Re: [RFC/PATCH] sched: Honor sync wake up in select_idle_sibling

[Mike Galbraith <efault@gmx.de>]

On Fri, 2012-08-10 at 16:25 +0900, Namhyung Kim wrote: 
> From: Namhyung Kim <namhyung.kim@lge.com>
> 
> When sync wakeup happens and there's the waker task running alone,
> select the target cpu as if it's already idle.

Ouch.  That defeats the purpose of select_idle_sibling().  Just because
we're doing a sync wakeup does not mean there's no convertible overlap,
nor that the waker is really really going to take a nap immediately.  A
lot of the places that benefit up to and including hugely from looking
for an idle shared cache to wake to do sync wakeups.

> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Suresh Siddha <suresh.b.siddha@intel.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  kernel/sched/fair.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d0cc03b3e70b..64b68bc82b52 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2632,7 +2632,7 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
>  /*
>   * Try and locate an idle CPU in the sched_domain.
>   */
> -static int select_idle_sibling(struct task_struct *p, int target)
> +static int select_idle_sibling(struct task_struct *p, int target, int sync)
>  {
>  	int cpu = smp_processor_id();
>  	int prev_cpu = task_cpu(p);
> @@ -2646,6 +2646,13 @@ static int select_idle_sibling(struct task_struct *p, int target)
>  		return cpu;
>  
>  	/*
> +	 * If the task is going to be woken-up on this cpu and there's only
> +	 * the sync-waker running, then it is the right target.
> +	 */
> +	if (target == cpu && sync && this_rq()->nr_running == 1)
> +		return cpu;
> +
> +	/*
>  	 * If the task is going to be woken-up on the cpu where it previously
>  	 * ran and if it is currently idle, then it the right target.
>  	 */
> @@ -2748,7 +2755,7 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  		if (cpu == prev_cpu || wake_affine(affine_sd, p, sync))
>  			prev_cpu = cpu;
>  
> -		new_cpu = select_idle_sibling(p, prev_cpu);
> +		new_cpu = select_idle_sibling(p, prev_cpu, sync);
>  		goto unlock;
>  	}
>