linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6)

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Date:	Thu, 13 May 2010 17:37:03 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tony Lindgren <tony@...mide.com>
cc:	Paul Walmsley <paul@...an.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Kevin Hilman <khilman@...prootsystems.com>,
	<magnus.damm@...il.com>, Theodore Ts'o <tytso@....edu>,
	mark gross <mgross@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Geoff Smith <geoffx.smith@...el.com>,
	Brian Swetland <swetland@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg@...hat.com>,
	Benoît Cousson <b-cousson@...com>,
	<linux-omap@...r.kernel.org>, Vitaly Wool <vitalywool@...il.com>,
	Linus Walleij <linus.walleij@...csson.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Liam Girdwood <lrg@...mlogic.co.uk>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6)

On Thu, 13 May 2010, Tony Lindgren wrote:

> * Alan Stern <stern@...land.harvard.edu> [100513 07:11]:
> > On Wed, 12 May 2010, Paul Walmsley wrote:
> > 
> > > Hello,
> > > 
> > > Some general comments on the suspend blockers/wakelock/opportunistic
> > > suspend v6 patch series, posted here:
> > > 
> > >     https://lists.linux-foundation.org/pipermail/linux-pm/2010-April/025146.html
> > > 
> > > The comments below are somewhat telegraphic in the interests of 
> > > readability - more specific comments to follow in later E-mails. I am 
> > > indebted to those of us who discussed these issues at LPC last year and 
> > > ELC this year for several stimulating discussions.
> > > 
> > > There are several general problems with the design of opportunistic
> > > suspend and suspend-blocks.
> > > 
> > > 1. The opportunistic suspend code bypasses existing Linux kernel code,
> > >    such as timers and the scheduler, that indicates when code
> > >    needs to run, and when the system is idle.
> > 
> > Whoa!  That's not my understanding at all.
> > 
> > As I see it, opportunistic suspend doesn't bypass any code that isn't 
> > already bypassed by the existing suspend code.  Users can do
> > 
> > 	echo mem >/sys/power/state
> > 
> > whenever they want, without regard to kernel timers and the scheduler 
> > (other than the fact that the user's thread must be running in order to 
> > carry out the write, of course).
> 
> The difference between echo mem > /sys/power/state and suspend blocks
> is that with suspend blocks the system keeps running.

Irrelevant.  Paul wasn't talking about the suspend blockers; he was 
talking about opportunistic suspend.  So what's the difference between 
opportunistic suspend and "echo mem >/sys/power/state"?  Especially 
when suspend blockers aren't being used?

> And that's why 
> it should be handled by runtime power management instead.

Runtime PM is not capable of freezing userspace and shutting down CPUs.  
More or less by definition -- if it could then it wouldn't be "runtime" 
any more, since the processor wouldn't be running.

> The suspend blocks seems like a hack to spam filter good and bad
> apps from timer usage point of view. Applications are categorized
> as good or bad depending if they grab a susped blocker or not.

You're referring just to the userspace part of the suspend blocker 
API.  What about the kernel part?

> I believe categorizing the apps should be instead done with some
> timer flags or cgroups instead.

That would be compatible with using suspend blockers.

Alan Stern

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