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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <1355842130-15482-1-git-send-email-meltedpianoman@gmail.com>
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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/