[PATCH 0/3] (Was: sched: start stopper early)

[Oleg Nesterov <oleg@redhat.com>]

To avoid the confusion, this has nothing to do with "stop_machine"
changes we discuss in another thread, but

On 10/09, Oleg Nesterov wrote:
>
> >  	case CPU_ONLINE:
> > +		stop_machine_unpark(cpu);
> >  		/*
> >  		 * At this point a starting CPU has marked itself as online via
> >  		 * set_cpu_online(). But it might not yet have marked itself
> > @@ -5337,7 +5340,7 @@ static int sched_cpu_active(struct notifier_block *nfb,
> >  		 * Thus, fall-through and help the starting CPU along.
> >  		 */
> >  	case CPU_DOWN_FAILED:
> > -		set_cpu_active((long)hcpu, true);
> > +		set_cpu_active(cpu, true);
>
> On a second thought, we can't do this (and your initial change has
> the same problem).
>
> We can not wakeup it before set_cpu_active(). This can lead to the
> same problem fixed by dd9d3843755da95f6 "sched: Fix cpu_active_mask/
> cpu_online_mask race".

OTOH, I don't understand why do we actually need this fix... Or, iow
I don't really understand the cpu_active() checks in select_fallback_rq().

Looks like we have some strange arch/ which has the "unsafe" online &&
!active window, but then it is not clear why it is safe to mark it active
in sched_cpu_active(CPU_ONLINE). Confused.

And I am even more confused by the fact that select_fallback_rq()
checks cpu_active(), but select_task_rq() doesn't. This can't be right
in any case.

Oleg.


 kernel/sched/core.c |   41 +++++++++++++++++++++--------------------
 1 files changed, 21 insertions(+), 20 deletions(-)