linux-kernel - Re: [PATCH RFC] sched/fair: let cpu's cfs

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Message-ID: <20160401194948.GN3448@twins.programming.kicks-ass.net>
Date:	Fri, 1 Apr 2016 21:49:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Leo Yan <leo.yan@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	linux-kernel@...r.kernel.org, eas-dev@...ts.linaro.org
Subject: Re: [PATCH RFC] sched/fair: let cpu's cfs_rq to reflect task
 migration

On Sat, Apr 02, 2016 at 12:38:37AM +0800, Leo Yan wrote:
> When task is migrated from CPU_A to CPU_B, scheduler will decrease
> the task's load/util from the task's cfs_rq and also add them into
> migrated cfs_rq. But if kernel enables CONFIG_FAIR_GROUP_SCHED then this
> cfs_rq is not the same one with cpu's cfs_rq. As a result, after task is
> migrated to CPU_B, then CPU_A still have task's stale value for
> load/util; on the other hand CPU_B also cannot reflect new load/util
> which introduced by the task.
> 
> So this patch is to operate the task's load/util to cpu's cfs_rq, so
> finally cpu's cfs_rq can really reflect task's migration.

Sorry but that is unintelligible.

What is the problem? Why do we care? How did you fix it? and at what
cost?