linux-kernel - Re: [PATCH 3/4] sched/fair: introduce core_vruntime and core_min

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Date:   Wed, 15 Nov 2023 21:42:13 +0800
From:   cruzzhao <cruzzhao@...ux.alibaba.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] sched/fair: introduce core_vruntime and
 core_min_vruntime

在 2023/11/15 下午8:20, Peter Zijlstra 写道:
> On Wed, Nov 15, 2023 at 07:33:40PM +0800, Cruz Zhao wrote:
>> To compare the priority of sched_entity from different cpus of a core,
>> we introduce core_vruntime to struct sched_entity and core_min_vruntime
>> to struct cfs_rq.
>>
>> cfs_rq->core->core_min_vruntime records the min vruntime of the cfs_rqs
>> of the same task_group among the core, and se->core_vruntime is the
>> vruntime relative to se->cfs_rq->core->core_min_vruntime.
> 
> But that makes absolutely no sense. vruntime of different RQs can
> advance at wildly different rates. Not to mention there's this random
> offset between them.
> 
> No, this cannot be.

Force idle vruntime snapshot does the same thing, comparing
sea->vruntime - cfs_rqa->min_vruntime_fi with seb->vruntime -
cfs_rqb->min_vruntime_fi, while sea and seb may have wildly different rates.

Actually, cfs_rq->core->core_min_vruntime does the same thing as
cfs_rq->min_vruntime_fi, providing a baseline, but
cfs_rq->core->core_min_vruntime is more accurate.

I've tried to implement a fair enough mechanism of core_vruntime, but
it's too complex because of the weight, and it costs a lot. So this is a
compromise solution.

BTW, is there any other solutions to solve this problem?

Best,
Cruz Zhao