Re: [RFC][PATCH 0/8] PM: Implement autosleep and "wake locks"

Rafael J. Wysocki
2012-02-07T01:09:57Z

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.
>
> [3/8] - This is something we can do no problem, although completely optional
> without the autosleep feature. Rather necessary with it, though.
>
> [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.
>
> 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.
>
> [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. :-)
>
> [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.
>
> [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.
>
> [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.
>
> All in all, it's not as much code as I thought it would be and it seems to be
> relatively simple (which rises the question why the Android people didn't
> even _try_ to do something like this instead of slapping the "real" wakelocks
> onto the kernel FWIW). IMHO it doesn't add anything really new to the kernel,
> except for the user space interfaces that should be maintainable. At least I
> think I should be able to maintain them. :-)
>
> 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.
>
> IOW, I need to know your opinions!

Ouch. Sorry for breaking the Greg's address. Please replace it with the
correct one when you reply.

Thanks,
Rafael