linux-kernel - Re: [patch v3.18+ regression fix] sched: Further improve spurious CPU

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Message-ID: <1472639782.3942.27.camel@gmail.com>
Date:   Wed, 31 Aug 2016 12:36:22 +0200
From:   Mike Galbraith <umgwanakikbuti@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [patch v3.18+ regression fix] sched: Further improve spurious
 CPU_IDLE active migrations

On Wed, 2016-08-31 at 12:18 +0200, Mike Galbraith wrote:
> On Wed, 2016-08-31 at 12:01 +0200, Peter Zijlstra wrote:

> > So 43f4d66637bc ("sched: Improve sysbench performance by fixing spurious
> > active migration") 's +1 made sense in that its a tie breaker. If you
> > have 3 tasks on 2 groups, one group will have to have 2 tasks, and
> > bouncing the one task around just isn't going to help _anything_.
> 
> Yeah, but frequently tasks don't come in ones, so, you end up with an
> endless tug of war between LB ripping communicating buddies apart, and
> select_idle_sibling() pulling them back together.. bouncing cow
> syndrome.

The whole business of trying to balance groups down to the single task
seems a bit illogical given we care enough to wake to shared cache in
the first place, creating the 'imbalance' we then try to correct. 
 'course that weakens your unrelated tasks (which may meet on a sleepin
g lock or whatever) argument not one bit, it's also valid.

hrm.

	-Mike