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 21:47:28 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     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 21:40, Li, Aubrey 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?

aha, no, select_idle_mask will be re-assigned in select_idle_cpu() by:

	cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);

So, yes, I guess we can remove the cpu_smt_mask(target) from select_idle_core() safely.

> 
> Thanks,
> -Aubrey
>