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SubjectRe: [PATCH] RT: Fix special-case exception for preempting the local CPU
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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@in.ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs,
Bangalore, India
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