Message-Id: <201202070213.42970.rjw@sisk.pl>
Date:	Tue, 7 Feb 2012 02:13:42 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linux PM list <linux-pm@...r.kernel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>, markgross@...gnar.org,
	Matthew Garrett <mjg@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Brian Swetland <swetland@...gle.com>,
	Neil Brown <neilb@...e.de>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 0/8] PM: Implement autosleep and "wake locks"

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
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