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Message-Id: <20090213160526.b5d619e6.akpm@linux-foundation.org>
Date: Fri, 13 Feb 2009 16:05:26 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: dmitry.torokhov@...il.com, dtor@...l.ru,
linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
fweisbec@...il.com
Subject: Re: [PATCH v2] psmouse: run kpsmoused only while needed
On Sat, 31 Jan 2009 16:27:22 -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 resync 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 resync is needed. Once the resync 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).
>
> Changes in V2:
>
> _ fix the "resync" mispelled in the patch and the changelog
> _ don't schedule more than one resync in case of concurrent interrupts
> _ if resync_pending is not cleared after a few time before disconnect, print a warning.
>
This patch seems complicated.
>
> diff --git a/drivers/input/mouse/psmouse-base.c b/drivers/input/mouse/psmouse-base.c
> index f8f86de..fc4490e 100644
> --- a/drivers/input/mouse/psmouse-base.c
> +++ b/drivers/input/mouse/psmouse-base.c
> @@ -20,6 +20,7 @@
> #include <linux/init.h>
> #include <linux/libps2.h>
> #include <linux/mutex.h>
> +#include <linux/kthread.h>
>
> #include "psmouse.h"
> #include "synaptics.h"
> @@ -104,8 +105,6 @@ static struct attribute_group psmouse_attribute_group = {
> */
> static DEFINE_MUTEX(psmouse_mutex);
>
> -static struct workqueue_struct *kpsmoused_wq;
> -
> struct psmouse_protocol {
> enum psmouse_type type;
> const char *name;
> @@ -203,11 +202,6 @@ static psmouse_ret_t psmouse_process_byte(struct psmouse *psmouse)
> return PSMOUSE_FULL_PACKET;
> }
>
> -void psmouse_queue_work(struct psmouse *psmouse, struct delayed_work *work,
> - unsigned long delay)
> -{
> - queue_delayed_work(kpsmoused_wq, work, delay);
> -}
>
> /*
> * __psmouse_set_state() sets new psmouse state and resets all flags.
> @@ -313,7 +307,10 @@ 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);
> + if (atomic_inc_return(&psmouse->resync_pending) == 1)
> + schedule_work(&psmouse->resync_work);
> + else
> + atomic_dec(&psmouse->resync_pending);
> goto out;
> }
This little trick looks inherently racy. Suppose
psmouse_resync_thread_helper() is concurrently fiddling with
->resync_pending.
Dunno - maybe it isn't racy. I can't really be bothered working it
out, because it should be obviosuly non-racy!
Can we remove ->resync_pending? Just unconditionally do the
schedule_work()? schedule_work() will take care of things
appropriately, won't it?
There is, I guess, a small possibility that we'll end up with two (or
more!) kernel threads running at the same time. But the psmouse code
should be able to handle that appropriately.
> @@ -350,7 +347,11 @@ static irqreturn_t psmouse_interrupt(struct serio *serio,
> time_after(jiffies, psmouse->last + psmouse->resync_time * HZ)) {
> psmouse->badbyte = psmouse->packet[0];
> __psmouse_set_state(psmouse, PSMOUSE_RESYNCING);
> - psmouse_queue_work(psmouse, &psmouse->resync_work, 0);
> +
> + if (atomic_inc_return(&psmouse->resync_pending) == 1)
> + schedule_work(&psmouse->resync_work);
> + else
> + atomic_dec(&psmouse->resync_pending);
> goto out;
> }
>
etc.
>
>
> /*
> - * psmouse_resync() attempts to re-validate current protocol.
> + * psmouse_resync_thread() attempts to re-validate current protocol.
> + * This thread is created on the fly when needed because its job can take too
> + * much time on events workqueues, and the resync is rare enough to avoid
> + * the need of a private workqueue.
> */
>
> -static void psmouse_resync(struct work_struct *work)
> +static int psmouse_resync_thread(void *v)
> {
> - struct psmouse *parent = NULL, *psmouse =
> - container_of(work, struct psmouse, resync_work.work);
> + struct psmouse *psmouse = v;
> + struct psmouse *parent = NULL;
> struct serio *serio = psmouse->ps2dev.serio;
> psmouse_ret_t rc = PSMOUSE_GOOD_DATA;
> int failed = 0, enabled = 0;
> @@ -1072,6 +1076,30 @@ static void psmouse_resync(struct work_struct *work)
> psmouse_activate(parent);
> out:
> mutex_unlock(&psmouse_mutex);
> +
> + /*
> + * While disconnecting, the driver wants to be sure all resync are done
> + */
> + atomic_dec(&psmouse->resync_pending);
> + wake_up(&psmouse->resync_pending_queue);
> + return 0;
> +}
> +
> +/* Launch the resync thread */
> +static void psmouse_resync_thread_helper(struct work_struct *work)
> +{
> + struct task_struct *t;
> + struct psmouse *psmouse;
> +
> + psmouse = container_of(work, struct psmouse, resync_work);
> +
> + t = kthread_run(psmouse_resync_thread, psmouse, "kpsmoused");
> + if (t == ERR_PTR(-ENOMEM)) {
> + printk(KERN_WARNING "psmouse.c: failed to create kpsmoused"
> + " thread\n");
> + atomic_dec(&psmouse->resync_pending);
> + wake_up(&psmouse->resync_pending_queue);
> + }
> }
I expect I asked before "can we use the kernel/async.c code for this".
It's such an obvious question that it should be covered in the
changelog.
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