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SubjectRe: [PATCH] sched: select eligible run-queue for RT task
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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@gmail.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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