linux-kernel - Re: [RFCv5 PATCH 36/46] sched: Prevent unnecessary active balance of single task in sched group

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Message-ID: <20150815094617.GD10304@worktop.programming.kicks-ass.net>
Date:	Sat, 15 Aug 2015 11:46:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, vincent.guittot@...aro.org,
	daniel.lezcano@...aro.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	yuyang.du@...el.com, mturquette@...libre.com, rjw@...ysocki.net,
	Juri Lelli <Juri.Lelli@....com>, sgurrappadi@...dia.com,
	pang.xunlei@....com.cn, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [RFCv5 PATCH 36/46] sched: Prevent unnecessary active balance of
 single task in sched group

On Tue, Jul 07, 2015 at 07:24:19PM +0100, Morten Rasmussen wrote:
> Scenarios with the busiest group having just one task and the local
> being idle on topologies with sched groups with different numbers of
> cpus manage to dodge all load-balance bailout conditions resulting the
> nr_balance_failed counter to be incremented. This eventually causes an
> pointless active migration of the task. This patch prevents this by not
> incrementing the counter when the busiest group only has one task.
> ASYM_PACKING migrations and migrations due to reduced capacity should
> still take place as these are explicitly captured by
> need_active_balance().
> 
> A better solution would be to not attempt the load-balance in the first
> place, but that requires significant changes to the order of bailout
> conditions and statistics gathering.

*groan*, and this is of course triggered by your 2+3 core TC2 thingy.

Yes, asymmetric groups like that are a pain.
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