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From: Mel Gorman <mgorman@techsingularity.net>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Barry Song <song.bao.hua@hisilicon.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Peter Ziljstra <peterz@infradead.org>,
	Aubrey Li <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 1/4] sched/fair: Remove SIS_AVG_CPU
Date: Tue, 8 Dec 2020 10:59:00 +0000	[thread overview]
Message-ID: <20201208105900.GG3371@techsingularity.net> (raw)
In-Reply-To: <25a8c4bd-792b-2851-b10a-c4375eb83dfe@arm.com>

On Tue, Dec 08, 2020 at 11:07:19AM +0100, Dietmar Eggemann wrote:
> On 07/12/2020 10:15, Mel Gorman wrote:
> > SIS_AVG_CPU was introduced as a means of avoiding a search when the
> > average search cost indicated that the search would likely fail. It
> > was a blunt instrument and disabled by 4c77b18cf8b7 ("sched/fair: Make
> > select_idle_cpu() more aggressive") and later replaced with a proportional
> > search depth by 1ad3aaf3fcd2 ("sched/core: Implement new approach to
> > scale select_idle_cpu()").
> > 
> > While there are corner cases where SIS_AVG_CPU is better, it has now been
> > disabled for almost three years. As the intent of SIS_PROP is to reduce
> > the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
> > on SIS_PROP as a throttling mechanism.
> > 
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > ---
> >  kernel/sched/fair.c     | 3 ---
> >  kernel/sched/features.h | 1 -
> >  2 files changed, 4 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 98075f9ea9a8..23934dbac635 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6161,9 +6161,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >  	avg_idle = this_rq()->avg_idle / 512;
> >  	avg_cost = this_sd->avg_scan_cost + 1;
> >  
> > -	if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
> > -		return -1;
> > -
> >  	if (sched_feat(SIS_PROP)) {
> >  		u64 span_avg = sd->span_weight * avg_idle;
> >  		if (span_avg > 4*avg_cost)
> 
> Nitpick:
> 
> Since now avg_cost and avg_idle are only used w/ SIS_PROP, they could go
> completely into the SIS_PROP if condition.
> 

Yeah, I can do that. In the initial prototype, that happened in a
separate patch that split out SIS_PROP into a helper function and I
never merged it back. It's a trivial change.

Thanks.

-- 
Mel Gorman
SUSE Labs

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WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mgorman@techsingularity.net>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
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>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Linux-ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU
Date: Tue, 8 Dec 2020 10:59:00 +0000	[thread overview]
Message-ID: <20201208105900.GG3371@techsingularity.net> (raw)
In-Reply-To: <25a8c4bd-792b-2851-b10a-c4375eb83dfe@arm.com>

On Tue, Dec 08, 2020 at 11:07:19AM +0100, Dietmar Eggemann wrote:
> On 07/12/2020 10:15, Mel Gorman wrote:
> > SIS_AVG_CPU was introduced as a means of avoiding a search when the
> > average search cost indicated that the search would likely fail. It
> > was a blunt instrument and disabled by 4c77b18cf8b7 ("sched/fair: Make
> > select_idle_cpu() more aggressive") and later replaced with a proportional
> > search depth by 1ad3aaf3fcd2 ("sched/core: Implement new approach to
> > scale select_idle_cpu()").
> > 
> > While there are corner cases where SIS_AVG_CPU is better, it has now been
> > disabled for almost three years. As the intent of SIS_PROP is to reduce
> > the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
> > on SIS_PROP as a throttling mechanism.
> > 
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > ---
> >  kernel/sched/fair.c     | 3 ---
> >  kernel/sched/features.h | 1 -
> >  2 files changed, 4 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 98075f9ea9a8..23934dbac635 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6161,9 +6161,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >  	avg_idle = this_rq()->avg_idle / 512;
> >  	avg_cost = this_sd->avg_scan_cost + 1;
> >  
> > -	if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
> > -		return -1;
> > -
> >  	if (sched_feat(SIS_PROP)) {
> >  		u64 span_avg = sd->span_weight * avg_idle;
> >  		if (span_avg > 4*avg_cost)
> 
> Nitpick:
> 
> Since now avg_cost and avg_idle are only used w/ SIS_PROP, they could go
> completely into the SIS_PROP if condition.
> 

Yeah, I can do that. In the initial prototype, that happened in a
separate patch that split out SIS_PROP into a helper function and I
never merged it back. It's a trivial change.

