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Message-Id: <200707180930.45564.rjw@sisk.pl> Date: Wed, 18 Jul 2007 09:30:44 +0200 From: "Rafael J. Wysocki" <rjw@...k.pl> To: Andrew Morton <akpm@...ux-foundation.org> Cc: LKML <linux-kernel@...r.kernel.org>, Nigel Cunningham <nigel@...el.suspend2.net>, Oleg Nesterov <oleg@...sign.ru>, Pavel Machek <pavel@....cz> Subject: [PATCH -mm 1/5] Freezer: Document relationship with memory shrinking From: Rafael J. Wysocki <rjw@...k.pl> One important reason to freeze tasks, which is that we don't want them to allocate memory after freeing it for the hibernation image, has not been documented. Fix it. Signed-off-by: Rafael J. Wysocki <rjw@...k.pl> Acked-by: Pavel Machek <pavel@....cz> --- Documentation/power/freezing-of-tasks.txt | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) Index: linux-2.6.22-rc6-mm1/Documentation/power/freezing-of-tasks.txt =================================================================== --- linux-2.6.22-rc6-mm1.orig/Documentation/power/freezing-of-tasks.txt 2007-07-11 20:48:04.000000000 +0200 +++ linux-2.6.22-rc6-mm1/Documentation/power/freezing-of-tasks.txt 2007-07-11 20:50:24.000000000 +0200 @@ -81,7 +81,16 @@ hibernation image has been created and b The majority of these are user space processes, but if any of the kernel threads may cause something like this to happen, they have to be freezable. -2. The second reason is to prevent user space processes and some kernel threads +2. Next, to create the hibernation image we need to free a sufficient amount of +memory (approximately 50% of available RAM) and we need to do that before +devices are deactivated, because we generally need them for swapping out. Then, +after the memory for the image has been freed, we don't want tasks to allocate +additional memory and we prevent them from doing that by freezing them earlier. +[Of course, this also means that device drivers should not allocate substantial +amounts of memory from their .suspend() callbacks before hibernation, but this +is e separate issue.] + +3. The third reason is to prevent user space processes and some kernel threads from interfering with the suspending and resuming of devices. A user space process running on a second CPU while we are suspending devices may, for example, be troublesome and without the freezing of tasks we would need some @@ -111,7 +120,7 @@ frozen before the driver's .suspend() ca thawed after the driver's .resume() callback has run, so it won't be accessing the device while it's suspended. -3. Another reason for freezing tasks is to prevent user space processes from +4. Another reason for freezing tasks is to prevent user space processes from realizing that hibernation (or suspend) operation takes place. Ideally, user space processes should not notice that such a system-wide operation has occurred and should continue running without any problems after the restore (or resume - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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