Date:	Fri, 13 Aug 2010 20:43:19 -0400
From:	James Bottomley <James.Bottomley@...e.de>
To:	Ted Ts'o <tytso@....edu>
Cc:	david@...g.hm, arjan@...radead.org, peterz@...radead.org,
	Brian Swetland <swetland@...gle.com>,
	Felipe Contreras <felipe.contreras@...il.com>,
	linux-kernel@...r.kernel.org, galibert@...ox.com,
	florian@...kler.org, tglx@...utronix.de, menage@...gle.com,
	paulmck@...ux.vnet.ibm.com, linux-pm@...ts.linux-foundation.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>, swmike@....pp.se
Subject: Re: [linux-pm] Attempted summary of suspend-blockers LKML thread,
 take three

On Fri, 2010-08-13 at 15:08 -0400, Ted Ts'o wrote: 
> On Fri, Aug 13, 2010 at 01:11:29PM -0400, James Bottomley wrote:
> > 
> > The facts are that suspend blockers identifies a race within our suspend
> > to ram system that permeates from top to bottom (that's from server to
> > mobile).  The problem is that resume events are racy with respect to
> > suspend and vice versa.  This manifests itself most annoyingly on my
> > laptop in the "double suspend" case: where I suspend with a pending
> > suspend event, my laptop will resume and then immediately re-suspend
> > (leading me to kick myself and remind myself to check it stayed up
> > before pushing unsuspend and walking away).  The other annoying case is
> > that if I accidentally close the lid before presenting, I have to wait
> > until the system is fully down before pressing resume.
> 
> This is all true, but it's also only one aspect of the problem.  I
> agree with you that this is the part of the problem which affects
> Linux at all scales, from Cloud servers in a data center that want to
> suspend themselves when there's no work to do (and then fail to
> respond to the WOL packet) to mobile platforms that are suspending
> much more frequently.
> 
> However, it doesn't follow that this is the _only_ problem that the
> Android folks might be interested in solving.  Opportunistic suspend
> is a different part of the problem space, which is generally believed
> by the Android developers as being far more efficient than a
> user-space suspend manager.  Rafael has stated his complete
> unwillingness to deal with this part of the problem.  OK, so that
> probably means that for Android, it will have to be an out-of-tree
> kernel patch.

OK, so I tried desperately to avoid the question of whether
opportunistic suspend is a good way of managing power.  However, it
seems to me that it is in use by several systems (android, olpc, etc).
I'll defer the question of whether it's better in user space or kernel
space to Rafael's investigations  ...  but I will point out that the
kernel space patch, once the suspend blockers issue is taken care of
looks like a single patch to one file, so should be locally containable
and should allow upstream to be useful as the driver base again.

> The question, then, is whether a solution which addresses the only
> part of the problem which Rafael is interested in dealing with at this
> point, is sufficient such that (a) the kernel-level opportunistic
> suspend can be done as an out-of-tree patch, while simultaneously (b)
> allowing device drivers for Android devices can utilize Rafael's
> interfaces to solve the race design bug currently found in our suspend
> subsystem, while (c) requiring minimal changes to the Android
> userspace, and (d) providing all of the statistics and debugging
> functionality required by the Android userspace.
> 
> If we can engineer a solution which meets (a), (b), (c), and (d)
> above, then everyone will be happy.

That's my goal.

James


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