linux-kernel - Re: [PATCH] sched: select eligible run-queue for RT task

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Date:	Thu, 16 Jun 2011 05:19:04 +0200
From:	Mike Galbraith <efault@....de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Hillf Danton <dhillf@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Yong Zhang <yong.zhang0@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: select eligible run-queue for RT task

On Wed, 2011-06-15 at 13:50 -0400, Steven Rostedt wrote:
> On Fri, 2011-06-03 at 22:06 +0800, Hillf Danton wrote:
> > When selecting run-queue for a given task, eligible run-queue should be
> > returned by checking the CPU affinity of task.
> > 
> > Signed-off-by: Hillf Danton <dhillf@...il.com>
> > ---
> >  kernel/sched_rt.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> > index 88725c9..45b3e0a 100644
> > --- a/kernel/sched_rt.c
> > +++ b/kernel/sched_rt.c
> > @@ -1006,7 +1006,8 @@ select_task_rq_rt(struct task_struct *p, int
> > sd_flag, int flags)
> >  	int cpu;
> > 
> >  	if (sd_flag != SD_BALANCE_WAKE)
> > -		return smp_processor_id();
> > +		return cpumask_test_cpu(smp_processor_id(), &p->cpus_allowed) ?
> > +			smp_processor_id() : task_cpu(p);
> 
> I wonder if we should bother even dhoing a test here. Perhaps a better
> solution is just:
> 
> 	if (sd_flag != SD_BALANCE_WAKE)
> 		return task_cpu(p);

Hm.  We shouldn't need to check the mask here for SD_BALANCE_WAKE, since
it will be checked when we return to select_task_rq().

For exec, it doesn't matter which we return, but for a preempted and
migrated parent waking it's child, it could.  task_cpu(parent) seems
better than task_cpu(child), since that is likely where the parent was
preempted, and may still be occupied by a higher priority task.  We'll
subsequently try to push, but we then fiddle with a higher priority rq
needlessly, no?

So to me, it looks like things are better as is.. but we could perhaps
do better by handling SD_BALANCE_FORK here as well, to avoid some 'queue
the child locally (overload) then push it away' overhead.

	-Mike

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