Date: Mon, 2 Aug 2010 15:27:52 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Arjan van de Ven <arjan@infradead.org>,
        linux-pm@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        arve@android.com, mjg59@srcf.ucam.org, pavel@ucw.cz,
        florian@mickler.org, stern@rowland.harvard.edu, swetland@google.com,
        peterz@infradead.org, tglx@linutronix.de, alan@lxorguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread
Message-ID: <20100802222752.GI2405@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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On Mon, Aug 02, 2010 at 11:33:32PM +0200, Rafael J. Wysocki wrote:
> On Monday, August 02, 2010, Paul E. McKenney wrote:
> > On Mon, Aug 02, 2010 at 03:52:20PM +0200, Rafael J. Wysocki wrote:
> > > On Monday, August 02, 2010, Paul E. McKenney wrote:
> > > > On Sun, Aug 01, 2010 at 03:47:08PM -0700, Arjan van de Ven wrote:
> > > > > On Sun, 1 Aug 2010 12:12:28 -0700
> > > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > > ...
> > > > > Another one: freezing whole cgroups..... we have that today. it
> > > > > actually works quite well.... of course the hard part is the decision
> > > > > what to put in which cgroup, and at what frequency and duration you let
> > > > > cgroups run.
> > > > 
> > > > Indeed, the Android guys seemed to be quite excited by cgroup freezing
> > > > until they thought about the application-classification problem.
> > > > Seems like it should be easy for some types of applications, but I do
> > > > admit that apps can have non-trivial and non-obvious dependencies.
> > > 
> > > This isn't more difficult than deciding which applications will be allowed to
> > > use wakelocks (in the wakelocks world).  It actually seems to be pretty much
> > > equivalent to me. :-)
> > 
> > If I understand correctly, the problem they were concerned about was
> > instead "given that a certain set of apps are permitted to use wakelocks,
> > which of the other apps can safely be frozen when the display blanks
> > itself."
> 
> I _think_ the problem should be reformulated as "which of the other apps
> can be safely frozen without causing the wakelocks-using ones to have
> problems" instead (the particular scenario is that one of the wakelocks-using
> apps may need one of the other apps to process something and therefore the
> other app cannot be frozen; however, that may be resolved by thawing all of
> the other apps in such situations IMO).

I agree that your statement is equivalent to mine.  From what I can see,
the current Android code resolves this by not freezing any app while
a wakelock is held.

Just out of curiosity, how are you detecting the situation in order to
decide when to thaw the apps in the cgroup?

> > > > > on the suspend blockers for drivers; the linux device runtime PM is
> > > > > effectively doing the same things; it allows drivers to suspend/resume
> > > > > individually (with a very nice API/programming model I should say) based
> > > > > on usage. And it works on a tree level, so that it's relatively easy
> > > > > to do things like "I want to go to <this magic deep idle state>, but
> > > > > only if <this set of devices is suspended already>". This is obviously
> > > > > an important functionality for all low power devices, ARM or x86. 
> > > > > Suspend blockers had this functionality as part of what it did (they do
> > > > > more obviously) but I'd wager that the current Linux infrastructure is
> > > > > outright nicer. 
> > > > 
> > > > This is what Rafael has been working on?
> > > 
> > > If you mean the runtime PM framework, then yes, I've been working on it.
> > > 
> > > > Of course, the Android guys also want to pay attention to which apps
> > > > are running as well as to the state of devices on the system.
> > > 
> > > In fact the runtime PM framework is also important to Android, because it
> > > can be used in there, for example, to implement the "early suspend" thing
> > > I referred to in one of my previous messages in this thread.
> > 
> > Now we just need to convince the Android guys of that.  ;-)
> 
> I believe there's no need for that, as we were talking about that a few months
> ago.

Here is hoping...

							Thanx, Paul
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