Re: [PATCH RT] fix rt-task scheduling issue

Mike Kravetz
2007-10-09T18:51:14Z

On Mon, Oct 08, 2007 at 10:46:21PM -0400, Steven Rostedt wrote:
> Mike,
>
> Can you attach your Signed-off-by to this patch, please.
>
>
> On Fri, Oct 05, 2007 at 07:15:48PM -0700, Mike Kravetz wrote:
> > Hi Ingo,
> >
> > After applying the fix to try_to_wake_up() I was still seeing some large
> > latencies for realtime tasks. Some debug code pointed out two additional
> > causes of these latencies. I have put fixes into my 'old' kernel and the
> > scheduler related latencies have gone away. I'm pretty confident that
> > one of these bugs still exist in the latest RT patch set. Not so sure
> > about the other. But, I wanted to describe in detail so that you could
> > address in the latest version of the code if applicable.
> >
> > finish_task_switch() contains the following code:
> >
> > #if defined(CONFIG_PREEMPT_RT) && defined(CONFIG_SMP)
> > /*
> > * If we pushed an RT task off the runqueue,
> > * then kick other CPUs, they might run it:
> > */
> > if (unlikely(rt_task(current) && prev->se.on_rq && rt_task(prev))) {
> > schedstat_inc(rq, rto_schedule);
> > smp_send_reschedule_allbutself_cpumask(current->cpus_allowed);
> > }
> > #endif
> >
> > My debug code found instances where more than one realtime task got
> > put on the runqueue before the __schedule() was invoked. So, current
> > would be a realtime task, but prev was not realtime. And, there was
> > another (lesser priority, or last in) realtime task on the queue. I
> > believe that in this case we would still want to send the IPIs. In my
> > kernel I changed the test to be:
> >
> > if (unlikely(rt_task(current) && rq->rt_nr_running > 1)) {
> >
> > After this change, I definitely saw some long latencies go away.

OK, not really a patch but

Signed-off-by: Mike Kravetz <kravetz@us.ibm.com>

--
Mike