linux-kernel - Re: [RFC] Cpu-hotplug: Using the Process Freezer (try2)

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Date:	Mon, 2 Apr 2007 18:12:00 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Gautham R Shenoy <ego@...ibm.com>, akpm@...ux-foundation.org,
	paulmck@...ibm.com, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>,
	"Rafael J. Wysocki" <rjw@...k.pl>, dipankar@...ibm.com,
	dino@...ibm.com, masami.hiramatsu.pt@...achi.com
Subject: Re: [RFC] Cpu-hotplug: Using the Process Freezer (try2)

On Mon, Apr 02, 2007 at 01:18:28PM +0200, Ingo Molnar wrote:
> > 	if (freezing(current))
> > 		freeze_process(p);	/* function exported by freezer */
> 
> yeah. (is that safe with tasklist_lock held?)

from my scan of the code, it appears to be safe ..

> i'm wondering whether we could do even better than the signal approach. 
> I _think_ the best approach would be to only wait for tasks that are _on 
> the runqueue_. I.e. any task that has scheduled away with 
> TASK_UNINTERRUPTIBLE (and might not be able to process signal events for 
> a long time) is still freezable because it scheduled away.

I am slightly uncomfortable with "not waiting for tasks inside the
kernel to get out" part, even if it that is done only for
TASK_UNINTERRUPTIBLE tasks. For ex: consider this:

flush_workqueue() <- One of biggest offenders of lock_cpu_hotplug() to date
	for_each_online_cpu(cpu)
		flush_cpu_workqueue
			TASK_UNINTERRUPTIBLE sleep

If we don't wait for this thread from being frozen "voluntarily" (because it is 
in TASK_UNINTERRUPTIBLE sleep), then flush_workqueue is clearly racy wrt
cpu hotplug.

I would imagine other situations like this are possible where "not waiting
for everyone to /voluntarily/ quiece" can break cpu hotplug. In fact,
the biggest reason why we are moving to freezer based hotplug is the
fact that it quiesces everyone, leading to (hopefully) zero race conditions.

-- 
Regards,
vatsa
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