Date:	Thu, 5 Aug 2010 18:29:27 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>, david@...g.hm,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, florian@...kler.org, stern@...land.harvard.edu,
	swetland@...gle.com, peterz@...radead.org, tglx@...utronix.de,
	alan@...rguk.ukuu.org.uk
Subject: Re: Attempted summary of suspend-blockers LKML thread

2010/8/5 Rafael J. Wysocki <rjw@...k.pl>:
> On Friday, August 06, 2010, Arve Hjønnevåg wrote:
>> 2010/8/5 Rafael J. Wysocki <rjw@...k.pl>:
>> > On Thursday, August 05, 2010, Arve Hjønnevåg wrote:
>> >> 2010/8/4 Rafael J. Wysocki <rjw@...k.pl>:
>> >> > On Thursday, August 05, 2010, Arve Hjønnevåg wrote:
>> >> >> On Wed, Aug 4, 2010 at 1:56 PM, Matthew Garrett <mjg59@...f.ucam.org> wrote:
>> >> >> > On Wed, Aug 04, 2010 at 10:51:07PM +0200, Rafael J. Wysocki wrote:
>> >> >> >> On Wednesday, August 04, 2010, Matthew Garrett wrote:
>> >> >> >> > No! And that's precisely the issue. Android's existing behaviour could
>> >> >> >> > be entirely implemented in the form of binary that manually triggers
>> >> >> >> > suspend when (a) the screen is off and (b) no userspace applications
>> >> >> >> > have indicated that the system shouldn't sleep, except for the wakeup
>> >> >> >> > event race. Imagine the following:
>> >> >> >> >
>> >> >> >> > 1) The policy timeout is about to expire. No applications are holding
>> >> >> >> > wakelocks. The system will suspend providing nothing takes a wakelock.
>> >> >> >> > 2) A network packet arrives indicating an incoming SIP call
>> >> >> >> > 3) The VOIP application takes a wakelock and prevents the phone from
>> >> >> >> > suspending while the call is in progress
>> >> >> >> >
>> >> >> >> > What stops the system going to sleep between (2) and (3)? cgroups don't,
>> >> >> >> > because the voip app is an otherwise untrusted application that you've
>> >> >> >> > just told the scheduler to ignore.
>> >> >> >>
>> >> >> >> I _think_ you can use the just-merged /sys/power/wakeup_count mechanism to
>> >> >> >> avoid the race (if pm_wakeup_event() is called at 2)).
>> >> >> >
>> >> >> > Yes, I think that solves the problem. The only question then is whether
>> >> >>
>> >> >> How? By passing a timeout to pm_wakeup_event when the network driver
>> >> >> gets the packet or by passing 0. If you pass a timeout it is the same
>> >> >> as using a wakelock with a timeout and should work (assuming the
>> >> >> timeout you picked is long enough). If you don't pass a timeout it
>> >> >> does not work, since the packet may not be visible to user-space yet.
>> >> >
>> >> > Alternatively, pm_stay_awake() / pm_relax() can be used.
>> >> >
>> >>
>> >> Which makes the driver and/or network stack changes identical to using
>> >> wakelocks, right?
>> >
>> > Please refer to the Matthew's response.
>> >
>> >> >> > it's preferable to use cgroups or suspend fully, which is pretty much up
>> >> >> > to the implementation. In other words, is there a reason we're still
>> >> >>
>> >> >> I have seen no proposed way to use cgroups that will work. If you
>> >> >> leave some processes running while other processes are frozen you run
>> >> >> into problems when a frozen process holds a resource that a running
>> >> >> process needs.
>> >> >>
>> >> >>
>> >> >> > having this conversation? :) It'd be good to have some feedback from
>> >> >> > Google as to whether this satisfies their functional requirements.
>> >> >> >
>> >> >>
>> >> >> That is "this"? The merged code? If so, no it does not satisfy our
>> >> >> requirements. The in kernel api, while offering similar functionality
>> >> >> to the wakelock interface, does not use any handles which makes it
>> >> >> impossible to get reasonable stats (You don't know which pm_stay_awake
>> >> >> request pm_relax is reverting).
>> >> >
>> >> > Why is that a problem (out of curiosity)?
>> >> >
>> >>
>> >> Not having stats or not knowing what pm_relax is undoing? We need
>> >> stats to be able to debug the system.
>> >
>> > You have the stats in struct device and they are available via sysfs.
>> > I suppose they are insufficient, but I'd like to know why exactly.
>> >
>>
>> Our wakelock stats currently have
>> (name,)count,expire_count,wake_count,active_since,total_time,sleep_time,max_time
>> and last_change. Not all of these are equally important (total_time is
>> most important followed by active_since), but you only have count.
>> Also as discussed before, many wakelocks/suspendblockers are not
>> associated with a struct device.
>
> OK
>
> How much of that is used in practice and what for exactly?

