[PATCH 3/3] RFC: Solved unnecessary schedule latency in the TTY layer (3/3)

[Ivo Sieben <meltedpianoman@xxxxxxxxx>]

Solved unnecessary schedule latency in the TTY layer when used in
realtime context:
The global wait_queue that is used for line discipline idle handling is
moved to a separate wait_queue for each line instance. This prevents
unnecessary blocking on one line, because of idle handling on another
line.

Note: In a PREEMPT_RT system "normal" spin locks behave like mutexes and
no interrupts (and therefor no scheduling) is disabled.

Signed-off-by: Ivo Sieben <meltedpianoman@xxxxxxxxx>
---
 drivers/tty/tty_ldisc.c   |    7 ++++---
 include/linux/tty_ldisc.h |    2 ++
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 30836cc..77eb299 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -28,7 +28,6 @@
 
 static DEFINE_RAW_SPINLOCK(tty_ldisc_lock);
 static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
-static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_idle);
 /* Line disc dispatch table */
 static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
 
@@ -65,7 +64,7 @@ static void put_ldisc(struct tty_ldisc *ld)
 		return;
 	}
 	raw_spin_unlock_irqrestore(&tty_ldisc_lock, flags);
-	wake_up(&tty_ldisc_idle);
+	wake_up(&ld->wq_idle);
 }
 
 /**
@@ -200,6 +199,8 @@ static struct tty_ldisc *tty_ldisc_get(int disc)
 
 	ld->ops = ldops;
 	atomic_set(&ld->users, 1);
+	init_waitqueue_head(&ld->wq_idle);
+
 	return ld;
 }
 
@@ -538,7 +539,7 @@ static void tty_ldisc_flush_works(struct tty_struct *tty)
 static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout)
 {
 	long ret;
-	ret = wait_event_timeout(tty_ldisc_idle,
+	ret = wait_event_timeout(tty->ldisc->wq_idle,
 			atomic_read(&tty->ldisc->users) == 1, timeout);
 	return ret > 0 ? 0 : -EBUSY;
 }
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index ff7dc08..fb79dd8 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -110,6 +110,7 @@
 #include <linux/fs.h>
 #include <linux/wait.h>
 #include <linux/pps_kernel.h>
+#include <linux/wait.h>
 
 struct tty_ldisc_ops {
 	int	magic;
@@ -154,6 +155,7 @@ struct tty_ldisc_ops {
 struct tty_ldisc {
 	struct tty_ldisc_ops *ops;
 	atomic_t users;
+	wait_queue_head_t wq_idle;
 };
 
 #define TTY_LDISC_MAGIC	0x5403
-- 
1.7.0.4


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