 at 04:23:48PM +0100, Vincent Guittot wrote:
> On Fri, 4 Dec 2020 at 15:31, Mel Gorman <mgorman@techsingularity.net> wrote:
> >
> > On Fri, Dec 04, 2020 at 02:47:48PM +0100, Vincent Guittot wrote:
> > > > IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
> > > > If the target's sibling is removed from select_idle_mask from select_idle_core(),
> > > > select_idle_cpu() will lose the chance to pick it up?
> > >
> > > This is only relevant for patch 10 which is not to be included IIUC
> > > what mel said in cover letter : "Patches 9 and 10 are stupid in the
> > > context of this series."
> > >
> >
> > Patch 10 was stupid in the context of the prototype because
> > select_idle_core always returned a CPU. A variation ended up being
> > reintroduced at the end of the Series Yet To Be Posted so that SMT siblings
> > are cleared during select_idle_core() but select_idle_cpu() still has a
> > mask with unvisited CPUs to consider if no idle cores are found.
> >
> > As far as I know, this would still be compatible with Aubrey's idle
> > cpu mask as long as it's visited and cleared between select_idle_core
> > and select_idle_cpu. It relaxes the contraints on Aubrey to some extent
> > because the idle cpu mask would be a hint so if the information is out
> > of date, an idle cpu may still be found the normal way.
> 
> But even without patch 10, just replacing sched_domain_span(sd) by
> sds_idle_cpus(sd->shared) will ensure that sis loops only on cpus that
> get a chance to be idle so select_idle_core is likely to return an
> idle_candidate
> 

Yes but if the idle mask is out of date for any reason then idle CPUs might
be missed -- hence the intent to maintain a mask of CPUs visited and use
the idle cpu mask as a hint to prioritise CPUs that are likely idle but
fall back to a normal scan if none of the "idle cpu mask" CPUs are
actually idle.

-- 
Mel Gorman
SUSE Labs

  reply	other threads:[~2020-12-04 15:41 UTC|newest]

Thread overview: 60+ 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 ` Mel Gorman
2020-12-03 14:11 ` [PATCH 01/10] sched/fair: Track efficiency " Mel Gorman
2020-12-03 14:11   ` 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   ` Mel Gorman
2020-12-03 14:11 ` [PATCH 03/10] sched/fair: Remove SIS_AVG_CPU Mel Gorman
2020-12-03 14:11   ` 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 14:11   ` Mel Gorman
2020-12-03 16:35   ` Vincent Guittot
2020-12-03 16:35     ` Vincent Guittot
2020-12-03 17:50     ` Mel Gorman
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   ` 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 14:11   ` Mel Gorman
2020-12-03 16:38   ` Vincent Guittot
2020-12-03 16:38     ` Vincent Guittot
2020-12-03 17:52     ` Mel Gorman
2020-12-03 17:52       ` Mel Gorman
2020-12-04 10:56       ` Vincent Guittot
2020-12-04 10:56         ` Vincent Guittot
2020-12-04 11:30         ` Mel Gorman
2020-12-04 11:30           ` Mel Gorman
2020-12-04 13:13           ` Vincent Guittot
2020-12-04 13:13             ` Vincent Guittot
2020-12-04 13:17             ` Vincent Guittot
2020-12-04 13:17               ` Vincent Guittot
2020-12-04 13:40               ` Li, Aubrey
2020-12-04 13:40                 ` Li, Aubrey
2020-12-04 13:47                 ` Li, Aubrey
2020-12-04 13:47                   ` Li, Aubrey
2020-12-04 13:47                 ` Vincent Guittot
2020-12-04 13:47                   ` Vincent Guittot
2020-12-04 14:07                   ` Li, Aubrey
2020-12-04 14:07                     ` Li, Aubrey
2020-12-04 14:31                   ` Mel Gorman
2020-12-04 14:31                     ` Mel Gorman
2020-12-04 15:23                     ` Vincent Guittot
2020-12-04 15:23                       ` Vincent Guittot
2020-12-04 15:40                       ` Mel Gorman [this message]
2020-12-04 15:40                         ` Mel Gorman
2020-12-04 15:43                         ` Vincent Guittot
2020-12-04 15:43                           ` Vincent Guittot
2020-12-04 18:41                           ` Mel Gorman
2020-12-04 18:41                             ` Mel Gorman
2020-12-04 14:27               ` 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   ` 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   ` Mel Gorman
2020-12-03 14:11 ` [PATCH 09/10] sched/fair: Limit the search for an idle core Mel Gorman
2020-12-03 14:11   ` Mel Gorman
2020-12-03 14:19 ` 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
2020-12-03 14:20   ` Mel Gorman

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