linux-kernel - Re: [PATCH 03/16] sched/fair: Disregard idle task wakee_flips in wake

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Date:	Mon, 23 May 2016 13:12:07 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Morten Rasmussen <morten.rasmussen@....com>, peterz@...radead.org,
	mingo@...hat.com
Cc:	dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/16] sched/fair: Disregard idle task wakee_flips in
 wake_wide

On Mon, 2016-05-23 at 11:58 +0100, Morten Rasmussen wrote:
> wake_wide() is based on task wakee_flips of the waker and the wakee to
> decide whether an affine wakeup is desirable. On lightly loaded systems
> the waker is frequently the idle task (pid=0) which can accumulate a lot
> of wakee_flips in that scenario. It makes little sense to prevent affine
> wakeups on an idle cpu due to the idle task wakee_flips, so it makes
> more sense to ignore them in wake_wide().

You sure?  What's the difference between a task flipping enough to
warrant spreading the load, and an interrupt source doing the same? 
 I've both witnessed firsthand, and received user confirmation of this
very thing improving utilization.

> cc: Ingo Molnar <mingo@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
>  kernel/sched/fair.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c49e25a..0fe3020 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5007,6 +5007,10 @@ static int wake_wide(struct task_struct *p)
>  	unsigned int slave = p->wakee_flips;
>  	int factor = this_cpu_read(sd_llc_size);
>  
> +	/* Don't let the idle task prevent affine wakeups */
> +	if (is_idle_task(current))
> +		return 0;
> +
>  	if (master < slave)
>  		swap(master, slave);
>  	if (slave < factor || master < slave * factor)