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

Yuyang Du
2016-05-24T07:02:00Z

On Mon, May 23, 2016 at 01:00:10PM +0100, Morten Rasmussen wrote:
> 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 :-)

On mobile, the factor is as small as 2 to 4, may easily be exceeded,
so decay at HZ may be too slow.

> >
> > > 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)