linux-kernel - Re: [PATCH] psmouse: run kpsmoused only while needed

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Message-ID: <20090127003412.GC4815@nowhere>
Date:	Tue, 27 Jan 2009 01:34:15 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dmitry.torokhov@...il.com, dtor@...l.ru,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-input@...r.kernel.org
Subject: Re: [PATCH] psmouse: run kpsmoused only while needed

On Mon, Jan 26, 2009 at 02:32:10PM -0800, Andrew Morton wrote:
> On Thu, 15 Jan 2009 16:16:19 -0800 (PST)
> Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > While looking at the workqueue tracer, I noticed that kpsmoused receives
> > rarely (if not never) events.
> > 
> > Currently, when a mouse has to resync, it uses the kpsmoused singlethreaded
> > workqueue. But recync are rare. While reading an old discussion, it seems
> > that usual workqueue events can't be used for that purpose because resync
> > can take too much time and could delay the other works in queue.
> > 
> > But if you have built psmouse driver, this workqueue will always be present
> > whether you have a ps/2 port or not. And its events are rare.
> > 
> > To avoid this pointless task, this patch makes the kpsmoused a kernel
> > thread only created on the fly when a recync is needed. Once the recync is done,
> > this thread will die. So you will almost never see it, and it will not be
> > an inactive task anymore.
> > 
> > This thread is created through a usual workqueue event (because we can't create
> > it from interrupt).
> > 
> 
> Seems like a reasonable objective.
> 
> >
> > ...
> >
> >  /*
> >   * __psmouse_set_state() sets new psmouse state and resets all flags.
> > @@ -313,7 +307,8 @@ static irqreturn_t psmouse_interrupt(struct serio *serio,
> >  		       psmouse->name, psmouse->phys, psmouse->pktcnt);
> >  		psmouse->badbyte = psmouse->packet[0];
> >  		__psmouse_set_state(psmouse, PSMOUSE_RESYNCING);
> > -		psmouse_queue_work(psmouse, &psmouse->resync_work, 0);
> > +		atomic_inc(&psmouse->nb_recync_pending);
> 
> The patch and the changelog consistently misspell "sync".
> 
> A code comment (in psmouse.h) which clearly spells out the role of
> nb_recync_pending would be useful.


Ok.

 
> > +		schedule_work(&psmouse->resync_work);
> >  		goto out;
> >  	}
> >  
> >
> > ...
> >
> > @@ -1131,7 +1155,13 @@ static void psmouse_disconnect(struct serio *serio)
> >  
> >  	/* make sure we don't have a resync in progress */
> >  	mutex_unlock(&psmouse_mutex);
> > -	flush_workqueue(kpsmoused_wq);
> > +
> > +	prepare_to_wait(&psmouse->recync_pending_queue, &wait,
> > +				TASK_UNINTERRUPTIBLE);
> > +	if (atomic_read(&psmouse->nb_recync_pending))
> > +		schedule();
> > +	finish_wait(&psmouse->recync_pending_queue, &wait);
> 
> So... we're requiring that nb_recync_pending is zero at this stage?
> 
> I wonder if the code manages to do that.  A little WARN_ON(), maybe?


Yes it is supposed to be zero since no new recync can be performed at this time.
But a workqueue (one that creates the kpsmoused thread) or the thread itself can
be still running, so we want to ensure all is completed.

Perhaps a schedule_timeout with a warn_on would be better to detect soft-lockups?

Thanks!

 
> >  	mutex_lock(&psmouse_mutex);
> >  
> 

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