linux-kernel - Re: [PATCH] PM: Make it possible to avoid wakeup events from being lost

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Date:	Sun, 27 Jun 2010 11:50:05 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	linux-pm@...ts.linux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Neil Brown <neilb@...e.de>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	mark gross <640e9920@...il.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Florian Mickler <florian@...kler.org>,
	<linux-pci@...r.kernel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>
Subject: Re: [PATCH] PM: Make it possible to avoid wakeup events from being
 lost

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.

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.

Alan Stern

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