Re: Attempted summary of suspend-blockers LKML thread, take three

Arve Hjønnevåg
2010-08-13T04:37:21Z

On Thu, Aug 12, 2010 at 8:28 PM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> On Thursday, August 12, 2010, Jesse Barnes wrote:
>> On Thu, 12 Aug 2010 12:19:34 -0700
>> Brian Swetland <swetland@google.com> wrote:
>> > Question though -- has every feature ever added to the kernel been a
>> > feature that there's pre-existing usage of?  Seems like a chicken and
>> > egg problem.  Also, some people seem to think there's value in being
>> > able to build kernels "out of the box" that work with the Android
>> > userspace -- given that there are a few devices out there that have
>> > that userspace on 'em.
>>
>> We generally try to merge new features like this along with code that
>> uses said feature, but there are always exceptions.  We've merged code
>> one release or more before the new code gets used for example, which is
>> fine IMO.  What we don't want to see is some new drop of code added and
>> abandoned, but you already knew that.
>>
>> At any rate, if Felipe is the only one arguing against including
>> suspend blockers in the kernel, you're probably in good shape.  Based
>> on my (rather cursory I admit) evaluation of this thread, it seems like
>> reasonable people agree that there's a place for a suspend blocker like
>> API in the kernel, and that dynamic power management is also highly
>> desirable.  So where's the git pull request already? :)
>
> In fact my patch going in that direction has been merged already and that
> code will likely be extended to cover some needs and cases I didn't have in
> mind when I was preparing it.
>
> However, having discussed the whole issue for many times and reconsidered it
> thoroughly, I think that it's inappropriate to identify the suspend blockers
> (or wakelocks) framework with the opportunistic suspend feature as proposed in
> the original submission of the "suspend blockers" patchset.  IMO they really
> are not the same thing and while the suspend blockers framework is used by
> Android to implement opportunistic suspend, I don't really believe this is the
> right approach.
>

Can you clarify this? Do you not believe using opportunistic suspend
is the right approach, or do you not believe linking suspend blockers
with opportunistic suspend is the right approach?

> We really need something similar to suspend blockers to avoid races between
> a suspend process and wakeup events, but it isn't necessary to provide user
> space with an interface allowing it to use these things directly.  Such an
> interface is only necessary in the specific implementation in which the system
> is suspended as soon as the number of "active" suspend blockers goes down to
> zero.

I don't think what you are saying here is correct. When you decide to
suspend has no impact on whether a user space interface to block
suspend is needed. The last suspend blocker patchset had this
interface as a separate patch and the reasons for providing it have
not changed with your interface. Android need the user space interface
because low level services that handle wakeup events are started
before the user space power manager. The other reason to have this
interface in the mainline kernel is to provide a safe way to handle
wakeup events on linux regardless of which user space power manager is
used on the system. For instance some devices have a user space
battery monitor, and there would be no need for this code to be
android specific if the kernel provided all the functionality it
needs.

>  Arguably, though, this isn't the only possible way to implement a
> mechanism allowing the system to be suspended automatically when it appears
> to be inactive.
>
> Namely, one can use a user space power manager for this purpose and actually
> the OLPC project has been doing that successfully for some time, which clearly
> demonstrates that the Android approach to this problem is not the only one
> possible.  Moreover, the kernel's system suspend (or hibernate for that matter)
> code has not been designed to be started from within the kernel.  It's been
> designed to allow a privileged user space process to request the kernel to
> put the system into a sleep state at any given time regardless of what the
> other user space processes are doing.  While it can be started from within the
> kernel, this isn't particularly nice and, in the Android case, starting it from
> within the kernel requires permission from multiple user space processes
> (given by not taking suspend blockers these processes are allowed to use).
>

Why is starting suspend from within the kernel not nice? Personally I
think reentering suspend from within the kernel is nicer than being
forced to wake up a user space thread for events that are fully
handled within the kernel (for instance the battery monitor on the
Nexus One).

> Since, quite clearly, user space input is necessary to make the decision
> whether or not to suspend the system, I think it is more appropriate to allow
> user space to start the entire operation and provide the kernel with a means
> to abort it in the case of a wakeup event.  Then, user space will be able to
> use arbitrary heuristics in deciding whether or not to suspend the system,
> possibly taking some kernel's input into account.
>
When we don't need these heuristics, this is just a burden.

> I'm not against the very idea of automatic system suspend, which IMO is a
> legitimate and reasonable thing to do in many usage scenarios, but I don't
> think that the kernel is the right place to start a suspend process.  For this
> reason I'm not going to take any code trying to start a suspend process from
> within the kernel, regardless of that code's purpose, unless somebody makes a
> really convincing case for that to me (basically proving the need for such a
> solution).

There is no absolute need to start the suspend process from within the
kernel, but it makes the user space code much simpler for what we
need.

> That said I'm willing to accept patches adding or improving code
> that will help us to avoid races between system suspend, initiated by user
> space, and wakeup events detected by the kernel.
>
> I hope that makes things clear.
>
> Thanks,
> Rafael
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--
Arve Hjønnevåg