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SubjectRe: [PATCH 1/8] PM: Opportunistic suspend support.
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Arve Hjønnevåg <arve@android.com> writes:

> 2010/5/25 Dmitry Torokhov <dmitry.torokhov@gmail.com>:
>> 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@gmail.com> 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.
>>
>
> How is that not polling? If the user is holding down a key, the keypad
> driver has to block suspend, and user space will try to suspend again
> and again and again...

Then the userspace suspend manager should be a little more clever
and should not blindly retry continuously.

It should be more like a governor which makes some simple decisions
based on previous events, simple heuristics, uses timeouts etc.,

Kevin

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