linux-kernel - Re: [PATCH] sched,rt,nohz: stop scheduler tick if running realtime task

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Date:	Tue, 3 Mar 2015 18:12:31 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	peterz@...radead.org, lcapitulino@...hat.com,
	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	mingo@...nel.org, fweisbec@...hat.com
Subject: Re: [PATCH] sched,rt,nohz: stop scheduler tick if running realtime
 task

On Mon, Feb 16, 2015 at 03:23:49PM -0500, Rik van Riel wrote:
> If the CPU is running a realtime task that does not round-robin with
> another realtime task of equal priority, there is no point in keeping
> the scheduler tick going. After all, whenever the scheduler tick runs,
> the kernel will just decide not to reschedule.
> 
> Extend sched_can_stop_tick to recognize these situations, and inform
> the rest of the kernel that the scheduler tick can be stopped.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Tested-by: Luiz Capitulino <lcapitulino@...hat.com>
> ---
>  kernel/sched/core.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ade2958a9197..ad985a632c4d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -745,6 +745,22 @@ static inline bool got_nohz_idle_kick(void)
>  bool sched_can_stop_tick(void)
>  {
>  	/*
> +	 * FIFO realtime policy runs the highest priority task. Other runnable
> +	 * tasks are of a lower priority. The scheduler tick does nothing.
> +	 */
> +	if (current->policy == SCHED_FIFO)
> +		return true;
> +
> +	/*
> +	 * Round-robin realtime tasks time slice with other tasks at the same
> +	 * realtime priority. Is this task the only one at this priority?
> +	 */
> +	if (current->policy == SCHED_RR) {
> +		struct sched_rt_entity *rt_se = &current->rt;
> +		return rt_se->run_list.prev == rt_se->run_list.next;
> +	}
> +
> +	/*

I think it should work yes.

There are still many things, that the tick updates, which are broken without it
(rq->rt_avg is supposed to be updated avery tick for example) but it goes way beyond
the scope of this change since SCHED_FIFO tasks are already allowed to stop the tick
when no SCHED_OTHER task is running so the problem is there already before this patch.

So it's a welcome fix, thanks.

>  	 * More than one running task need preemption.
>  	 * nr_running update is assumed to be visible
>  	 * after IPI is sent from wakers.
> --
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