[PATCH 1/3] wait: introduce wait_event_cmd_exclusive

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

It's just a variant of wait_event_cmd, with exclusive flag being set.

For cases like RAID5, which puts many processes to sleep until 1/4
resources are free, a wake_up wakes up all processes to run, but
there is one process being able to get the resource as it's protected
by a spin lock. That ends up introducing heavy lock contentions, and
hurts performance badly.

Here introduce wait_event_cmd_exclusive to relieve the lock contention
naturally by letting wake_up() just wake up one process.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
---
 include/linux/wait.h | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 2db8334..6c3b4de 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -358,10 +358,18 @@ do {									\
 	__ret;								\
 })
 
-#define __wait_event_cmd(wq, condition, cmd1, cmd2)			\
-	(void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
+#define __wait_event_cmd(wq, condition, cmd1, cmd2, exclusive)		\
+	(void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, exclusive, 0, \
 			    cmd1; schedule(); cmd2)
 
+
+#define wait_event_cmd_exclusive(wq, condition, cmd1, cmd2)			\
+do {									\
+	if (condition)							\
+		break;							\
+	__wait_event_cmd(wq, condition, cmd1, cmd2, 1);			\
+} while (0)
+
 /**
  * wait_event_cmd - sleep until a condition gets true
  * @wq: the waitqueue to wait on
@@ -380,7 +388,7 @@ do {									\
 do {									\
 	if (condition)							\
 		break;							\
-	__wait_event_cmd(wq, condition, cmd1, cmd2);			\
+	__wait_event_cmd(wq, condition, cmd1, cmd2, 0);			\
 } while (0)
 
 #define __wait_event_interruptible(wq, condition)			\
-- 
1.9.0