linux-kernel - Re: [RFC][PATCH][EXPERIMENTAL] Make kernel threads nonfreezable by default

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Message-Id: <200705281009.06685.rjw@sisk.pl>
Date:	Mon, 28 May 2007 10:09:05 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz>,
	Maximilian Engelhardt <maxi@...monizer.de>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [RFC][PATCH][EXPERIMENTAL] Make kernel threads nonfreezable by default

On Sunday, 27 May 2007 00:12, Rafael J. Wysocki wrote:
> Hi,
> 
> Following the "Freezing of kernel threads" discussion
> (http://lkml.org/lkml/2007/5/12/162) I have created a patch that changes the
> freezer's behavior with respect to kernel threads.  Namely, with the patch
> applied all kernel threads are nonfreezable by default (have PF_NOFREEZE set)
> and the ones that want to be frozen need to call set_freezable() (which clears
> PF_NOFREEZE for them) and try_to_freeze().  The other (nonfreezable) kernel
> threads don't need to call try_to_freeze() any more.
> 
> I have removed try_to_freeze() from a handful of kernel threads that I think
> need not be freezable, but in many cases I wasn't quite sure whether or not
> it was a good idea to change the current behavior.  For this reason I've added
> set_freezable() to the majority of (currently freezable) kernel threads that
> belong to device drivers and filesystems.
> 
> Of course, I have removed the setting of PF_NOFREEZE from the kernel threads
> that are currently nonfreezable, since with the other changes in the patch it
> isn't necessary any more.
> 
> This patch is on top of the "Freezer: Avoid freezing kernel threads prematurely"
> patch that I posted yestarday, available at http://lkml.org/lkml/2007/5/25/199
> (updated version that applies cleanly on top of 2.6.22-rc3, is available at
> http://www.sisk.pl/kernel/hibernation_and_suspend/2.6.22-rc3/patches/07-freezer-avoid-freezing-kernel-threads-prematurely.patch).
> It has been tested on a couple of machines and doesn't seem to break anything.
> 
> [As you can see there are quite a lot of files affected, so I didn't add all
> maintainers of them to the CC list.  In fact, I'm not sure how to handle
> notifying them of the change, so please advise.]

Does the lack of comments mean that everyone on the CC list agrees with this
approach? ;-)

In the meantime, it turns out that this patch fixes the hibernation/suspend
problem with cryptd discussed in the thread at
http://lkml.org/lkml/2007/5/26/24 .

The problem is that cryptd doesn't call try_to_freeze() and doesn't set
PF_NOFREEZE for itself, so the freezer cannot handle it properly.  In principle
we can add either try_to_freeze() or the setting of PF_NOFREEZE to it, but if
the approach in the $subject patch is acceptable, we'll need to remove that
soon.  So, what should we do?

Rafael

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