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

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Message-ID: <20160523120010.GB27946@e105550-lin.cambridge.arm.com>
Date:	Mon, 23 May 2016 13:00:10 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Mike Galbraith <mgalbraith@...e.de>
Cc:	peterz@...radead.org, mingo@...hat.com, 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, May 23, 2016 at 01:12:07PM +0200, Mike Galbraith wrote:
> 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.

Right, I didn't consider the interrupt source scenario, my fault.

The problem then seems to be distinguishing truly idle and busy doing
interrupts. The issue that I observe is that wake_wide() likes pushing
tasks around in lightly scenarios which isn't desirable for power
management. Selecting the same cpu again may potentially let others
reach deeper C-state.

With that in mind I will if I can do better. Suggestions are welcome :-)

> 
> > 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)