Re: [PATCH 2/3] sched,fair: Fix local starvation

[Mike Galbraith]

On Sun, 2016-05-22 at 15:42 +0800, Wanpeng Li wrote:
> 2016-05-22 15:32 GMT+08:00 Mike Galbraith <mgalbraith@xxxxxxx>:
> > On Sun, 2016-05-22 at 15:27 +0800, Wanpeng Li wrote:
> > 
> > > What's the meaning of 'x-cpu wakeup'? ;-)
> > 
> > Generally, cross CPU, as in waker/wakee reside on different CPUs,
> > but
> > in this case, it's cross socket wakeup.
> 
> Do you mean wakeup wakees on remote socket don't imply
> migration/normalized, why?

Because the wakee is NOT necessarily migrated simply because it lives
in some remote cache domain.  It was a simple but nasty booboo.

ttwu():
        cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
        if (task_cpu(p) != cpu) {
                wake_flags |= WF_MIGRATED;
                set_task_cpu(p, cpu);

set_task_cpu():
                if (task_cpu(p) != new_cpu) {
                if (p->sched_class->migrate_task_rq)
                        p->sched_class->migrate_task_rq(p);
                                        ^^^^^^^^^^^^^^^^^^

migrate_task_rq_fair() normalizes wakee, those wakees that did not
migrate have NOT been normalized, leaving two flavors of wakee on the
wake_list, with no discriminator.  Store class information internally,
and the x-socket information disconnect evaporates.

	-Mike