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

Arve Hjønnevåg
2010-05-26T00:04:27Z

2010/5/25 Kevin Hilman <khilman@deeprootsystems.com>:
> 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.,
>

So instead of the kernel suspending as soon as the last driver stops
blocking suspend, you want to add heuristics in user-space to guess
when suspend will succeed. This would, in my opinion, be a much worse
solution.

--
Arve Hjønnevåg