linux-kernel - Re: [PATCH 2/3] sched: Unloop sched avg decaying

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Date:	Wed, 6 Jul 2016 13:53:17 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/3] sched: Unloop sched avg decaying

On Thu, Jun 30, 2016 at 03:20:37PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 30, 2016 at 02:52:26PM +0200, Frederic Weisbecker wrote:
> > On Tue, Jun 14, 2016 at 05:58:42PM +0200, Peter Zijlstra wrote:
> 
> > > Why not add the division to the nohz exit path only?
> > 
> > It would be worse I think because we may exit much more often from nohz
> > than we reach a sched_avg_period().
> > 
> > So the only safe optimization I can do for now is:
> 
> How about something like this then?
> 
> ---
> 
>  kernel/sched/core.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3387e4f14fc9..fd1ae4c4105f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -665,9 +665,23 @@ bool sched_can_stop_tick(struct rq *rq)
>  
>  void sched_avg_update(struct rq *rq)
>  {
> -	s64 period = sched_avg_period();
> +	s64 delta, period = sched_avg_period();
>  
> -	while ((s64)(rq_clock(rq) - rq->age_stamp) > period) {
> +	delta = (s64)(rq_clock(rq) - rq->age_stamp);
> +	if (likely(delta < period))
> +		return;
> +
> +	if (unlikely(delta > 3*period)) {
> +		int pending;
> +		u64 rem;
> +
> +		pending = div64_u64_rem(delta, period, &rem);
> +		rq->age_stamp += delta - rem;
> +		rq->rt_avg >>= pending;
> +		return;
> +	}
> +
> +	while (delta > period) {
>  		/*
>  		 * Inline assembly required to prevent the compiler
>  		 * optimising this loop into a divmod call.
> @@ -675,6 +689,7 @@ void sched_avg_update(struct rq *rq)
>  		 */
>  		asm("" : "+rm" (rq->age_stamp));
>  		rq->age_stamp += period;
> +		delta -= period;
>  		rq->rt_avg /= 2;
>  	}
>  }

Makes sense. I'm going to do some tests. We might want to precompute 3*period maybe.

Thanks.