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

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Message-ID: <YtGbvaia8oOulMEo@lorien.usersys.redhat.com>
Date:   Fri, 15 Jul 2022 12:54:21 -0400
From:   Phil Auld <pauld@...hat.com>
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, peterz@...radead.org, mingo@...hat.com,
        mtosatti@...hat.com
Subject: Re: [PATCH] nohz/full, sched/rt: Fix missed tick-reenabling bug in
 dequeue_task_rt

On Tue, Jun 28, 2022 at 11:22:59AM +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>

That's been around for a while. This change looks right and makes sense
to me.

Thanks,
Phil

Reviewed-by: Phil Auld <pauld@...hat.com>

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