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From: Mel Gorman <mgorman@suse.de>
To: Julia Lawall <julia.lawall@inria.fr>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	kernel-janitors@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	linux-kernel@vger.kernel.org,
	Valentin Schneider <valentin.schneider@arm.com>,
	Gilles.Muller@inria.fr
Subject: Re: [PATCH] sched/fair: check for idle core
Date: Wed, 21 Oct 2020 13:18:27 +0000	[thread overview]
Message-ID: <20201021131827.GF32041@suse.de> (raw)
In-Reply-To: <alpine.DEB.2.22.394.2010211452100.8475@hadrien>

On Wed, Oct 21, 2020 at 02:56:06PM +0200, Julia Lawall wrote:
> 
> 
> On Wed, 21 Oct 2020, Mel Gorman wrote:
> 
> > On Wed, Oct 21, 2020 at 02:25:32PM +0200, Vincent Guittot wrote:
> > > > I see Vincent already agreed with the patch so I could be wrong.  Vincent,
> > > > did I miss something stupid?
> > >
> > > This patch fixes the problem that we don't favor anymore the prev_cpu when it is idle since
> > > commit 11f10e5420f6ce because load is not null when cpu is idle whereas runnable_load was
> > > And this is important because this will then decide in which LLC we will looks for a cpu
> > >
> >
> > Ok, that is understandable but I'm still concerned that the fix simply
> > trades one problem for another by leaving related tasks remote to each
> > other and increasing cache misses and remote data accesses.
> >
> > wake_affine_weight is a giant pain because really we don't care about the
> > load on the waker CPU or its available, we care about whether it has idle
> > siblings that can be found quickly. As tempting as ripping it out is,
> > it never happened because sometimes it makes the right decision.
> 
> My goal was to restore the previous behavior, when runnable load was used.
> The patch removing the use of runnable load (11f10e5420f6) presented it
> basically as that load balancing was using it, so wakeup should use it
> too, and any way it didn't matter because idle CPUS were checked for
> anyway.
> 

Which is fair.

> Is your point of view that the proposed change is overkill?  Or is it that
> the original behavior was not desirable?
> 

I worry it's overkill because prev is always used if it is idle even
if it is on a node remote to the waker. It cuts off the option of a
wakee moving to a CPU local to the waker which is not equivalent to the
original behaviour.

-- 
Mel Gorman
SUSE Labs

  reply	other threads:[~2020-10-21 13:18 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-20 16:37 [PATCH] sched/fair: check for idle core Julia Lawall
2020-10-21  7:29 ` Vincent Guittot
2020-10-21 11:13   ` Peter Zijlstra
2020-10-21 12:27   ` Vincent Guittot
2020-10-21 11:20 ` Mel Gorman
2020-10-21 11:56   ` Julia Lawall
2020-10-21 12:19     ` Peter Zijlstra
2020-10-21 12:42       ` Julia Lawall
2020-10-21 12:52         ` Peter Zijlstra
2020-10-21 18:18           ` Rafael J. Wysocki
2020-10-21 18:15         ` Rafael J. Wysocki
2020-10-21 19:47           ` Julia Lawall
2020-10-21 20:25             ` Rafael J. Wysocki
2020-10-21 13:10       ` Peter Zijlstra
2020-10-21 18:11         ` Rafael J. Wysocki
2020-10-22  4:53           ` Viresh Kumar
2020-10-22  7:11           ` Peter Zijlstra
2020-10-22 10:59             ` Viresh Kumar
2020-10-22 11:45               ` Rafael J. Wysocki
2020-10-22 12:02                 ` default cpufreq gov, was: " Peter Zijlstra
2020-10-22 12:19                   ` Rafael J. Wysocki
2020-10-22 12:29                     ` Peter Zijlstra
2020-10-22 14:52                       ` Mel Gorman
2020-10-22 14:58                         ` Colin Ian King
2020-10-22 15:12                           ` Phil Auld
2020-10-22 16:35                             ` Mel Gorman
2020-10-22 17:59                               ` Rafael J. Wysocki
2020-10-22 20:32                                 ` Mel Gorman
2020-10-22 20:39                                   ` Phil Auld
2020-10-22 15:25                         ` Peter Zijlstra
2020-10-22 15:55                           ` Rafael J. Wysocki
2020-10-22 16:29                           ` Mel Gorman
2020-10-22 20:10                           ` Giovanni Gherdovich
2020-10-22 20:16                             ` Giovanni Gherdovich
2020-10-23  7:03                             ` Peter Zijlstra
2020-10-23 17:46                               ` Tom Lendacky
2020-10-26 19:52                                 ` Fontenot, Nathan
2020-10-22 15:45                       ` A L
2020-10-22 15:55                         ` Vincent Guittot
2020-10-23  5:23                           ` Viresh Kumar
2020-10-22 16:23                   ` [PATCH] cpufreq: Avoid configuring old governors as default with intel_pstate Rafael J. Wysocki
2020-10-23  6:29                     ` Viresh Kumar
2020-10-23 11:59                       ` Rafael J. Wysocki
2020-10-23 15:15                     ` [PATCH v2] " Rafael J. Wysocki
2020-10-27  3:13                       ` Viresh Kumar
2020-10-27 11:11                   ` default cpufreq gov, was: [PATCH] sched/fair: check for idle core Qais Yousef
2020-10-27 11:26                     ` Valentin Schneider
2020-10-27 11:42                       ` Qais Yousef
2020-10-27 11:48                         ` Viresh Kumar
2020-10-23  6:24                 ` Viresh Kumar
2020-10-23 15:06                   ` Rafael J. Wysocki
2020-10-27  3:13                     ` Viresh Kumar
2020-10-22 11:21             ` AW: " Walter Harms
2020-10-21 12:28     ` Mel Gorman
2020-10-21 12:25   ` Vincent Guittot
2020-10-21 12:47     ` Mel Gorman
2020-10-21 12:56       ` Julia Lawall
2020-10-21 13:18         ` Mel Gorman [this message]
2020-10-21 13:24           ` Julia Lawall
2020-10-21 15:08             ` Mel Gorman
2020-10-21 15:18               ` Julia Lawall
2020-10-21 15:23                 ` Vincent Guittot
2020-10-21 15:33                   ` Julia Lawall
2020-10-21 15:19               ` Vincent Guittot
2020-10-21 17:00                 ` Mel Gorman
2020-10-21 17:39                   ` Julia Lawall
2020-10-21 13:48           ` Julia Lawall
2020-10-21 15:26             ` Mel Gorman

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