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.

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).

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.

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.

total_time, total time the wake lock has been active. This one should
be obvious.

sleep_time, total time the wake lock has been active when the screen was off.

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.


>
> Do you _really_ have to debug the wakelocks in drivers that much?
>

Wake locks in drivers sometimes need to be debugged. If the api has no
accountability, then these problems would take forever to fix.

>> >> If the system does not suspend
>> >> at all or is awake for too long, the wakelock stats tells us which
>> >> component is at fault. Since pm_stay_awake and pm_relax does not
>> >> operate on a handle, you cannot determine how long it prevented
>> >> suspend for.
>> >
>> > Well, if you need that, you can add a counter of "completed events" into
>>
>> We need more than that (see above).
>>
>> > struct dev_pm_info and a function similar to pm_relax() that
>> > will update that counter.  I don't think anyone will object to that change.
>> >
>>
>> What about adding a handle that is passed to all three functions?
>
> I don't think that will fly at this point.
>

Why not? I think allowing drivers to modify a global reference count
with no accountability is a terrible idea.

>> >> >> The proposed in user-space interface
>> >> >> of calling into every process that receives wakeup events before every
>> >> >> suspend call
>> >> >
>> >> > Well,  you don't really need to do that.
>> >> >
>> >>
>> >> Only if the driver blocks suspend until user-space has read the event.
>> >> This means that for android to work we need to block suspend when
>> >> input events are not processed, but a system using your scheme needs a
>> >> pm_wakeup_event call when the input event is queued. How to you switch
>> >> between them? Do we add separate ioctls in the input device to enable
>> >> each scheme? If someone has a single threaded user space power manager
>> >> that also reads input event it will deadlock if you block suspend
>> >> until it reads the input events since you block when reading the wake
>> >> count.
>> >
>> > Well, until someone actually tries to implement a power manager in user space
>> > it's a bit vague.
>> >
>>
>> Not having clear rules for what the drivers should do is a problem.
>> The comments in your code seem to advocate using timeouts instead of
>> overlapping pm_stay_awake/pm_relax sections. I find this
>> recommendation strange given all the opposition to
>> wakelock/suspendblocker timeouts.
>
> There's no recommendation either way.

I'm referring to this paragraph:

  * Second, a wakeup event may be detected by one functional unit and processed
  * by another one.  In that case the unit that has detected it cannot really
  * "close" the "no suspend" period associated with it, unless it knows in
  * advance what's going to happen to the event during processing.  This
  * knowledge, however, may not be available to it, so it can simply
specify time
  * to wait before the system can be suspended and pass it as the second
  * argument of pm_wakeup_event().

>
>> But more importantly, calling
>> pm_wakeup_event with a timeout of 0 is incompatible with the android
>> user space code,
>
> Which I don't find really relevant, sorry.
>
>> 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.
>
> Well, that are your personal preferences, which I respect.  I also have some
> personal preferences that are not necessarily followed by the kernel code.
>
> Thanks,
> Rafael
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-- 
Arve Hjønnevåg
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