linux-kernel - Re: NULL pointer dereference in pick_next_task

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Message-ID: <20191107154239.GE4114@hirez.programming.kicks-ass.net>
Date:   Thu, 7 Nov 2019 16:42:39 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Quentin Perret <qperret@...gle.com>, linux-kernel@...r.kernel.org,
        aaron.lwe@...il.com, valentin.schneider@....com, mingo@...nel.org,
        pauld@...hat.com, jdesfossez@...italocean.com,
        naravamudan@...italocean.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, juri.lelli@...hat.com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        kernel-team@...roid.com, john.stultz@...aro.org
Subject: Re: NULL pointer dereference in pick_next_task_fair

On Thu, Nov 07, 2019 at 06:12:07PM +0300, Kirill Tkhai wrote:
> On 07.11.2019 16:26, Peter Zijlstra wrote:

> > Urgh... throttling.

> One more thing about current code in git. After rq->lock became able to
> be unlocked after put_prev_task() is commited, we got a new corner case.
> We usually had the same order for running task:
> 
>   dequeue_task()
>   put_prev_task()
> 
> Now the order may be reversed (this is also in case of throttling):
> 
>   put_prev_task() (called from pick_next_task())
>   dequeue_task()  (called from another cpu)
> 
> This is more theoretically, since I don't see a problem here. But there are
> too many statistics and counters in sched_class methods, that it is impossible
> to be sure all of them work as expected.

Hmm,.. where does throttling happen on a remote CPU? I through both
cfs-bandwidth and dl throttle locally.

Or are you talking about NO_HZ_FULL's sched_remote_tick() ?