Date:	Tue, 22 Jun 2010 10:35:38 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Florian Mickler <florian@...kler.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Neil Brown <neilb@...e.de>, mark gross <640e9920@...il.com>
Subject: Re: [RFC][PATCH] PM: Avoid losing wakeup events during suspend

On Tue, 22 Jun 2010, Rafael J. Wysocki wrote:

> Having reconsidered that I think there's more to it.
> 
> Take the PCI subsystem as an example, specifically pcie_pme_handle_request().
> This is the place where wakeup events are started, but it has no idea about
> how they are going to be handled.  Thus in the suspend blocker scheme it would
> need to activate a blocker, but it wouldn't be able to release it.  So, it
> seems, we would need to associate a suspend blocker with each PCIe device
> that can generate wakeup events and require all drivers of such devices to
> deal with a blocker activated by someone else (PCIe PME driver in this
> particular case).  That sounds cumbersome to say the least.

Maybe this means pcie_pme_handle_request() isn't a good place to note
the arrival of a wakeup event.  Doing it in the individual driver might
work out better.

Or look at this from a different point of view.  Adopting Mark's 
terminology, let's say that the kernel is in a "critical section" at 
times when we don't want the system to suspend.  Currently there's no 
way to tell the PM core when the kernel enters or leaves a critical 
section, so there's no way to prevent the system from suspending at the 
wrong time.

Most wakeup events indicate the start of a critical section, in the 
sense that you hardly ever say: "I want the computer to wake up when I 
press this button, but I don't care what it does afterward -- it can go 
right back to sleep without doing anything if it wants."  Much more 
common is that a wakeup event requires a certain amount of processing, 
and you don't want the system to go back to sleep until the processing 
is over.  Of course, if the processing is simple enough that it can all 
be done in an interrupt handler or a resume method, then nothing 
extra is needed since obviously the system won't suspend while an 
interrupt handler or a resume method is running.  But for more 
complicated cases, we need to do more.

The problem in your example is that pcie_pme_handle_request() has no 
idea about the nature or extent of the critical section to follow.  
Therefore it's not in a good position to mark the beginning of the 
critical section, even though it is in an excellent position to mark 
the receipt of a wakeup event.

> Moreover, even if we do that, it still doesn't solve the entire problem,
> because the event may need to be delivered to user space and processed by it.
> While a driver can check if user space has already read the event, it has
> no way to detect whether or not it has finished processing it.  In fact,
> processing an event may involve an interaction with a (human) user and there's
> no general way by which software can figure out when the user considers the
> event as processed.

Is this the kernel's problem?  Once userspace has read the event, we
can safely say that the kernel's critical section is over.  Perhaps a
userspace critical section has begun, perhaps not; either way, it's no
longer the kernel's responsibility.

> It looks like user space suspend blockers only help in some special cases
> when the user space processing of a wakeup event is simple enough, but I don't
> think they help in general.  For an extreme example, a user may want to wake up
> a system using wake-on-LAN to log into it, do some work and log out, so
> effectively the initial wakeup event has not been processed entirely until the
> user finally logs out of the system.  Now, after the system wakeup (resulting
> from the wake-on-LAN signal) we need to give the user some time to log in, but
> if the user doesn't do that in certain time, it may be reasonable to suspend
> and let the user wake up the system again. 

I agree.  This is a case where there is no clear-cut end to the
kernel's critical section, because the event is not handed over to
userspace.  A reasonable approach would be to use a timeout, perhaps
also with some heuristic like: End the critical section early if we
receive 100(?) more network packets before the timeout expires.

> Similar situation takes place when the system is woken up by a lid switch.
> Evidently, the user has opened the laptop lid to do something, but we don't
> even know what the user is going to do, so there's no way we can say when
> the wakeup event is finally processed.

In this case, the kernel could inform an appropriate user process (some
part of DeviceKit? or the power-manager process?) about the lid-switch
event.  Once that information had been passed on, the kernel's critical
section would be over.  The process could start its own critical
section or not, as it sees fit.

If there is no process to send the information to, the kernel could 
again end the critical section after a reasonable timeout (1 minute?).

> So, even if we can say when the kernel has finished processing the event
> (although that would be complicated in the PCIe case above), I don't think
> it's generally possible to ensure that the entire processing of a wakeup event
> has been completed.  This leads to the question whether or not it is worth
> trying to detect the ending of the processing of a wakeup event.

As Arve pointed out, in some cases it definitely is worthwhile (the
gpio keypad matrix example).  In other cases there may be no reasonable
way to tell.  That doesn't mean we have to give up entirely.

> Now, going back to the $subject patch, I didn't really think it would be
> suitable for opportunistic suspend, so let's focus on the "forced" suspend
> instead.  It still has the problem that wakeup events occuring while
> /sys/power/state is written to (or even slightly before) should cause the
> system to cancel the suspend, but they generally won't.  With the patch
> applied that can be avoided by (a) reading from /sys/power/wakeup_count,
> (b) waiting for certain time (such that if a suspend event is not entirely
> processed within that time, it's worth suspending and waking up the
> system again) and (c) writing to /sys/power/wakeup_count right before writing
> to /sys/power/state (where the latter is only done if the former succeeds).

In other words, you could detect if a critical section begins after the 
user process has decided to initiate a suspend.  Yes, that's so.

On the other hand, we should already be able to abort a suspend if a
wakeup event arrives after tasks are frozen (to pick a reasonable
boundary point).  If we can't -- if some wakeup events are able to slip
through our fingers -- I would say it's a bug and the relevant drivers
need to be fixed.  In the end this probably will require adding a
function to notify the PM core that a wakeup event occurred and
therefore a suspend-in-progress should be aborted -- almost exactly
what pm_wakeup_event() does.

So I'm not opposed to the new function.  But it doesn't solve the 
entire problem of avoiding suspends during critical sections.

Alan Stern

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