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

Mike Galbraith
2016-05-23T11:12:12Z

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@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)