linux-kernel - Re: suspend blockers & Android integration

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Date:	Fri, 4 Jun 2010 22:35:06 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, tytso@....edu,
	Brian Swetland <swetland@...gle.com>,
	Neil Brown <neilb@...e.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	James Bottomley <James.Bottomley@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kevin Hilman <khilman@...prootsystems.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: suspend blockers & Android integration

2010/6/4 Matt Helsley <matthltc@...ibm.com>:
> On Fri, Jun 04, 2010 at 05:39:17PM -0700, Arve Hjønnevåg wrote:
>> On Fri, Jun 4, 2010 at 5:05 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> > On Sat, 5 Jun 2010, Rafael J. Wysocki wrote:
>
> <snip>
>
>>
>> >     With the cgroup freezer you can "suspend" them right away and
>> >     just keep the trusted background task(s) alive which allows us to
>> >     go into deeper idle states instead of letting the crapplications
>> >     run unconfined until the download finished and the suspend
>> >     blocker goes away.
>> >
>>
>> Yes this would be better, but I want it in addition to suspend, not
>> instead of it. It is also unclear if our user-space code could easily
>> make use of it since our trusted code calls into untrusted code.
>>
>
> Perhaps I'm misunderstanding, but suspend and the cgroup freezer
> interoperate well today -- you don't have to choose one or the other.
> If you've discovered otherwise I'd consider it a bug and would like to
> hear more about it.
>

I'm not aware of any bug with combining both, but we cannot use
suspend at all without suspend blockers in the kernel (since wakeup
events may be ignored) and I don't know how we can safely freeze
cgroups without funneling all potential wakeup events through a
process that never gets frozen.

> <snip>
>
>> it can handle bad apps better (assuming you don't combine
>> opportunistic suspend and cgroup freezing).
>
> I don't see why that would be a problem. The cgroup freezer works
> independently of the suspend freezer -- even with suspend blockers.
> So my hunch is this is really the same as the next problem you refer to:
>
>> The biggest hurdle is how
>> to handle dependencies between processes that gets frozen and
>> processes that don't get frozen.
>
> I'm not sure it covers everything you want, but it should be possible to
> identify some of those so long as you know which process you're
> communicating with.
>
> A trusted app can look up the freezer cgroup of a target app in /proc, then
> look at the cgroup's freezer.state file. If it's FREEZING or FROZEN then
> you've very likely got a "bad" dependency.
>

I don't think they are "bad" dependencies. Our framework has to
communicate with apps.

> For example, say a trusted app plans on doing a blocking read() to fetch
> the output of an untrusted app via a pipe. Assuming we know the untrusted
> app's pid we could then check the dependency and determine that we're likely
> to block because the untrusted app's freezer cgroup is FREEZING or FROZEN.
> (certain to block if we see FROZEN)
>
> That said, it involves quite a few system calls compared to a simple read()
> from the pipe. So my guess is it would be a debugging tool at best -- not
> something you always have enabled.
>
> It may even be possible to make an lsof-like debugging tool to do that from
> outside both apps.
>
> Cheers,
>        -Matt Helsley
>

-- 
Arve Hjønnevåg
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