linux-kernel - Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

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Date:	Sun, 18 Feb 2007 14:31:24 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
	vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>
Subject: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

On 02/18, Rafael J. Wysocki wrote:
>
> On Sunday, 18 February 2007 00:47, Oleg Nesterov wrote:
> > 
> > However, this means that sys_vfork() makes impossible to freeze processes
> > until child exits/execs. Not good.
> 
> Yes, but this also is the current behavior.

Yes, yes, I see.

I forgot to say that we have another problem: coredumping.

A thread which does do_coredump() send SIGKILL to ->mm users, and sleeps
on ->mm->core_startup_done. Now it can't be frozen if sub-thread goes to
refrigerator. I think this could be solved easily if we add a check to
refrigerator() as you suggested for ->vfork_donw.

> I think the real solution would be to use an interruptible completion in the
> vfork code.  It was discussed some time ago and, IIRC, Ingo had an experimental
> patch that implemented it.  Still, for the suspend this really is not an issue
> in practice, so it wasn't merged.

It is not (afaics) so trivial to do rightly, and with this change the parent
will be seen as TASK_INTERRUPTIBLE even without freezer in progress.

A very vague idea: what if parent will do

	current->flags |= PF_PLEASE_CONSIDER_ME_AS_FROZEN_BUT_SET_TIF_FREEZE
	wait_for_completion(&vfork);
	try_to_freeze();

?

> It may be a good time to solve this problem now. :-)

Yes, I think so :)

Oleg.

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