linux-kernel - [PATCH] tty: Only wakeup the line discipline idle queue when queue is active

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Date:	Tue, 18 Dec 2012 15:48:50 +0100
From:	Ivo Sieben <meltedpianoman@...il.com>
To:	<linux-serial@...r.kernel.org>, Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>
CC:	Oleg Nesterov <oleg@...hat.com>, <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Ivo Sieben <meltedpianoman@...il.com>
Subject: [PATCH] tty: Only wakeup the line discipline idle queue when queue is active

Before waking up the tty line discipline idle queue first check if the queue is
active (non empty). This prevents unnecessary entering the critical section in
the wake_up() function and therefore avoid needless scheduling overhead on a
PREEMPT_RT system caused by two processes being in the same critical section.

Signed-off-by: Ivo Sieben <meltedpianoman@...il.com>
---

 Remark:
 This patch has kind of a long history... I first tried to prevent the critical
 section in the wakeup() function itself by a change in the scheduler. But after
 review remarks from Oleg Nesterov it turned out that using the
 waitqueue_active() was a much nicer way to prevent it. See also
 https://lkml.org/lkml/2012/10/25/159

 drivers/tty/tty_ldisc.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index c578229..e96d187 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -64,7 +64,9 @@ static void put_ldisc(struct tty_ldisc *ld)
 		return;
 	}
 	raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-	wake_up(&ld->wq_idle);
+
+	if (waitqueue_active(&ld->wq_idle))
+		wake_up(&ld->wq_idle);
 }

 /**
-- 
1.7.9.5

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