Re: Attempted summary of suspend-blockers LKML thread

Paul E. McKenney
2010-08-04T21:15:21Z

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
completes.

But we do need to hear from the Android guys on these points.

Thanx, Paul