linux-kernel - Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

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Date:   Fri, 4 Dec 2020 22:07:11 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask
 of CPUs searched

On 2020/12/4 21:47, Vincent Guittot wrote:
> On Fri, 4 Dec 2020 at 14:40, Li, Aubrey <aubrey.li@...ux.intel.com> wrote:
>>
>> On 2020/12/4 21:17, Vincent Guittot wrote:
>>> On Fri, 4 Dec 2020 at 14:13, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>>>>
>>>> On Fri, 4 Dec 2020 at 12:30, Mel Gorman <mgorman@...hsingularity.net> wrote:
>>>>>
>>>>> On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote:
>>>>>>> The intent was that the sibling might still be an idle candidate. In
>>>>>>> the current draft of the series, I do not even clear this so that the
>>>>>>> SMT sibling is considered as an idle candidate. The reasoning is that if
>>>>>>> there are no idle cores then an SMT sibling of the target is as good an
>>>>>>> idle CPU to select as any.
>>>>>>
>>>>>> Isn't the purpose of select_idle_smt ?
>>>>>>
>>>>>
>>>>> Only in part.
>>>>>
>>>>>> select_idle_core() looks for an idle core and opportunistically saves
>>>>>> an idle CPU candidate to skip select_idle_cpu. In this case this is
>>>>>> useless loops for select_idle_core() because we are sure that the core
>>>>>> is not idle
>>>>>>
>>>>>
>>>>> If select_idle_core() finds an idle candidate other than the sibling,
>>>>> it'll use it if there is no idle core -- it picks a busy sibling based
>>>>> on a linear walk of the cpumask. Similarly, select_idle_cpu() is not
>>>>
>>>> My point is that it's a waste of time to loop the sibling cpus of
>>>> target in select_idle_core because it will not help to find an idle
>>>> core. The sibling  cpus will then be check either by select_idle_cpu
>>>> of select_idle_smt
>>>
>>> also, while looping the cpumask, the sibling cpus of not idle cpu are
>>> removed and will not be check
>>>
>>
>> IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)?
>> If the target's sibling is removed from select_idle_mask from select_idle_core(),
>> select_idle_cpu() will lose the chance to pick it up?
> 
> This is only relevant for patch 10 which is not to be included IIUC
> what mel said in cover letter : "Patches 9 and 10 are stupid in the
> context of this series."

So the target's sibling can be removed from cpumask in select_idle_core
in patch 6, and need to be added back in select_idle_core in patch 10, :)