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

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Date:   Fri, 01 Jul 2022 12:25:28 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        linux-kernel@...r.kernel.org, fweisbec@...il.com
Cc:     bristot@...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,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>
Subject: Re: [PATCH] nohz/full, sched/rt: Fix missed tick-reenabling bug in
 dequeue_task_rt

On 28/06/22 11:22, 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.
>

I can't see anything wrong with your approach, though I did have to spend
some time re-learning RT_GROUP_SCHED. The designated Fixes: commit looks
about right too.

> Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mask model")
> Signed-off-by: Nicolas Saenz Julienne <nsaenzju@...hat.com>

Reviewed-by: Valentin Schneider <vschneid@...hat.com>