linux-kernel - Re: [PATCH] RT: Fix special-case exception for preempting the local CPU

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Message-ID: <20071011105123.GE6721@in.ibm.com>
Date:	Thu, 11 Oct 2007 16:21:23 +0530
From:	ankita@...ibm.com (Ankita Garg)
To:	mike kravetz <kravetz@...ibm.com>
Cc:	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	peterz@...radead.org, rostedt@...dmis.org,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RT: Fix special-case exception for preempting the local CPU

On Wed, Oct 10, 2007 at 09:22:48AM -0700, mike kravetz wrote:
> On Wed, Oct 10, 2007 at 10:49:35AM -0400, Gregory Haskins wrote:
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 3e75c62..b7f7a96 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -1869,7 +1869,8 @@ out_activate:
> >  		 * extra locking in this particular case, because
> >  		 * we are on the current CPU.)
> >  		 */
> > -		if (TASK_PREEMPTS_CURR(p, this_rq))
> > +		if (TASK_PREEMPTS_CURR(p, this_rq)
> > +		    && cpu_isset(this_cpu, p->cpus_allowed))
> >  			set_tsk_need_resched(this_rq->curr);
> >  		else
> >  			/*
> 
> I wonder if it might better to explicitly take the rq lock and try to
> put the task on this_rq in this situation?  Rather than waiting for
> schedule to pull it from a remote rq as part of balance_rt_tasks.
> 
> A question that has passed through my head a few times is:  When waking
> a RT task is it better to:
> 1) run on current CPU if possible
> 2) run on CPU task previously ran on
> 
> I think #1 may result in lower latency.  But, if the task has lots of
> cache warmth the lower wakeup latency may be negated by running on a
> 'remote' cpu.

Could we use task_hot() routine to find if the task is cache hot? If it
isn't, if possible, we could run on current CPU, else, if possible, on
the CPU it last ran on?


-- 
Regards,
Ankita Garg (ankita@...ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs, 
Bangalore, India   
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