Re: [PATCH 1/8] PM: Opportunistic suspend support.

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Wednesday 26 May 2010, Matthew Garrett wrote:
> On Wed, May 26, 2010 at 02:57:45PM +0200, Peter Zijlstra wrote:
> 
> > I fail to see why. In both cases the woken userspace will contact a
> > central governing task, either the kernel or the userspace suspend
> > manager, and inform it there is work to be done, and please don't
> > suspend now.
> 
> Thinking about this, you're right - we don't have to wait, but that does 
> result in another problem. Imagine we get two wakeup events 
> approximately simultaneously. In the kernel-level universe the kernel 
> knows when both have been handled. In the user-level universe, we may 
> have one task schedule, bump the count, handle the event, drop the count 
> and then we attempt a suspend again because the second event handler 
> hasn't had an opportunity to run yet. We'll then attempt a suspend and 
> immediately bounce back up. That's kind of wasteful, although it'd be 
> somewhat mitigated by checking that right at the top of suspend entry 
> and returning -EAGAIN or similar.

I still think it would cause a loop-alike behavior between the user space
power manager and the kernel PM core to happen, because the power manager
will always have to check the user space counter after a failing suspend
attempt.

Rafael