linux-kernel - RE: [PATCH v8 03/10] sched: move cfs task on a CPU with higher capacity

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Message-ID: <011801cff82e$78cd2c90$6a6785b0$@alibaba-inc.com>
Date:	Tue, 04 Nov 2014 20:54:29 +0800
From:	"Hillf Danton" <hillf.zj@...baba-inc.com>
To:	"'Vincent Guittot'" <vincent.guittot@...aro.org>
Cc:	"'Wanpeng Li'" <kernellwp@...il.com>,
	"'Peter Zijlstra'" <peterz@...radead.org>,
	"'Ingo Molnar'" <mingo@...nel.org>,
	"'linux-kernel'" <linux-kernel@...r.kernel.org>,
	"'Preeti U Murthy'" <preeti@...ux.vnet.ibm.com>,
	"'Morten Rasmussen'" <Morten.Rasmussen@....com>,
	"'Kamalesh Babulal'" <kamalesh@...ux.vnet.ibm.com>
Subject: RE: [PATCH v8 03/10] sched: move cfs task on a CPU with higher capacity

> 
> On 4 November 2014 13:07, Hillf Danton <hillf.zj@...baba-inc.com> wrote:
> >> >>   +     /*
> >> >> +        * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
> >> >
> >> >
> >> > Why specify one task instead of not less than one?
> >>
> >> if cfs.h_nr_running = 0 (which should not occurs at that point), we
> >> don't need to do more test to check if it's worth moving the task
> >> because there is no task to move.
> >>
> > I wonder if you can please shed light on the case that
> > the dst_cpu is newly idle.
> 
> The main problem if we do the test only for newly idle case, is that
> we are not sure to move the task because we must rely on the
> wakeup/sleep sequence of other tasks on an idle CPU in order to trig
> the migration (periodic background task as an example). So we might
> never move the task whereas idle CPUs are available
> 
So no task is migrated in the newly idle case, if I understand the
above correctly.

Hillf

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