Thanks.

-- 
Mel Gorman
SUSE Labs

  reply	other threads:[~2020-12-08 11:00 UTC|newest]

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-07  9:15 [RFC PATCH 0/4] Reduce worst-case scanning of runqueues in select_idle_sibling Mel Gorman
2020-12-07  9:15 ` Mel Gorman
2020-12-07  9:15 ` [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU Mel Gorman
2020-12-07  9:15   ` Mel Gorman
2020-12-07 15:05   ` Vincent Guittot
2020-12-07 15:05     ` Vincent Guittot
2020-12-08 10:07   ` Dietmar Eggemann
2020-12-08 10:07     ` Dietmar Eggemann
2020-12-08 10:59     ` Mel Gorman [this message]
2020-12-08 10:59       ` Mel Gorman
2020-12-08 13:24       ` Vincent Guittot
2020-12-08 13:24         ` Vincent Guittot
2020-12-08 13:36         ` Mel Gorman
2020-12-08 13:36           ` Mel Gorman
2020-12-08 13:43           ` Vincent Guittot
2020-12-08 13:43             ` Vincent Guittot
2020-12-08 13:53             ` Mel Gorman
2020-12-08 13:53               ` Mel Gorman
2020-12-08 14:47               ` Vincent Guittot
2020-12-08 14:47                 ` Vincent Guittot
2020-12-08 15:12                 ` Mel Gorman
2020-12-08 15:12                   ` Mel Gorman
2020-12-08 15:19                   ` Vincent Guittot
2020-12-08 15:19                     ` Vincent Guittot
2020-12-07  9:15 ` [PATCH 2/4] sched/fair: Do not replace recent_used_cpu with the new target Mel Gorman
2020-12-07  9:15   ` Mel Gorman
2020-12-08  9:57   ` Dietmar Eggemann
2020-12-08  9:57     ` Dietmar Eggemann
2020-12-08 11:02     ` Mel Gorman
2020-12-08 11:02       ` Mel Gorman
2020-12-07  9:15 ` [PATCH 3/4] sched/fair: Return an idle cpu if one is found after a failed search for an idle core Mel Gorman
2020-12-07  9:15   ` Mel Gorman
2020-12-07 15:06   ` Vincent Guittot
2020-12-07 15:06     ` Vincent Guittot
2020-12-07  9:15 ` [PATCH 4/4] sched/fair: Avoid revisiting CPUs multiple times during select_idle_sibling Mel Gorman
2020-12-07  9:15   ` Mel Gorman
2020-12-07 15:04 ` [RFC PATCH 0/4] Reduce worst-case scanning of runqueues in select_idle_sibling Vincent Guittot
2020-12-07 15:04   ` Vincent Guittot
2020-12-07 15:42   ` Mel Gorman
2020-12-07 15:42     ` Mel Gorman
2020-12-08  2:06     ` Li, Aubrey
2020-12-08  2:06       ` Li, Aubrey
  -- strict thread matches above, loose matches on Subject: below --
2020-12-08 15:34 [PATCH 0/4] Reduce " Mel Gorman
2020-12-08 15:34 ` [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU Mel Gorman
2020-12-08 15:34   ` Mel Gorman
2020-12-08 16:13   ` Vincent Guittot
2020-12-08 16:13     ` Vincent Guittot
2021-01-25  8:59 [PATCH v4 0/4] Scan for an idle sibling in a single pass Mel Gorman
2021-01-25  8:59 ` [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU Mel Gorman
2021-01-27 13:51 [PATCH v5 0/4] Scan for an idle sibling in a single pass Mel Gorman
2021-01-27 13:52 ` [PATCH 1/4] sched/fair: Remove SIS_AVG_CPU Mel Gorman

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