linux-kernel - Re: [PATCH] kernel/sched/rt: Add a rescheduling point

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Date:   Wed, 25 Jan 2017 13:16:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] kernel/sched/rt: Add a rescheduling point

On Tue, Jan 24, 2017 at 03:40:06PM +0100, Sebastian Andrzej Siewior wrote:
> Since the change in commit fd7a4bed1835 ("sched, rt: Convert
> switched_{from, to}_rt() / prio_changed_rt() to balance callbacks") we
> don't reschedule a task under certain circumstances:
> 
> Lets say taskA, SCHED_OTHER, is running on CPU0 (and it may run only on
> CPU0) and holds a PI lock. This task is removed from the CPU because it
> used up its time slice and another SCHED_OTHER task is running. TaskB on
> CPU1 runs at RT priority and asks for the lock owned by taskA. This
> results in a priority boost for taskA. TaskB goes to sleep until the
> lock has been made available. TaskA is already runable (but not active)
> so it receives no wake up.
> The reality now is that taskA gets on the CPU once the scheduler decides
> to remove the current task despite the fact that a high priority task is
> enqueued and waiting. This may take a long time.
> The desired behaviour is that CPU0 immediately reschedules after the
> priority boost which made taskA the task with the lowest priority.
> 
> Fixes: fd7a4bed1835 ("sched, rt: Convert switched_{from, to}_rt() /
> 		     prio_changed_rt() to balance callbacks")
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/sched/rt.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 88254be118b0..cdba8d58dbc5 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2198,10 +2198,10 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
>  #ifdef CONFIG_SMP
>  		if (tsk_nr_cpus_allowed(p) > 1 && rq->rt.overloaded)
>  			queue_push_tasks(rq);
> -#else
> +		else

I killed that "else" as well, because the queue_push_tasks() can fail to
actually push the task, in which case we'd still miss the preemption.

> +#endif /* CONFIG_SMP */
>  		if (p->prio < rq->curr->prio)
>  			resched_curr(rq);
> -#endif /* CONFIG_SMP */
>  	}
>  }
>  
> -- 
> 2.11.0
>