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

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Date:	Tue, 4 Nov 2014 14:10:29 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Hillf Danton <hillf.zj@...baba-inc.com>
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 13:54, Hillf Danton <hillf.zj@...baba-inc.com> wrote:
>>
>> 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.

A task can be moved in both idle and newly idle. If we rely only on
newly idle and we  have only idle CPUs, we can never move task. In the
same way, if we rely only on idle case and a CPU never stays idle long
enough to trig the idle load balance, we will never move the task. I
agree that for the latter, we might wonder if it's worth moving the
task. This is your concern ?

Vincent

>
> Hillf
>
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