Message-ID: <AANLkTinnSH_egFYowkOQiYHL87QfIH332i1__OaTem-s@mail.gmail.com>
Date:	Tue, 25 May 2010 15:37:48 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Len Brown <len.brown@...el.com>,
	Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Ming Lei <tom.leiming@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.

2010/5/25 Dmitry Torokhov <dmitry.torokhov@...il.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@...il.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...

-- 
Arve Hjønnevåg
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