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

> 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...