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