linux-kernel - Re: [PATCH -tip 09/32] sched/fair: Snapshot the min

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Date:   Sun, 22 Nov 2020 22:44:42 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
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        Alexandre Chartre <alexandre.chartre@...cle.com>,
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        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 09/32] sched/fair: Snapshot the min_vruntime of CPUs
 on force idle

On Tue, Nov 17, 2020 at 06:19:39PM -0500, Joel Fernandes (Google) wrote:
> During force-idle, we end up doing cross-cpu comparison of vruntimes
> during pick_next_task. If we simply compare (vruntime-min_vruntime)
> across CPUs, and if the CPUs only have 1 task each, we will always
> end up comparing 0 with 0 and pick just one of the tasks all the time.
> This starves the task that was not picked. To fix this, take a snapshot
> of the min_vruntime when entering force idle and use it for comparison.
> This min_vruntime snapshot will only be used for cross-CPU vruntime
> comparison, and nothing else.
> 
> This resolves several performance issues that were seen in ChromeOS
> audio usecase.
> 
> NOTE: Note, this patch will be improved in a later patch. It is just
>       kept here as the basis for the later patch and to make rebasing
>       easier. Further, it may make reverting the improvement easier in
>       case the improvement causes any regression.
>

This seems cumbersome, is there no way to track the min_vruntime via
rq->core->min_vruntime?

Balbir Singh.