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Message-ID: <Ykbk4MSSNSxsQoMs@hirez.programming.kicks-ass.net>
Date: Fri, 1 Apr 2022 13:41:20 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
vincent.guittot@...aro.org
Cc: LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
joel@...lfernandes.org, dietmar.eggemann@....com,
bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com
Subject: Re: [PATCH] sched/core: Fix forceidle balancing
On Wed, Mar 30, 2022 at 06:05:35PM +0200, Peter Zijlstra wrote:
>
> Steve reported that ChromeOS encounters the forceidle balancer being
> ran from rt_mutex_setprio()'s balance_callback() invocation and
> explodes.
>
> Now, the forceidle balancer gets queued every time the idle task gets
> selected, set_next_task(), which is strictly too often.
> rt_mutex_setprio() also uses set_next_task() in the 'change' pattern:
>
> queued = task_on_rq_queued(p); /* p->on_rq == TASK_ON_RQ_QUEUED */
> running = task_current(rq, p); /* rq->curr == p */
>
> if (queued)
> dequeue_task(...);
> if (running)
> put_prev_task(...);
>
> /* change task properties */
>
> if (queued)
> enqueue_task(...);
> if (running)
> set_next_task(...);
>
> However, rt_mutex_setprio() will explicitly not run this pattern on
> the idle task (since priority boosting the idle task is quite insane).
> Most other 'change' pattern users are pidhash based and would also not
> apply to idle.
>
> Also, the change pattern doesn't contain a __balance_callback()
> invocation and hence we could have an out-of-band balance-callback,
> which *should* trigger the WARN in rq_pin_lock() (which guards against
> this exact anti-pattern).
>
> So while none of that explains how this happens, it does indicate that
> having it in set_next_task() might not be the most robust option.
>
> Instead, explicitly queue the forceidle balancer from pick_next_task()
> when it does indeed result in forceidle selection. Having it here,
> ensures it can only be triggered under the __schedule() rq->lock
> instance, and hence must be ran from that context.
>
> This also happens to clean up the code a little, so win-win.
So I couldn't figure out how this could happen without triggering other
warnings, because as I mentioned elsewhere, commit 565790d28b1e ("sched:
Fix balance_callback()") should've caused a different splat.
But then Dietmar reminded me that ChromeOS is probably running some
ancient crud with backports on :/ and will most likely not have that
commit.
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