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

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Message-ID: <20090306170834.GA11271@redhat.com>
Date:	Fri, 6 Mar 2009 18:08:34 +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:
>
> .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!

Oleg.

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