linux-kernel - Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick

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Date:   Sat, 24 Oct 2020 08:27:16 -0400
From:   Vineeth Pillai <viremana@...ux.microsoft.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Joel Fernandes <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>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Thomas Glexiner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <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,
        Dario Faggioli <dfaggioli@...e.com>,
        Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Patrick Bellasi <derkling@...gle.com>,
        benbjiang(蒋彪) <benbjiang@...cent.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        "Hyser,Chris" <chris.hyser@...cle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>,
        "Ning, Hongyu" <hongyu.ning@...ux.intel.com>
Subject: Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()



On 10/24/20 7:10 AM, Vineeth Pillai wrote:
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 93a3b874077d..4cae5ac48b60 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4428,12 +4428,14 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct 
> sched_entity *curr)
>                         se = second;
>         }
>
> -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) 
> < 1) {
> +       if (left && cfs_rq->next &&
> +                       wakeup_preempt_entity(cfs_rq->next, left) < 1) {
>                 /*
>                  * Someone really wants this to run. If it's not 
> unfair, run it.
>                  */
>                 se = cfs_rq->next;
> -       } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, 
> left) < 1) {
> +       } else if (left && cfs_rq->last &&
> +                       wakeup_preempt_entity(cfs_rq->last, left) < 1) {
>                 /*
>                  * Prefer last buddy, try to return the CPU to a 
> preempted task.
>
>
> There reason for left being NULL needs to be investigated. This was
> there from v1 and we did not yet get to it. I shall try to debug later
> this week.

Thinking more about it and looking at the crash, I think that
'left == NULL' can happen in pick_next_entity for core scheduling.
If a cfs_rq has only one task that is running, then it will be
dequeued and 'left = __pick_first_entity()' will be NULL as the
cfs_rq will be empty. This would not happen outside of coresched
because we never call pick_tack() before put_prev_task() which
will enqueue the task back.

With core scheduling, a cpu can call pick_task() for its sibling while
the sibling is still running the active task and put_prev_task has yet
not been called. This can result in 'left == NULL'. So I think the
above fix is appropriate when core scheduling is active. It could be
cleaned up a bit though.

Thanks,
Vineeth