linux-kernel - Re: [PATCH] nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task

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Message-ID: <Ytf6B19HjYGPvJZ/@worktop.programming.kicks-ass.net>
Date:   Wed, 20 Jul 2022 14:50:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     linux-kernel@...r.kernel.org, fweisbec@...il.com,
        bristot@...hat.com, vschneid@...hat.com, cmetcalf@...hip.com,
        mgorman@...e.de, bsegall@...gle.com, rostedt@...dmis.org,
        dietmar.eggemann@....com, vincent.guittot@...aro.org,
        juri.lelli@...hat.com, mingo@...hat.com, mtosatti@...hat.com
Subject: Re: [PATCH] nohz/full, sched/rt: Fix missed tick-reenabling bug in
 dequeue_task_rt

On Wed, Jul 13, 2022 at 07:18:42PM +0200, Nicolas Saenz Julienne wrote:
> On Tue, 2022-06-28 at 11:22 +0200, Nicolas Saenz Julienne wrote:
> > dequeue_task_rt() only decrements 'rt_rq->rt_nr_running' after having
> > called sched_update_tick_dependency() preventing it from re-enabling the
> > tick on systems that no longer have pending SCHED_RT tasks but have
> > multiple runnable SCHED_OTHER tasks:
> > 
> >   dequeue_task_rt()
> >     dequeue_rt_entity()
> >       dequeue_rt_stack()
> >         dequeue_top_rt_rq()
> > 	  sub_nr_running()	// decrements rq->nr_running
> > 	    sched_update_tick_dependency()
> > 	      sched_can_stop_tick()	// checks rq->rt.rt_nr_running,
> > 	      ...
> >         __dequeue_rt_entity()
> >           dec_rt_tasks()	// decrements rq->rt.rt_nr_running
> > 	  ...
> > 
> > Every other scheduler class performs the operation in the opposite
> > order, and sched_update_tick_dependency() expects the values to be
> > updated as such. So avoid the misbehaviour by inverting the order in
> > which the above operations are performed in the RT scheduler.
> > 
> > Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mask model")
> > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>
> > ---
> 
> Ping :)

Thanks! got stuck in the retbleed backlog :/