Re: Attempted summary of suspend-blockers LKML thread

From: Rafael J. Wysocki
Date: Sat Aug 07 2010 - 04:47:17 EST

On Saturday, August 07, 2010, Arve Hjønnevåg wrote:
> 2010/8/6 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
> > On Thu, 5 Aug 2010, Arve Hjønnevåg wrote:
> >
> >> count, tells you how many times the wakelock was activated. If a
> >> wakelock prevented suspend for a long time a large count tells you it
> >> handled a lot of events while a small count tells you it took a long
> >> time to process the events, or the wakelock was not released properly.
> >
> > As noted, we already have this.
> >
> Almost. We have it when a device is passed in.

Sure. And what are the other cases (details, please)?

> >> expire_count, tells you how many times the timeout expired. For the
> >> input event wakelock in the android kernel (which has a timeout) an
> >> expire count that matches the count tells you that someone opened an
> >> input device but is not reading from it (this has happened several
> >> times).
> >
> > This is a little tricky. Rafael's model currently does not allow
> > wakeup events started by pm_wakeup_event() to be cancelled any way
> > other than by having their timer expire. This essentially means that
> > for some devices, expire_count will always be the same as count and for
> > others it will always be 0. To change this would require adding an
> > extra timer struct, which could be done (in fact, an earlier version of
> > the code included it). It would be nice if we could avoid the need.
> >
> > Does Android use any kernel-internal wakelocks both with a timer and
> > with active cancellation?
> >
> I don't know if they are all kernel-internal but these drivers appear
> to use timeouts and active cancellation on the same wakelock:
> wifi driver, mmc core, alarm driver, evdev (suspend blocker version
> removes the timeout).

You previously said you didn't need timeouted wakelocks in the kernel, so
I guess that was incorrect.

> >> wake_count, tells you that this is the first wakelock that was
> >> acquired in the resume path. This is currently less useful than I
> >> would like on the Nexus One since it is usually "SMD_RPCCALL" which
> >> does not tell me a lot.
> >
> > This could be done easily enough, but if it's not very useful then
> > there's no point.
> >
> It is useful there is no other way to tell what triggered a wakeup,
> but it would probably be better to just track wakeup interrupts/events
> elsewhere.
> >> active_since, tells you how long a a still active wakelock has been
> >> active. If someone activated a wakelock and never released it, it will
> >> be obvious here.
> >
> > Easily added. But you didn't mention any field saying whether the
> > wakelock is currently active. That could be added too (although it
> > would be racy -- but for detecting unreleased wakelocks you wouldn't
> > care).
> >
> These are the reported stats, not the fields in the stats structure.
> The wakelock code has an active flag. If we want to keep the
> pm_stay_wake nesting (which I would argue against), we would need an
> active count. It would also require a handle, which is a change Rafael
> said would not fly.
> >> total_time, total time the wake lock has been active. This one should
> >> be obvious.
> >
> > Also easily added.
> >
> Only with a handle passed to all the calls.

Well, I'm kind of tired of this "my solution is the only acceptable one"
mindset. IMHO, it's totally counter productive.

> >> sleep_time, total time the wake lock has been active when the screen was off.
> >
> > Not applicable to general systems. Is there anything like it that
> > _would_ apply in general?
> >
> The screen off is how it is used on android, the stats is keyed of
> what user space wrote to /sys/power/state. If "on" was written the
> sleep time is not updated.
> >> max_time, longest time the wakelock was active uninterrupted. This
> >> used less often, but the battery on a device was draining fast, but
> >> the problem went away before looking at the stats this will show if a
> >> wakelock was active for a long time.
> >
> > Again, easily added. The only drawback is that all these additions
> > will bloat the size of struct device. Of course, that's why you used
> > separately-allocated structures for your wakelocks. Maybe we can
> > change to do the same; it seems likely that the majority of device
> > structures won't ever be used for wakeup events.
> >
> Since many wakelocks are not associated with s struct device we need a
> separate object for this anyway.
> >> >> and I would prefer that the kernel interfaces would
> >> >> encourage drivers to block suspend until user space has consumed the
> >> >> event, which works for the android user space, instead of just long
> >> >> enough to work with a hypothetical user space power manager.
> >
> > Rafael doesn't _discourage_ drivers from doing this. However you have
> > to keep in mind that many kernel developers are accustomed to working
> > on systems (mostly PCs) with a different range of hardware devices from
> > embedded systems like your phones. With PCI devices(*), for example,
> > there's no clear point where a wakeup event gets handed off to
> > userspace.
> >
> > On the other hand, there's no reason the input layer shouldn't use
> > pm_stay_awake and pm_relax. It simply hasn't been implemented yet.
> ...
> The merged user space interface makes this unclear to me. When I first
> used suspend on android I had a power manager process that opened all
> the input devices and reset a screen off timeout every time there was
> an input event. If the input layer uses pm_stay_awake to block suspend
> when the queue is not empty, this will deadlock with the current
> interface since reading the wake count will block forever if an input
> event occurred right after the power manager decides to suspend.

No, in that case suspend will be aborted, IIUC.

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