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

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Message-ID: <20090306145002.GA5828@redhat.com>
Date:	Fri, 6 Mar 2009 15:50:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, mingo@...e.hu,
	andi@...stfloor.org, venkatesh.pallipadi@...el.com,
	vatsa@...ux.vnet.ibm.com, arjan@...radead.org,
	svaidy@...ux.vnet.ibm.com
Subject: Re: [v2 PATCH 4/4] timers: logic to enable timer migration.

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.

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