Re: [PATCH 1/2] sched: allow SCHED_BATCH to preempt SCHED_IDLE tasks

[Mike Galbraith <efault@gmx.de>]

On Tue, 2011-02-22 at 13:04 -0800, Darren Hart wrote:
> Perform the test for SCHED_IDLE before testing for SCHED_BATCH (and ensure idle
> tasks don't preempt idle tasks) so the non-interactive, but still important,
> SCHED_BATCH tasks will run in favor of the very low priority SCHED_IDLE tasks.

Yeah, that could be construed as a fairness fix for light SCHED_BATCH vs
a heavy SCHED_IDLE.  It should lower latencies for both when mixed.

Acked-by: Mike Galbraith <efault@gmx.de>

Nit below.

> Signed-off-by: Darren Hart <dvhart@linux.intel.com>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Ingo Molnar <mingo@elte.hu>
> CC: Richard Purdie <richard.purdie@linuxfoundation.org>
> ---
>  kernel/sched_fair.c |   12 +++++++-----
>  1 files changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> index 0c26e2d..ff04bbd 100644
> --- a/kernel/sched_fair.c
> +++ b/kernel/sched_fair.c
> @@ -1857,16 +1857,18 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>  	if (test_tsk_need_resched(curr))
>  		return;
>  
> +	/* Idle tasks are by definition preempted by non-idle tasks. */
> +	if (unlikely(curr->policy == SCHED_IDLE) &&
> +	    likely(p->policy != SCHED_IDLE))
> +		goto preempt;
> +

if (unlikely(curr->policy == SCHED_IDLE && p->policy != curr->policy))
	goto preempt;

Looks better to me.

	-Mike