linux-kernel - Re: [PATCH V2] sched: Prevent balance

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Message-ID: <20210830091212.GG4353@worktop.programming.kicks-ass.net>
Date:   Mon, 30 Aug 2021 11:12:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH V2] sched: Prevent balance_push() on remote runqueues

On Sat, Aug 28, 2021 at 03:55:52PM +0200, Thomas Gleixner wrote:
> sched_setscheduler() and rt_mutex_setprio() invoke the run-queue balance
> callback after changing priorities or the scheduling class of a task. The
> run-queue for which the callback is invoked can be local or remote.
> 
> That's not a problem for the regular rq::push_work which is serialized with
> a busy flag in the run-queue struct, but for the balance_push() work which
> is only valid to be invoked on the outgoing CPU that's wrong. It not only
> triggers the debug warning, but also leaves the per CPU variable push_work
> unprotected, which can result in double enqueues on the stop machine list.
> 
> Remove the warning and validate that the function is invoked on the
> outgoing CPU.
> 
> Fixes: ae7927023243 ("sched: Optimize finish_lock_switch()")
> Reported-by: Sebastian Siewior <bigeasy@...utronix.de>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Thanks!