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

[Darren Hart]

On 02/22/2011 08:20 PM, Mike Galbraith wrote:
> 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@xxxxxx>
>
> Nit below.
>
> > Signed-off-by: Darren Hart<dvhart@xxxxxxxxxxxxxxx>
> > CC: Peter Zijlstra<peterz@xxxxxxxxxxxxx>
> > CC: Ingo Molnar<mingo@xxxxxxx>
> > CC: Richard Purdie<richard.purdie@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   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.

I have no opinion on the unlikely/likely optimizations. I chose the way 
I did as I thought it was more consistent with the existing code. I'll 
leave that to Peter and Ingo - let me know if I should resend.

--
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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