Re: possible migration bug with hotplug cpu

[Lucas De Marchi <lucas.de.marchi@gmail.com>]

No, because the tasks are executed only after the CPUs become offline.

Lucas De Marchi


On Wed, Jul 8, 2009 at 17:55, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, 2009-07-08 at 17:48 +0200, Lucas De Marchi wrote:
> > I was doing some analysis with the number of migrations in my application and
> > I think there's a bug in this accounting or even worse, in the migrations
> > mechanism when used together with cpu hotplug.
> >
> > I turned off all CPUs except one using the hotplug mechanism, after what I
> > launghed my application that has 8 threads. Before they finish they print the
> > file /proc/<tid>/sched. I have only 1 online CPU and there are ~ 200
> > migrations per thread. The function set_task_cpu is responsible for updating
> > the migrations counter and is called by 9 other functions. With some tests I
> > discovered that 95% of these migrations come from try_to_wake_up and the other
> > 5% from pull_task and __migrate_task.
> >
> > Looking at try_to_wake_up:
> >
> > ....
> >       cpu = task_cpu(p);
> >       orig_cpu = cpu;
> >       this_cpu = smp_processor_id();
> >
> > #ifdef CONFIG_SMP
> >       if (unlikely(task_running(rq, p)))
> >               goto out_activate;
> >
> >       cpu = p->sched_class->select_task_rq(p, sync);  //<<<<===
> >       if (cpu != orig_cpu) {                          //<<<<===
> >               set_task_cpu(p, cpu);
> > ....
> >
> > p->sched_class->select_task_rq(p, sync)  is returning a different cpu of
> > task_cpu(p) even if I have only 1 online CPU. In my tests this behavior is
> > similar for rt and normal tasks. For RT, the only possible problem could be on
> > find_lowest_rq, but I'm still rying to find out why. Since you have more
> > experience with this code, if you could give it a look I'd appreciate.
> >
> > Is there any obscure reason why this behavior could be right?
>
> If the task last ran on a now unplugged cpu this would be correct, is
> this indeed what happens?