Re: Attempted summary of suspend-blockers LKML thread
From: Paul E. McKenney
Date: Wed Aug 04 2010 - 17:15:41 EST
On Wed, Aug 04, 2010 at 09:56:55PM +0100, Matthew Garrett wrote:
> On Wed, Aug 04, 2010 at 10:51:07PM +0200, Rafael J. Wysocki wrote:
> > On Wednesday, August 04, 2010, Matthew Garrett wrote:
> > > No! And that's precisely the issue. Android's existing behaviour could
> > > be entirely implemented in the form of binary that manually triggers
> > > suspend when (a) the screen is off and (b) no userspace applications
> > > have indicated that the system shouldn't sleep, except for the wakeup
> > > event race. Imagine the following:
> > >
> > > 1) The policy timeout is about to expire. No applications are holding
> > > wakelocks. The system will suspend providing nothing takes a wakelock.
> > > 2) A network packet arrives indicating an incoming SIP call
> > > 3) The VOIP application takes a wakelock and prevents the phone from
> > > suspending while the call is in progress
> > >
> > > What stops the system going to sleep between (2) and (3)? cgroups don't,
> > > because the voip app is an otherwise untrusted application that you've
> > > just told the scheduler to ignore.
> > I _think_ you can use the just-merged /sys/power/wakeup_count mechanism to
> > avoid the race (if pm_wakeup_event() is called at 2)).
> Yes, I think that solves the problem. The only question then is whether
> it's preferable to use cgroups or suspend fully, which is pretty much up
> to the implementation. In other words, is there a reason we're still
> having this conversation? :) It'd be good to have some feedback from
> Google as to whether this satisfies their functional requirements.
The issue with cgroup freezer as currently defined is that it can freeze
processes that hold user-level resources (pthread mutexes, SysV semas,
...). If some non-frozen process attempts to acquire that resource, you
get a hang. There might be some ways to work around this, for example,
Arjan suggests momentarily unfreezing periodically, and I suggested doing
the freeze in user-space code, but we don't know if either of these will
really do what is required.
Also, I believe that Android's use of cgroups would be in addition to
suspending fully rather than instead of. Freezing a subset of the
applications allows cutting power drain from output-only apps when
the screen blanks but where some app such as a download needs to keep
the system active. They still would need to suspend once the download
But we do need to hear from the Android guys on these points.
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