linux-kernel - Re: [RFC PATCH v2 09/17] sched: Introduce sched_class::pick

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Date:   Fri, 26 Apr 2019 16:10:56 +0000
From:   Vineeth Remanan Pillai <vpillai@...italocean.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Aaron Lu <aaron.lwe@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [RFC PATCH v2 09/17] sched: Introduce sched_class::pick_task()

>
> I didn't get around to reading the original discussion here, but how can
> that possibly be?
>
> I can see !se, in that case curr is still selected.
>
> I can also see !curr, in that case curr is put.
>
> But I cannot see !se && !curr, per the above check we know
> cfs_rq->nr_running != 0, so there must be a cfs task to find. This means
> either curr or se must exist.

This fix was suggested as a quick fix for a crash seen in v1. But
I agree with you that this should be a bug if it happens and should
be investigated. I have tried in v2 and can no longer reproduce the
crash. Will remove the check in v3.

Thanks