linux-kernel - Re: [bisected] pty performance problem

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Date:	Mon, 23 Nov 2009 06:00:31 +0100
From:	Mike Galbraith <efault@....de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Ingo Molnar <mingo@...e.hu>, Robert Swan <swan.r.l@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [bisected] pty performance problem

On Sun, 2009-11-22 at 12:23 +0000, Alan Cox wrote:
> On Sun, 22 Nov 2009 07:39:26 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > (Cc:-ed Alan and Linus - mail repeated below. eot.)
> 
> I saw it and replied already. The kernel now queues the work for the pty
> as it does normal tty devices. If the CPU load is that low and nothing
> else is happening it makes me wonder what the scheduler thinks it is
> doing ?

Hm.  Looks to me like it's doing what it was told to do.

diff --git a/drivers/char/tty_buffer.c b/drivers/char/tty_buffer.c
index 66fa4e1..92a0864 100644
--- a/drivers/char/tty_buffer.c
+++ b/drivers/char/tty_buffer.c
@@ -495,7 +495,7 @@ void tty_flip_buffer_push(struct tty_struct *tty)
 	if (tty->low_latency)
 		flush_to_ldisc(&tty->buf.work.work);
 	else
-		schedule_delayed_work(&tty->buf.work, 1);
+		schedule_delayed_work(&tty->buf.work, 0);
 }
 EXPORT_SYMBOL(tty_flip_buffer_push);
 
Telling it to execute now made test proggy happy.. and likely broke tons
of things that need a delay there.  So, what's wrong with delaying, when
that's what the customer asked for?  /me must be missing something.  It
could know that no delay is needed?

	-Mike

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