linux-kernel - Re: [RFC] Core Scheduling unnecessary force idle?

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Date:   Wed, 29 Nov 2023 19:35:59 +0800
From:   Benjamin Tang <tangsong8264@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org,
        Benjamin Tang <tangsong8264@...il.com>
Subject: Re: [RFC] Core Scheduling unnecessary force idle?

In general scenarios, the number of tagged tasks should be less.

Is it feasible to maintain the leftmost untagged node?

在 2023/11/29 下午6:17, Peter Zijlstra 写道:
> On Wed, Nov 29, 2023 at 04:53:35PM +0800, Benjamin Tang wrote:
>> When I'm reading the code related to "core scheduling", I have a question.
>>
>> Say the RQs in a particular core look like this:
>> Let CFS1 and CFS4 be 2 untagged CFS tasks.
>> Let CFS2 and CFS3 be 2 untagged CFS tasks.
>>
>>           rq0                      rq1
>>      CFS1(no tag)     CFS3(tagged)
>>      CFS2(tagged)    CFS4(no tag)
>>
>> Say schedule() runs on rq0. In the core-wide pick logic, if I'm not
>> mistaken,
>> the end result of the selection will be (say prio(CFS1) > prio(CFS3)):
>>
>>           rq0                 rq1
>>      CFS1(no tag)    IDLE
>>
>> Why not consider trying to find untagged tasks for rq1 here?
>> Is it because it seems less fair, or are there other considerations?
>>
>> I would be very grateful if someone could give me some suggestions.
>> Thanks!
> Because it's expensive to unconditionally track the untagged tasks. I
> suppose it could be fixed by iterating the task-set when we
> enable/disable core-scheduling, but that's going to be somewhat painful.
>
> A work-around would be to always tag everything, eg. have an explicit
> 'rest' tag.