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

[Dmitry Torokhov]

On Tue, May 25, 2010 at 03:23:23PM -0700, Arve Hjønnevåg wrote:
> On Tue, May 25, 2010 at 11:47 AM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> > On Tue, May 25, 2010 at 02:35:17PM -0400, Alan Stern wrote:
> >> On Tue, 25 May 2010, Dmitry Torokhov wrote:
> >>
> >> > > Here's the scenario:
> >> > >
> >> > > The system is awake, and the user presses a key. The keyboard driver
> >> > > processes the keystroke and puts it in an input queue.  A user process
> >> > > reads it from the event queue, thereby emptying the queue.
> >> > >
> >> > > At that moment, the system decides to go into opportunistic suspend.
> >> > > Since the input queue is empty, there's nothing to stop it.  As the
> >> > > first step, userspace is frozen -- before the process has a chance to
> >> > > do anything with the keystroke it just read.  As a result, the system
> >> > > stays asleep until something else wakes it up, even though the
> >> > > keystroke was important and should have prevented it from sleeping.
> >> > >
> >> > > Suspend blockers protect against this scenario.  Here's how:
> >> > >
> >> > > The user process doesn't read the input queue directly; instead it
> >> > > does a select or poll.  When it sees there is data in the queue, it
> >> > > first acquires a suspend blocker and then reads the data.
> >> > >
> >> > > Now the system _can't_ go into opportunistic suspend, because a suspend
> >> > > blocker is active.  The user process can do whatever it wants with the
> >> > > keystroke.  When it is finished, it releases the suspend blocker and
> >> > > loops back to the select/poll call.
> >> > >
> >> >
> >> > What you describe can be done in userspace though, via a "suspend manager"
> >> > process. Tasks reading input events will post "busy" events to stop the
> >> > manager process from sending system into suspend. But this can be confined to
> >> > Android userspace, leaving the kernel as is (well, kernel needs to be modified
> >> > to not go into suspend with full queues, but that is using existing kernel
> >> > APIs).
> >>
> >> I think that could be made to work.  And it might remove the need for
> >> the userspace suspend-blocker API, which would be an advantage.  It
> >> could even remove the need for the opportunistic-suspend workqueue --
> >> opportunistic suspends would be initiated by the "suspend manager"
> >> process instead of by the kernel.
> >>
> >> However you still have the issue of modifying the kernel drivers to
> >> disallow opportunistic suspend if their queues are non-empty.  Doing
> >> that is more or less equivalent to implementing kernel-level suspend
> >> blockers.  (The suspend blocker approach is slightly more efficient,
> >> because it will prevent a suspend from starting if a queue is
> >> non-empty, instead of allowing the suspend to start and then aborting
> >> it partway through.)
> >>
> >> Maybe I'm missing something here...  No doubt someone will point it out
> >> if I am.
> >>
> >
> > Well, from my perspective that would limit changes to the evdev driver
> > (well, limited input core plumbing will be needed) but that is using the
> > current PM infrastructure. The HW driver changes will be limited to what
> > you described "type 2" in your other e-mail.
> >
> > Also, not suspending while events are in progress) is probably
> > beneficial for platforms other than Android as well. So unless I am
> > missing something this sounds like a win.
> >
> 
> How would this limit the changes you need in the evdev driver? It need
> to block suspend when there are unprocessed events in some queues.
> Suspend blockers gives you an api to do this, without it, you check
> the queues in your suspend hook and abort suspend if they are not
> empty. Without suspend blockers you have no api to signal that it is
> OK to suspend again, so you are forcing the thread that tried to
> suspend to poll until you stop aborting suspend.

No, you do not need to poll. You just set a timeout (short or long,
depending on your needs) and if no userspace task blocked suspend
durng that time you attempt to initiate suspend from your manager
process. If it succeeds - good, if not that means that more events came
your way and you have to do it later.

-- 
Dmitry
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