On Sunday, June 27, 2010, Alan Stern wrote:
> On Sat, 26 Jun 2010, Rafael J. Wysocki wrote:
>
> > +void pm_relax(void)
> > +{
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&events_lock, flags);
> > + if (events_in_progress) {
> > + event_count++;
> > + if (!--events_in_progress)
> > + wake_up_all(&events_wait_queue);
> > + }
> > + spin_unlock_irqrestore(&events_lock, flags);
> > +}
>
> > +bool pm_get_wakeup_count(unsigned long *count)
> > +{
> > + bool ret;
> > +
> > + spin_lock_irq(&events_lock);
> > + if (capable(CAP_SYS_ADMIN))
> > + events_check_enabled = false;
> > +
> > + if (events_in_progress) {
> > + DEFINE_WAIT(wait);
> > +
> > + do {
> > + prepare_to_wait(&events_wait_queue, &wait,
> > + TASK_INTERRUPTIBLE);
> > + if (!events_in_progress)
> > + break;
> > + spin_unlock_irq(&events_lock);
> > +
> > + schedule();
> > +
> > + spin_lock_irq(&events_lock);
> > + } while (!signal_pending(current));
> > + finish_wait(&events_wait_queue, &wait);
> > + }
> > + *count = event_count;
> > + ret = !events_in_progress;
> > + spin_unlock_irq(&events_lock);
> > + return ret;
> > +}
>
> Here's a thought. Presumably pm_relax() will end up getting called a
> lot more often than pm_get_wakeup_count(). Instead of using a wait
> queue, you could make pm_get_wakeup_count() poll at 100-ms intervals.
> The total overhead would be smaller.
For that I'd need a separate kernel thread or a work item that would reschedule
itself periodically, because pm_get_wakeup_count() is only called via
/sys/power/wakeup_count. It would complicate things quite a bit which I'm not
sure is worth it at this point.
> Here's another thought. If event_count and events_in_progress were
> atomic_t then the new spinlock wouldn't be needed at all. (But you
> would need an appropriate pair of memory barriers, to guarantee that
> when a writer decrements events_in_progress to 0 and increments
> event_count, a reader won't see events_in_progress == 0 without also
> seeing the incremented event_count.) Overall, this may not be a
> significant improvement.
No, I don't think it would be significant. Still, we can go back to this
if the spinlock turns out to be a problem in future.
Rafael
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