linux-kernel - Re: Attempted summary of suspend-blockers LKML thread

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Date:	Mon, 2 Aug 2010 07:09:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	david@...g.hm
Cc:	Arjan van de Ven <arjan@...radead.org>, "Ted Ts'o" <tytso@....edu>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	arve@...roid.com, mjg59@...f.ucam.org, pavel@....cz,
	florian@...kler.org, rjw@...k.pl, stern@...land.harvard.edu,
	swetland@...gle.com, peterz@...radead.org, tglx@...utronix.de,
	alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

On Sun, Aug 01, 2010 at 10:06:34PM -0700, david@...g.hm wrote:
> On Sun, 1 Aug 2010, Arjan van de Ven wrote:
> 
> >I'm a little worried that this whole "I need to block suspend" is
> >temporary. Yes today there is silicon from ARM and Intel where suspend
> >is a heavy operation, yet at the same time it's not all THAT heavy
> >anymore.... at least on the Intel side it's good enough to use pretty
> >much all the time (when the screen is off for now, but that's a memory
> >controller issue more than anything else). I'm pretty sure the ARM guys
> >will not be far behind.
> 
> remember that this 'block suspend' is really 'block overriding the
> fact that there are still runable processes and suspending anyway"
> 
> having it labeled as 'suspend blocker' or even 'wakelock' makes it
> sound as if it blocks any attempt to suspend, and I'm not sure
> that's what's really intended. Itsounds like the normal syspend
> process would continue to work, just this 'ignore if these other
> apps are busy' mode of operation would not work.
> 
> which makes me wonder, would it be possible to tell the normal idle
> detection mechanism to ignore specific processes when deciding if it
> should suspend or not? how about only considering processes in one
> cgroup when deciding to suspend and ignoring all others?

Why not flesh this out and compare it to the draft requirements?
(I expect to be sending another version by end of day Pacific Time.)

The biggest issue I see right off-hand is that a straightforward
implementation of your idea would require moving processes from one
cgroup to another when acquiring or releasing a suspend blocker, which
from what I understand would be way to heavyweight.  On the other hand,
if acquiring and releasing a suspend blocker does not move the process
from one cgroup to another, then you need something very like the
suspend-blocker mechanism to handle those processes that are permitted
to acquire suspend blockers, and which are thus not a member of the
cgroup in question.

That said, I did see some hint from the Android guys that it -might-
be possible to leverage cgroups in the way that you suggest might help
save power during times when suspend was blocked but (for example) the
screen was turned off.  The idea would be to freeze the cgroup whenever
the screen blanked, even if suspend was blocked.  The biggest issue
here is that any process that can hold a suspend blocker must never to
an unconditional wait on any process in this cgroup.  Seems to me that
this should be possible in theory, but the devil would be in the details.

If I am misunderstanding your proposal, please enlighten me!

							Thanx, Paul
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