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tty: stop using "delayed_work" in the tty layer

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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
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14
drivers/tty/tty_ldisc.c
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@ -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();