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Subject[RFC][PATCH 0/8] PM: Implement autosleep and "wake locks", take 3
Hi all,

Following is the third update of the autosleep patchset.

Patches [1-4/8] are regarded as v3.5 material, the rest - depending on
the feedback I get (lack of feedback will be understood as no objections,

On Wednesday, February 22, 2012, Rafael J. Wysocki wrote:
> Hi all,
> After the feedback so far I've decided to follow up with a refreshed patchset.
> The first two patches from the previous one went to linux-pm/linux-next
> and I included the recent evdev patch from Arve (with some modifications)
> to this patchset for completness.
> On Tuesday, February 07, 2012, Rafael J. Wysocki wrote:
> > Hi all,
> >
> > This series tests the theory that the easiest way to sell a once rejected
> > feature is to advertise it under a different name.
> >
> > Well, there actually are two different features, although they are closely
> > related to each other. First, patch [6/8] introduces a feature that allows
> > the kernel to trigger system suspend (or more generally a transition into
> > a sleep state) whenever there are no active wakeup sources (no, they aren't
> > called wakelocks). It is called "autosleep" here, but it was called a few
> > different names in the past ("opportunistic suspend" was probably the most
> > popular one). Second, patch [8/8] introduces "wake locks" that are,
> > essentially, wakeup sources which may be created and manipulated by user
> > space. Using them user space may control the autosleep feature introduced
> > earlier.
> >
> > This also is a kind of a proof of concept for the people who wanted me to
> > show a kernel-based implementation of automatic suspend, so there you go.
> > Please note, however, that it is done so that the user space "wake locks"
> > interface is compatible with Android in support of its user space. I don't
> > really like this interface, but since the Android's user space seems to rely
> > on it, I'm fine with using it as is. YMMV.
> >
> > Let me say a few words about every patch in the series individually.
> >
> > [1/8] - This really is a bug fix, so it's v3.4 material. Nobody has stepped
> > on this bug so far, but it should be fixed anyway.
> >
> > [2/8] - This is a freezer cleanup, worth doing anyway IMO, so v3.4 material too.

The two patches above have been merged.

> The above two are in linux-pm/linux-next now. There are a few more fixes
> related to wakeup sources in there and the patches below are based on that
> branch.
> > [3/8] - This is something we can do no problem, although completely optional
> > without the autosleep feature. Rather necessary with it, though.
> Now [1/7] - Look for wakeup events in later stages of device suspend.

[1/8] now - Look for wakeup events later down the suspend code path.

> > [4/8] - This kind of reintroduces my original idea of using a wait queue for
> > waiting until there are no wakeup events in progress. Alan convinced me that
> > it would be better to poll the counter to prevent wakeup_source_deactivate()
> > from having to call wake_up_all() occasionally (that may be costly in fast
> > paths), but then quite some people told me that the wait queue migh be
> > better. I think that the polling will make much less sense with autosleep
> > and user space "wake locks". Anyway, [4/8] is something we can do without
> > those things too.
> Now [2/7] - Use wait queue to signal "no wakeup events in progress"
> With a couple of improvements suggested by Neil.

[2/8] now - Use wait queue to signal "no wakeup events in progress" condition.

> > The patches above were given Sign-off-by tags, because I think they make some
> > sense regardless of the features introcuded by the remaining patches that in
> > turn are total RFC.
> This time all of the patches are signed-off and include the requisite
> documentation changes (hopefully, I haven't forgotten about anything).
> > [5/8] - This changes wakeup source statistics so that they are more similar to
> > the statistics collected for wakelocks on Android. The file those statistics
> > may be read from is still located in debugfs, though (I don't think it
> > belongs to proc and its name is different from the analogous Android's file
> > name anyway). It could be done without autosleep, but then it would be a bit
> > pointless. BTW, this changes interfaces that _in_ _theory_ may be used by
> > someone, but I'm not aware of anyone using them. If you are one, I'll be
> > pleased to learn about that, so please tell me who you are. :-)
> Now [3/7] - Change wakeup source statistics to follow Android.
> Rebased and reworked in accordance with the Arve's feedback.

[3/8] now - Change wakeup source statistics to follow Android.

[4/8] - Add tracepoints to wakeup_source_{de}activate()

[5/8] - Teach epoll to use wakeup sources if requested

This should be sufficient to ensure that a wakeup source will be kept active
after a wakeup event all the way up to user space without a need to make a
number of random drivers use wakeup sources.

> > [6/8] - Autosleep implementation. I think the changelog explains the idea
> > quite well and the code is really nothing special. It doesn't really add
> > anything new to the kernel in terms of infrastructure etc., it just uses
> > the existing stuff to implement an alternative method of triggering system
> > sleep transitions. Note, though, that the interface here is different
> > from the Android's one, because Android actually modifies /sys/power/state
> > to trigger something called "early suspend" (that is never going to be
> > implemented in the "stock" kernel as long as I have any influence on it) and
> > we simply can't do that in the mainline.
> Now [5/7] - Implement opportunistic sleep
> Rebased and simplified (most notably, I've dropped the "main" wakeup source,
> since it wasn't really necessary).

[6/8] now - Implement apportunistic sleep.

> > [7/8] - This adds a wakeup source statistics that only makes sense with
> > autosleep and (I believe) is analogous to the Android's prevent_suspend_time
> > statistics. Nothing really special, but I didn't want
> > wakeup_source_activate/deactivate() to take a common lock to avoid
> > congestion.
> Now [6/7] - Add "prevent autosleep time" statistics to wakeup sources.
> Rebased.

[7/8] now - Add "prevent autosleep time" statistics to wakeup sources.

> > [8/8] - This adds a user space interface to create, activate and deactivate
> > wakeup sources. Since the files it consists of are called wake_lock and
> > wake_unlock, to follow Android, the objects the wakeup sources are wrapped
> > into are called "wakelocks" (for added confusion). Since the interface
> > doesn't provide any means to destroy those "wakelocks", I added a garbage
> > collection mechanism to get rid of the unused ones, if any. I also tought
> > it might be a good idea to put a limit on the number of those things that
> > user space can operate simultaneously, so I did that too.
> Now [7/7] - Add user space interface for manipulating wakeup sources.

[8/8] now - Add user space interface for manipulating wakeup sources.

> > All of the above has been tested very briefly on my test-bed Mackerel board
> > and it quite obviously requires more thorough testing, but first I need to know
> > if it makes sense to spend any more time on it.
> The above is still accurate, but I also verified that the patches don't break
> my PC test boxes (at least as long as the new features aren't used ;-)).

Nothing has changed in that respect, as far as I can say.

The patches in the following series are available from the autosleep branch in
the linux-pm tree.


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