Re: [update 2] Re: [RFC][PATCH] PM: Avoid losing wakeup events duringsuspend

From: Alan Stern
Date: Thu Jun 24 2010 - 13:09:33 EST

On Thu, 24 Jun 2010, Rafael J. Wysocki wrote:

> > This is slightly different from the wakelock design. Each call to
> > pm_stay_awake() must be paired with a call to pm_relax(), allowing one
> > device to have multiple concurrent critical sections, whereas calls to
> > pm_wakeup_event() must not be paired with anything. With wakelocks,
> > you couldn't have multiple pending events for the same device.
> You could, but you needed to define multiple wakelocks for the same device for
> this purpose.

Yeah, okay, but who's going to do that?

> > I'm not sure which model is better in practice. No doubt the Android people
> > will prefer their way.
> I suppose so.

It may not make a significant difference in the end. You can always
emulate the wakelock approach by not calling pm_stay_awake() when you
know there is an earlier call still pending.

> > This requires you to define an explicit PCI_WAKEUP_COOLDOWN delay. I
> > think that's okay; I had to do something similar with USB and SCSI.
> > (And I still think it would be a good idea to prevent workqueue threads
> > from freezing until their queues are empty.)
> I guess you mean the freezable ones?

Yes. The unfreezable workqueue threads don't have to worry about
getting frozen while their queues are non-empty. :-)

> I'm not sure if that helps a lot, because
> new work items may still be added after the workqueue thread has been frozen.

That's not the point. If a wakeup handler queues a work item (for
example, by calling pm_request_resume) then it wouldn't need to guess a
timeout. The work item would be guaranteed to run before the system
could suspend again.

> > Instead of allocating the work structures dynamically, would you be
> > better off using a memory pool?
> Well, it would be kind of equivalent to defining my own slab cache for that,
> wouldn't it?

I suppose so. It would make the GFP_ATOMIC allocations a little more

Alan Stern

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