linux-kernel - Re: [linux-pm] Re: Hibernation considerations

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Date:	Fri, 20 Jul 2007 14:34:02 -0700 (PDT)
From:	david@...g.hm
To:	Alan Stern <stern@...land.harvard.edu>
cc:	Milton Miller <miltonm@....com>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Ying Huang <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	Jeremy Maitin-Shepard <jbms@....edu>
Subject: Re: [linux-pm] Re: Hibernation considerations

On Fri, 20 Jul 2007, Alan Stern wrote:

> On Fri, 20 Jul 2007 david@...g.hm wrote:
>
>>> Userspace can submit I/O requests.  Someone will have to audit every
>>> driver to make sure that such I/O requests don't cause a quiesced
>>> device to become active.  If the device is active, it will make the
>>> memory snapshot inconsistent with the on-device data.
>>
>> assuming this is the suspend-from-ram after a kexec back from the
>> write-to-disk kernel I don't think you are correct.
>>
>> when doing a suspend-to-ram you get to a point where you just don't use
>> any userspace.
>
> What do you mean?  How can you prevent user tasks from running?  That's
> basically what the freezer does, and the whole point of this approach
> is to eliminate the freezer.  Right?
>
>> from that point on you are just walking the device tree
>> putting things into low-power mode. This is the point where we are talking
>> about jumping to.
>
> Yes.  And putting things into low-power mode requires the ability to
> run the scheduler, which means that user tasks can be scheduled, which
> means that they can run.

I did not know that getting into low-power mode required scheduling.

does it require userspace?

if so this is a problem and I say punt on suspend-to-disk-and-ram until 
suspend-to-ram is working independantly ;-)

if not, then can you schedule but not consider non-kernel tasks runnable?

freezing all of userspace is easy (see above)

freezing all of kernelspace is easy (unplug all non-boot CPU's and don't 
schedule)

where freezing gets hard is when you need to partially freeze either one 
of these.

David Lang
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