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SubjectRe: [RFC][PATCH] PM: Avoid losing wakeup events during suspend
On Sun, Jun 20, 2010 at 10:23:38PM -0400, Alan Stern wrote:
> On Sun, 20 Jun 2010, Rafael J. Wysocki wrote:
>
> > > > Generally, there are two problems in that area. First, if a wakeup event
> > > > occurs exactly at the same time when /sys/power/state is being written to,
> > > > the even may be delivered to user space right before the freezing of it,
> > > > in which case the user space consumer of the event may not be able to process
> > > > it before the system is suspended.
> > >
> > > Indeed, the same problem arises if the event isn't delivered to
> > > userspace until after userspace is frozen.
> >
> > In that case the kernel should abort the suspend so that the event can be
> > delivered to the user space.
>
> Yes.
>
> > > Of course, the underlying issue here is that the kernel has no direct way
> > > to know when userspace has finished processing an event. Userspace would
> > > have to tell it, which generally would mean rewriting some large number of user
> > > programs.
> >
> > I'm not sure of that. If the kernel doesn't initiate suspend, it doesn't
> > really need to know whether or not user space has already consumed the event.
>
> That's true. But it only shifts the onus: When a userspace program has
> finished processing an event, it has to tell the power-manager process.
> Clearly this sort of thing is unavoidable, one way or another.
>
> > > > The following patch illustrates my idea of how these two problems may be
> > > > addressed. It introduces a new global sysfs attribute,
> > > > /sys/power/wakeup_count, associated with a running counter of wakeup events
> > > > and a helper function, pm_wakeup_event(), that may be used by kernel subsystems
> > > > to increment the wakeup events counter.
> > >
> > > In what way is this better than suspend blockers?
> >
> > It doesn't add any new framework and it doesn't require the users of
> > pm_wakeup_event() to "unblock" suspend, so it is simpler. It also doesn't add
> > the user space interface that caused so much opposition to appear.
>
> Okay. A quick comparison shows that in your proposal:
>
> There's no need to register and unregister suspend blockers.
> But instead you create the equivalent of a suspend blocker
> inside every struct device.
>
> Drivers (or subsystems) don't have to activate suspend
> blockers. But instead they have to call pm_wakeup_event().
>
> Drivers don't have to deactivate suspend blockers. You don't
> have anything equivalent, and as a result your scheme is
> subject to the race described below.
>
> There are no userspace suspend blockers and no opportunistic
> suspend. Instead a power-manager process takes care of
> initiating or preventing suspends as needed.
>
> In short, you have eliminated the userspace part of the suspend blocker
> approach just as in some of the proposals posted earlier, and you have
> replaced the in-kernel suspend blockers with new data in struct device
> and a new PM API. On the whole, it doesn't seem very different from
> the in-kernel part of suspend blockers. The most notable difference is
> the name: pm_wake_event() vs. suspend_blocker_activate(), or whatever
> it ended up being called.
>
> This is the race I was talking about:

Your confused about what problem this patch attempts to solve. There is
a pm_qos patch in the works to address the suspend blocker
functionality.
http://lists.linux-foundation.org/pipermail/linux-pm/2010-June/026760.html

--mgross



>
> > > What happens if an event arrives just before you read
> > > /sys/power/wakeup_count, but the userspace consumer doesn't realize
> > > there is a new unprocessed event until after the power manager checks
> > > it?
>
> > I think this is not the kernel's problem. In this approach the kernel makes it
> > possible for the user space to avoid the race. Whether or not the user space
> > will use this opportunity is a different matter.
>
> It is _not_ possible for userspace to avoid this race. Help from the
> kernel is needed.
>
> > > Your plan is missing a critical step: the "handoff" whereby
> > > responsibility for handling an event passes from the kernel to
> > > userspace.
>
> > > With suspend blockers, this handoff occurs when an event queue is
> > > emptied and its associate suspend blocker is deactivated. Or with some
> > > kinds of events for which the Android people have not written an
> > > explicit handoff, it occurs when a timer expires (timed suspend
> > > blockers).
> >
> > Well, quite frankly, I don't see any difference here. In either case there is
> > a possibility for user space to mess up things and the kernel can't really help
> > that.
>
> With suspend blockers, there is also the possibility for userspace to
> handle races correctly. But with your scheme there isn't -- that's the
> difference.
>
> > > This shares with the other alternatives posted recently the need for a
> > > central power-manager process. And like in-kernel suspend blockers, it
> > > requires changes to wakeup-capable drivers (the wakeup-events counter
> > > has to be incremented).
> >
> > It doesn't really require changes to drivers, but to code that knows of wakeup
> > events, like the PCI runtime wakeup code.
>
> Just like in-kernel suspend blockers.
>
> > Moreover, it doesn't require kernel
> > subsystems to know or even care when it is reasonable to allow suspend to
> > happen. The only thing they need to do is to call pm_wakeup_event() whenever
> > they see a wakeup event.
>
> That's just semantics. Obviously a wakeup event should prevent suspend
> from happening, so if subsystems know or care about one then they know
> or care about the other.
>
> > I don't really think it is too much of a requirement
> > (and quite frnakly I can't imagine anything simpler than that).
>
> This is because you have omitted the part about allowing suspends again
> (or if you prefer, about notifying the PM core that a wakeup event has
> been handed off to userspace). As a result of leaving this out, you
> haven't eliminated all the races.
>
> > Yes, it does, but I have an idea about how to implement such a power manager
> > and I'm going to actually try it.
>
> A logical design would be to use dbus for disseminating PM-related
> information. Does your idea work that way?
>
> > I don't think any of the approaches that don't use suspend blockers allows
> > one to avoid the race between the process that writes to /sys/power/state
> > and a wakeup event happening at the same time. They attempt to address another
> > issue, which is how to prevent untrusted user space processes from keeping the
> > system out of idle, but that is a different story.
>
> Well, there was one approach that didn't use suspend blockers and did
> solve the race: the original wakelocks proposal. Of course, that was
> just suspend blockers under a different name. One could make a very
> good case that your scheme is also suspend blockers under a different
> name (and with an important part missing).
>
> Alan Stern
>


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