Re: Attempted summary of suspend-blockers LKML thread

[Rafael J. Wysocki]

On Saturday, August 07, 2010, Arve Hjønnevåg wrote:
> 2010/8/7 Rafael J. Wysocki <rjw@xxxxxxx>:
> > 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)?
> >
> 
> The suspend blockers I added I my suspend blocker patchset were not
> directly associated with a device. The evdev changes could be modified
> to share a device, but it would give less detail since a separate
> queue is created for each client that opens the device.

OK, that's a good argument.

> The suspend blocking work api would have to change so the caller to passes
> a device in, which I think would make that api less flexible. Mostly the
> problem is that we need separate stats for wakelocks created by a
> single driver. For instance we will still need a user-space interface
> to block suspend on android devices (lower level services than the
> power manager need to block suspend), with the stats in the device
> struct we have to create a new device for every wakelock user space
> creates in the kernel.

Well, what about managing these stats in user space?

> There is also the issue of reading the stats. It is a lot easier to
> read a single stats file, than looping though every device on the
> system (when most of the devices never block suspend).

It seems you can have a list of the "interesting" ones.

> >> >> 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.
> >
> 
> I don't know what you are reffering to. We have always stated that we
> need timeouts in the kernel to pass events through other kernel layers
> that do not use wakelocks (that list is much longer than the list
> above which mixes timeouts and unlock on the same wakelock). The only
> feature we do not use is the timeout feature in the user space
> interface to kernel wakelocks.

Now that's more clear, thanks.

> >> >> 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.
> >
> 
> How do you propose to track how long a driver has blocked suspend when
> you have an unblock call that takes no arguments.

You can extend pm_relax() to take a dev argument and measure the time between
pm_stay_awake() and pm_relax() called for the same device.

> >> >> 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.
> >
> 
> How? Your pm_get_wakeup_count function loops until events_in_progress becomes 0.

So, to deadlock with it you'd have to call pm_stay_awake() and wait for it to
complete.  However, right now there are no means by which user space can call
pm_stay_awake(), so this can't happen.

Of course, if you add pm_stay_awake() to an ioctl() code path, you should make
sure that whoever uses that ioctl() won't be waiting for the power manager to
read from /sys/power/wakeup_count.  I guess your point is that this isn't
possible to achieve?

Thanks,
Rafael
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