Re: [PATCH 03/16] sched/fair: Disregard idle task wakee_flips in wake_wide

Morten Rasmussen
2016-05-23T11:59:34Z

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@redhat.com>
> > cc: Peter Zijlstra <peterz@infradead.org>
> >
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.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)