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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: Jef Driesen <jefdriesen@telenet.be>
Acked-by: Greg KH <gregkh@suse.de>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2011-03-22 16:17:32 -07:00
parent f741a79e98
commit f23eb2b2b2
4 changed files with 16 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
drivers/tty/tty_buffer.c
View File

@ -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);
}

14
drivers/tty/tty_ldisc.c
View File

@ -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 @@ enable:
/* 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();

2
include/linux/kbd_kern.h
View File

@ -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

2
include/linux/tty.h
View File

@ -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 */