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

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Date:	Mon, 29 Jan 2007 08:54:36 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	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 Mon, 29 Jan 2007, Oleg Nesterov wrote:

> Now,
> 	static void __devinit start_cpu_timer(int cpu)
> 	{
> 		struct delayed_work *reap_work = &per_cpu(reap_work, cpu);
> 
> 		if (keventd_up() && reap_work->work.func == NULL) {
> 			init_reap_node(cpu);
> 			INIT_DELAYED_WORK(reap_work, cache_reap);
> 			schedule_delayed_work_on(cpu, reap_work,
> 						__round_jiffies_relative(HZ, cpu));
> 		}
> 	}
> 
> This is wrong. Suppose we have a CPU_UP,CPU_DOWN,CPU_UP sequence. The last
> CPU_UP will not restart a per-cpu "cache_reap timer".

Why?

> With or without recent changes, it is possible that work->func() will run on
> another CPU (not that to which it was submitted) if CPU goes down. In fact,
> this can happen while work->func() is running, so even smp_processor_id()
> is not safe to use in work->func().

But the work func was scheduled by schedule_delayed_work_on(). Isnt that a 
general problem with schedule_delayed_work_on() and keventd?

> However, cache_reap() seems to wrongly assume that smp_processor_id() is stable,
> this is the second problem.
> 
> Is my understanding correct?

cache_reap assumes that the processor id is stable based on the kevent 
thread being pinned to a processor.
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