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

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Message-ID: <20070402092818.GE2456@in.ibm.com>
Date:	Mon, 2 Apr 2007 14:58:18 +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 08:16:12AM +0200, Ingo Molnar wrote:
> hm, shouldnt the make be frozen immediately?
> 
> doesnt the 'please freeze ASAP' flag get propagated to all tasks, 
> immediately? After that point any cloning activity should duplicate that 
> flag too, resulting in any new child freezing immediately too.

afaics, setting the 'please freeze asap' flag is racy wrt dup_task_struct
(where the child's tsk->thread_info->flags are copied from its parent?). 
Secondly, from what I understand, it takes a 'flag to be set + signal marked
pending' for the child task to be frozen. If that is the case, then
copy_process may not propogae the signal to the child, which could mean
mean that we can be in a catch-up game in freeze_processes, trying to
freeze processes we didnt see in earlier passes.

I think copy_process() can check for something like this:

	write_lock_irq(&tasklist_lock);

	...

	if (freezing(current))
		freeze_process(p);	/* function exported by freezer */

	...

	write_unlock_irq(&tasklist_lock);


-- 
Regards,
vatsa
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