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

Neil Brown
2010-05-30T08:09:11Z

On Fri, 28 May 2010 21:04:53 -0700
Arve Hjønnevåg <arve@android.com> wrote:

> On Fri, May 28, 2010 at 7:52 PM, mark gross <640e9920@gmail.com> wrote:
> > 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.
> >
>
> Can you be more specific? If you are talking about only letting
> drivers abort suspend, not block it, then the main argument against
> that is that you are forcing user-space to poll until the driver stops
> aborting suspend (which according to people arguing against us using
> suspend would make the power-manager a "bad" process). Or are you
> talking about blocking the request from user-space until all other
> suspend-blockers have been released and then doing a single suspend
> cycle before returning. This would not be as bad, but it would force
> the user-space power manager to be multi-threaded since it now would
> have way to cancel the request. Either way, what problem are you
> trying to solve by making it a one-shot request?
>

I don't know exactly what Mark has in mind, but I would advocate 1-shot
simply because what we currently have (echo mem > /sys/power/state) is
1-shot and I don't believe you need to do more than fix the bugs in that.

Your question of whether to abort or block suspend in central I think - the
answer to that question will make or break a possible solution.

Simply aborting the suspend cannot work as you rightly say - the suspend
daemon would then spin until other user-space processes get into action.
Simply blocking while there are any unhandled 'wakeup events' - then aborting
if there were any - is how I think it should work. However as it
doesn't work that way now I don't think it is safe to make it work that way
unconditionally. If we did we could find that existing configurations always
block suspend indefinitely with would clearly be a regression.

I think we still need some sort of "suspend_prepare". This would have two
particular effects.
1/ it sets the start time for interpreting the word "were" above. i.e. the
suspend would abort of there were any unhandled wakeup events since the
"suspend_prepare" was issued.
2/ It would allow unhandled wakeup events to abort the suspend. If no
suspend_prepare had been issued, then only "new" wakeup events would
be allowed to abort the suspend (i.e. the old racy version of suspend).

So the suspend daemon does:

wait for there to be no user-space suspend blocks
issue suspend_prepare
check there are still no suspend blocks
if there are, loop (possibly issue suspend_abort if needed)
issue suspend request
loop

processes that handle wakeup events would

poll for event to be available
request suspend-block
consume event
release suspend-block
loop

(where consuming the event would quite possibly cause some other
suspend-block to become active - e.g. it might request that the display
be unlocked which would block suspends for a time).

NeilBrown