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Subject: Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()
To:     Vineeth Pillai <viremana@linux.microsoft.com>,
        Joel Fernandes <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Thomas Glexiner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, vineeth@bitbyteword.org,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Agata Gruza <agata.gruza@intel.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
        graf@amazon.com, konrad.wilk@oracle.com,
        Dario Faggioli <dfaggioli@suse.com>,
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        Steven Rostedt <rostedt@goodmis.org>,
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 <4241e5ac-ecdf-8634-fa0d-dd6759e477e1@linux.microsoft.com>
 <8230ada7-839f-2335-9a55-b09f6a813e91@linux.microsoft.com>
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <a422116c-edef-53f6-8956-0d7525958e87@linux.intel.com>
Date:   Sun, 25 Oct 2020 07:48:15 +0800
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On 2020/10/24 20:27, Vineeth Pillai wrote:
> 
> 
> 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.

This patch works, thanks Vineeth for the quick fix!