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

[Darren Hart <dvhart@linux.intel.com>]

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@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.

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