Re: [v2 PATCH 4/4] timers: logic to enable timer migration.

[Oleg Nesterov <oleg@redhat.com>]

On 03/06, Gautham R Shenoy wrote:
>
> On Thu, Mar 05, 2009 at 05:23:29PM +0100, Oleg Nesterov wrote:
> > On 03/04, Arun R Bharadwaj wrote:
> > >
> > > +++ linux.trees.git/kernel/sched.c
> > > @@ -4009,6 +4009,11 @@ static struct {
> > >  	.load_balancer = ATOMIC_INIT(-1),
> > >  };
> > >
> > > +inline int get_nohz_load_balancer(void)
> >
> > inline?
> > 
> > > +{
> > > +	return atomic_read(&nohz.load_balancer);
> > > +}
> >
> > Shouldn't we reset .load_balancer when this CPU is CPU_DOWN'ed ?
> > Otherwise the timer can migrate to the dead CPU.
>
> In the select_nohz_load_balancer() code, we check if this CPU is in the
> cpu_active_map. If no, then this CPU relinquishes being the idle
> load balancer.

I don't understand this code, but I am not sure select_nohz_load_balancer()
is always called on cpu_down() path before migrate_timers/migrate_hrtimers.

If this is true, then there is no problem.

> Also, the timer migration code in the CPU down path would
> migrate any timers queued onto this CPU, right ?

Yes, but if .load_balancer is not cleared before the timer migration,
mod_timer() can move the timer to the dead CPU after migration.

Oleg.