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

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Message-ID: <20090306172654.GA18600@in.ibm.com>
Date:	Fri, 6 Mar 2009 22:56:54 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.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 Fri, Mar 06, 2009 at 06:08:34PM +0100, Oleg Nesterov wrote:
> On 03/06, Gautham R Shenoy wrote:
> >
> > .load_balancer is the idle load balancer CPU which performs
> > load balancing on behalf of all the idle cpus in the system.
> > It's the only idle CPU which doesn't turn off it's ticks.
> >
> > This CPU relinquishes it's role as the idle load balancer when
> > a) it finds a runnable task in it's runqueue, i.e before exiting idle
> >    state.
> > OR
> > b) all the CPUs in the system go idle.
> >
> > Since it doesn't turn off it's ticks, it calls
> > select_nohz_load_balancer() every scheduler tick, and thus can observe
> > that it's no longer set in cpu_active_map within a tick or two.
> >
> > Also, the cpu_down() path calls migrate_timers() in CPU_DEAD:,i.e after
> > the CPU has gone down. But to take the CPU down, we would have invoked
> > the stop_machine_run() code on that CPU, which by itself would make the
> > CPU relinquish it's role as the idle load balancer owing to condition (a).
> >
> > Thus I believe, mod_timer() can never migrate a timer on to a DEAD CPU,
> > during/_after_ we invoke migrate_timers() from the CPU_DEAD callpath.
> 
> OK, thanks a lot for your explanation!

Glad it helped :-)
> 
> Oleg.

-- 
Thanks and Regards
gautham
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