Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Wed Aug 04 2010 - 19:20:01 EST

On Wed, 4 Aug 2010, Paul E. McKenney wrote:

On Wed, Aug 04, 2010 at 03:56:42PM -0700, david@xxxxxxx wrote:
On Wed, 4 Aug 2010, Arve Hj?nnev?g wrote:

On Wed, Aug 4, 2010 at 3:31 PM, <david@xxxxxxx> wrote:
On Wed, 4 Aug 2010, 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
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 proposal that I nade was not to use cgroups to freeze some processes and
not others, but to use cgroups to decide to ignore some processes when
deciding if the system is idle, stop everything or nothing. cgroups are just
a way of easily grouping processes (and their children) into different

That does not avoid the dependency problem. A process may be waiting
on a resource that a process you ignore owns. I you ignore the process
that owns the resource and enter idle when it is ready to run (or
waiting on a timer), you are still effectively blocking the other

and if you don't have a wakelock the same thing will happen. If you
expect the process to take a while you can set a timeout to wake up
every 30 seconds or so and wait again, this would be enough to
prevent you from going to sleep (or am I misunderstanding how long
before you go into suspend without a wakelock set, see my other
e-mail for the full question)

The difference between the Android scheme and your proposal is that the
Android scheme freezes -all- the processes, not just a subset of them.
Therefore, in the Android scheme, the case of one process attempting to
acquire a resource held by a frozen process. In contrast, any scheme
that attempts to freeze only a subset of the processes must somehow
either avoid or properly handle the situation where a frozen process is
holding a resource that a running process is trying to acquire.

My proposal would never freeze a subset of processes.

what my proposal:

only consider the activity of a subset of processes when deciding if we should suspend or not. If the decision is to suspend, freeze everything.

you (and many other people) are confusing what I've proposed (use cgroups to indicate what processes to care about and what ones to not care about when deciding to suspend/go to idle) with the prior cgroup proposal (use cgroups to freeze a subset of tasks while leaving others runnable)

David Lang
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