Re: [linux-pm] [PATCH 1/8] PM: Opportunistic suspend support.

mark gross
2010-05-29T02:52:09Z

On Thu, May 27, 2010 at 05:23:54PM +1000, Neil Brown wrote:
> On Wed, 26 May 2010 14:20:51 +0100
> Matthew Garrett <mjg59@srcf.ucam.org> wrote:
>
> > On Wed, May 26, 2010 at 02:57:45PM +0200, Peter Zijlstra wrote:
> >
> > > I fail to see why. In both cases the woken userspace will contact a
> > > central governing task, either the kernel or the userspace suspend
> > > manager, and inform it there is work to be done, and please don't
> > > suspend now.
> >
> > Thinking about this, you're right - we don't have to wait, but that does
> > result in another problem. Imagine we get two wakeup events
> > approximately simultaneously. In the kernel-level universe the kernel
> > knows when both have been handled. In the user-level universe, we may
> > have one task schedule, bump the count, handle the event, drop the count
> > and then we attempt a suspend again because the second event handler
> > hasn't had an opportunity to run yet. We'll then attempt a suspend and
> > immediately bounce back up. That's kind of wasteful, although it'd be
> > somewhat mitigated by checking that right at the top of suspend entry
> > and returning -EAGAIN or similar.
> >
>
> (I'm coming a little late to this party, so excuse me if I say something that
> has already been covered however...)
>
> The above triggers a sequence of thoughts which (When they settled down) look
> a bit like this.
>
> At the hardware level, there is a thing that we could call a "suspend
> blocker". It is an interrupt (presumably level-triggered) that causes the
> processor to come out of suspend, or not to go into it.
>
> Maybe it makes sense to export a similar thing from the kernel to user-space.
> When any event happens that would wake the device (and drivers need to know
> about these already), it would present something to user-space to say that
> the event happened.
>
> When user-space processes the event, it clears the event indicator.

we did I proposed making the suspend enabling a oneshot type of thing
and all sorts of weak arguments came spewing forth. I honestly couldn't
tell if I was reading valid input or fanboy BS.

--mgross

>
> When there are no more current event indicators, userspace is allowed to
> request a suspend. Obviously this could fail as an event could happen at any
> moment, but the same is true when the kernel asks the device to suspend, an
> interrupt might happen immediately to stop it. But in either case an event
> will be reported. So when userspace requests a suspend and it fails, it
> will see events reported and so will wait for them to be handled.
>
> I imagine a sysfs directory with files that appear when events are pending.
> We could have some separate mechanism for user-space processes to request
> that the suspend-daemon not suspend. Then it suspends whenever there are no
> pending requests from user-space or from the kernel.
>
> The advantage of this model of suspend-blockers is that it is a close
> analogue for something that already exists in hardware so it isn't really
> creating new concepts, just giving the Linux virtual-machine features that
> have proved themselves in physical machines.
>
> The cost is that any wake-up event needs to not only be handled, but also
> explicitly acknowledged by clearing the relevant suspend-blocker (i.e.
> removing the file from sysfs, or whatever interface was ultimately chosen).
> I'm hoping that isn't a big cost.
>
> NeilBrown
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