linux-kernel - Re: slab: start_cpu_timer/cache_reap CONFIG_HOTPLUG

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Date:	Tue, 20 Feb 2007 23:05:50 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Christoph Lameter <clameter@....com>
Cc:	Max Krasnyansky <maxk@...lcomm.com>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Gautham shenoy <ego@...ibm.com>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org
Subject: Re: slab: start_cpu_timer/cache_reap CONFIG_HOTPLUG_CPU problems

On 02/20, Christoph Lameter wrote:
>
> On Tue, 20 Feb 2007, Max Krasnyansky wrote:
> 
> > > > Well seems that we have a set of unresolved issues with workqueues and cpu
> > > > hotplug.
> > 
> > How about storing 'cpu' explicitly in the work queue instead of relying on the
> > smp_processor_id() and friends ? That way there is no ambiguity when
> > threads/timers get
> > moved around.
> 
> The slab functionality is designed to work on the processor with the 
> queue. These tricks will only cause more trouble in the future. The 
> cache_reaper needs to be either disabled or run on the right processor. It 
> should never run on the wrong processor.

I personally agree. Besides, cache_reaper is not alone. Note the comment
in debug_smp_processor_id() about cpu-bound processes. The slab does correct
thing right now, stops the timer on CPU_DEAD. Other problems imho should be
solved by fixing cpu-hotplug. Gautham and Srivatsa are working on that.

Oleg.

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