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Commit f23eb2b2 authored by Linus Torvalds's avatar Linus Torvalds
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tty: stop using "delayed_work" in the tty layer 



Using delayed-work for tty flip buffers ends up causing us to wait for
the next tick to complete some actions.  That's usually not all that
noticeable, but for certain latency-critical workloads it ends up being
totally unacceptable.

As an extreme case of this, passing a token back-and-forth over a pty
will take two ticks per iteration, so even just a thousand iterations
will take 8 seconds assuming a common 250Hz configuration.

Avoiding the whole delayed work issue brings that ping-pong test-case
down to 0.009s on my machine.

In more practical terms, this latency has been a performance problem for
things like dive computer simulators (simulating the serial interface
using the ptys) and for other environments (Alan mentions a CP/M emulator).

Reported-by: default avatarJef Driesen <jefdriesen@telenet.be>
Acked-by: default avatarGreg KH <gregkh@suse.de>
Acked-by: default avatarAlan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f741a79e

drivers/tty/tty_buffer.c

+7 −7
Original line number Diff line number Diff line
@@ -322,7 +322,7 @@ void tty_schedule_flip(struct tty_struct *tty) 
	if (tty->buf.tail != NULL)

		tty->buf.tail->commit = tty->buf.tail->used;

	spin_unlock_irqrestore(&tty->buf.lock, flags);

	schedule_delayed_work(&tty->buf.work, 1);

	schedule_work(&tty->buf.work);

}

EXPORT_SYMBOL(tty_schedule_flip);
@@ -402,7 +402,7 @@ EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags); 
static void flush_to_ldisc(struct work_struct *work)

{

	struct tty_struct *tty =

		container_of(work, struct tty_struct, buf.work.work);

		container_of(work, struct tty_struct, buf.work);

	unsigned long 	flags;

	struct tty_ldisc *disc;
@@ -443,7 +443,7 @@ static void flush_to_ldisc(struct work_struct *work) 
			if (test_bit(TTY_FLUSHPENDING, &tty->flags))

				break;

			if (!tty->receive_room || seen_tail) {

				schedule_delayed_work(&tty->buf.work, 1);

				schedule_work(&tty->buf.work);

				break;

			}

			if (count > tty->receive_room)
@@ -481,7 +481,7 @@ static void flush_to_ldisc(struct work_struct *work) 
 */

void tty_flush_to_ldisc(struct tty_struct *tty)

{

	flush_delayed_work(&tty->buf.work);

	flush_work(&tty->buf.work);

}



/**
@@ -506,9 +506,9 @@ void tty_flip_buffer_push(struct tty_struct *tty) 
	spin_unlock_irqrestore(&tty->buf.lock, flags);



	if (tty->low_latency)

		flush_to_ldisc(&tty->buf.work.work);

		flush_to_ldisc(&tty->buf.work);

	else

		schedule_delayed_work(&tty->buf.work, 1);

		schedule_work(&tty->buf.work);

}

EXPORT_SYMBOL(tty_flip_buffer_push);
@@ -529,6 +529,6 @@ void tty_buffer_init(struct tty_struct *tty) 
	tty->buf.tail = NULL;

	tty->buf.free = NULL;

	tty->buf.memory_used = 0;

	INIT_DELAYED_WORK(&tty->buf.work, flush_to_ldisc);

	INIT_WORK(&tty->buf.work, flush_to_ldisc);

}

drivers/tty/tty_ldisc.c

+7 −7
Original line number Diff line number Diff line
@@ -529,7 +529,7 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) 
static int tty_ldisc_halt(struct tty_struct *tty)

{

	clear_bit(TTY_LDISC, &tty->flags);

	return cancel_delayed_work_sync(&tty->buf.work);

	return cancel_work_sync(&tty->buf.work);

}



/**
@@ -542,7 +542,7 @@ static void tty_ldisc_flush_works(struct tty_struct *tty) 
{

	flush_work_sync(&tty->hangup_work);

	flush_work_sync(&tty->SAK_work);

	flush_delayed_work_sync(&tty->buf.work);

	flush_work_sync(&tty->buf.work);

}



/**
@@ -722,9 +722,9 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc) 
	/* Restart the work queue in case no characters kick it off. Safe if

	   already running */

	if (work)

		schedule_delayed_work(&tty->buf.work, 1);

		schedule_work(&tty->buf.work);

	if (o_work)

		schedule_delayed_work(&o_tty->buf.work, 1);

		schedule_work(&o_tty->buf.work);

	mutex_unlock(&tty->ldisc_mutex);

	tty_unlock();

	return retval;
@@ -830,12 +830,12 @@ void tty_ldisc_hangup(struct tty_struct *tty) 


	/*

	 * this is like tty_ldisc_halt, but we need to give up

	 * the BTM before calling cancel_delayed_work_sync,

	 * which may need to wait for another function taking the BTM

	 * the BTM before calling cancel_work_sync, which may

	 * need to wait for another function taking the BTM

	 */

	clear_bit(TTY_LDISC, &tty->flags);

	tty_unlock();

	cancel_delayed_work_sync(&tty->buf.work);

	cancel_work_sync(&tty->buf.work);

	mutex_unlock(&tty->ldisc_mutex);



	tty_lock();

include/linux/kbd_kern.h

+1 −1
Original line number Diff line number Diff line
@@ -159,7 +159,7 @@ static inline void con_schedule_flip(struct tty_struct *t) 
	if (t->buf.tail != NULL)

		t->buf.tail->commit = t->buf.tail->used;

	spin_unlock_irqrestore(&t->buf.lock, flags);

	schedule_delayed_work(&t->buf.work, 0);

	schedule_work(&t->buf.work);

}



#endif

include/linux/tty.h

+1 −1
Original line number Diff line number Diff line
@@ -82,7 +82,7 @@ struct tty_buffer { 




struct tty_bufhead {

	struct delayed_work work;

	struct work_struct work;

	spinlock_t lock;

	struct tty_buffer *head;	/* Queue head */

	struct tty_buffer *tail;	/* Active buffer */