Re: Async suspend-resume patch w/ rwsems (was: Re: [GIT PULL] PM updates for 2.6.33)

Rafael J. Wysocki
2009-12-09T22:17:34Z

On Wednesday 09 December 2009, Alan Stern wrote:
> On Tue, 8 Dec 2009, Rafael J. Wysocki wrote:
>
> > For completness, below is the full async suspend/resume patch with rwlocks,
> > that has been (very slightly) tested and doesn't seem to break things.
> >
> > [Note to Alan: lockdep doesn't seem to complain about the not annotated nested
> > locks.]
>
> I can't imagine why not. And wouldn't lockdep get confused by the fact
> that in the async case, the rwsems are released by a different process
> from the one that acquired them?

/me looks at the .config

I have CONFIG_LOCKDEP_SUPPORT set, is there anything else I need to set
in .config?

> > Index: linux-2.6/drivers/base/power/main.c
> > ===================================================================
> > --- linux-2.6.orig/drivers/base/power/main.c
> > +++ linux-2.6/drivers/base/power/main.c
>
> Should we have an attribute under /sys/power to disable async
> suspend/resume? It would make testing easier and give people a way to
> work around problems.

I have a separate patch adding that, but I'd prefer to focus on the core
feature first, if possible.

> > @@ -334,25 +337,53 @@ static void pm_dev_err(struct device *de
> > * The driver of @dev will not receive interrupts while this function is being
> > * executed.
> > */
> > -static int device_resume_noirq(struct device *dev, pm_message_t state)
> > +static int __device_resume_noirq(struct device *dev, pm_message_t state)
> > {
>
> Do you want to use async tasks in the late-suspend/early-resume stages?
> I know that USB won't use it, not even for the PCI host controllers --
> not unless the PCI core specifically wants it. Doing just the regular
> suspend/resume stages may be enough.

I guess so. It's a leftover from the time I thought PCI might use async
suspend, but it didn't really speed up things at all AFAICS.

I think I'll remove it for now and it's going to be trivial to add it back if
desired.

> > +static int device_resume_noirq(struct device *dev)
> > +{
> > + down_write(&dev->power.rwsem);
> > +
> > + if (dev->power.async_suspend && !pm_trace_is_enabled()) {
>
> If the sysfs attribute exists, then maybe we _should_ allow async with
> PM tracing enabled. I don't know; it's your decision.

I don't think it would be reliable in that case, because the RTC might be
written to by two concurrent threads at the same time.

> atomic_set(&async_error, error);
> }
>
>
> > @@ -683,10 +835,12 @@ static int dpm_suspend(pm_message_t stat
> >
> > INIT_LIST_HEAD(&list);
> > mutex_lock(&dpm_list_mtx);
> > + pm_transition = state;
> > while (!list_empty(&dpm_list)) {
> > struct device *dev = to_device(dpm_list.prev);
> >
> > get_device(dev);
> > + dev->power.status = DPM_OFF;
>
> What's that for? dev->power.status is supposed to be DPM_SUSPENDING
> until the suspend method is successfully completed.

If the suspend is run asynchronoysly, the main thread will always get a
"success" from device_suspend(), so it can't change power.status on this
basis. I thought we could set power.status to DPM_OFF upfront and change
it back when error is returned.

The alternative would be to move the modification of power.status to
device_suspend() and async_suspend(). Well, maybe that's better.

> > mutex_unlock(&dpm_list_mtx);
> >
> > error = device_suspend(dev, state);
> > @@ -694,16 +848,22 @@ static int dpm_suspend(pm_message_t stat
> > mutex_lock(&dpm_list_mtx);
> > if (error) {
> > pm_dev_err(dev, state, "", error);
> > + dev->power.status = DPM_SUSPENDING;
>
> And then this isn't needed.
>
> > put_device(dev);
> > break;
> > }
> > - dev->power.status = DPM_OFF;
>
> This line has to be moved into __device_suspend(), even though it won't
> be protected by dpm_list_mtx. The same sort of thing applies to
> dpm_suspend_noirq() (although nothing needs to be moved if you don't
> make it async).
>
> The rest looks okay.

Still, I think I'd rework it to use completions for the reason described in the
message I've just sent (in short, because of the off-tree dependencies
problem).

> How about exporting a wait_for_device_to_resume() routine? Drivers
> could call it for non-tree resume constraints:
>
> void wait_for_device_to_resume(struct device *other)
> {
> down_read(&other->power.rwsem);
> up_read(&other->power.rwsem);
> }
>
> Unfortunately there is no equivalent for non-tree suspend constraints.

If we use completions, it will be possible to just export something like

dpm_wait(dev)
{
if (dev)
wait_for_completion(dev->power.completion);
}

I think. It appears that will also work for suspend, unless I'm missing
something.

Rafael