[RFC v2 1/2] swait: add idle variants which don't contribute to load average

[Luis R. Rodriguez]

There are cases where folks are using an interruptible swait when
using kthreads. This is rather confusing given you'd expect
interruptible waits to be -- interruptible, but kthreads are not
interruptible ! The reason for such practice though is to avoid
having these kthreads contribute to the system load average.

When systems are idle some kthreads may spend a lot of time blocking if
using swait_event_timeout(). This would contribute to the system load
average. On systems without preemption this would mean the load average
of an idle system is bumped to 2 instead of 0. On systems with PREEMPT=y
this would mean the load average of an idle system is bumped to 3
instead of 0.

This adds proper API using TASK_IDLE to make such goals explicit and
avoid confusion.

Suggested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
---
 include/linux/swait.h | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/linux/swait.h b/include/linux/swait.h
index 2c700694d50a..105c70e23286 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -194,4 +194,29 @@ do {									\
 	__ret;								\
 })
 
+#define __swait_event_idle(wq, condition)				\
+	___swait_event(wq, condition, TASK_IDLE, 0, schedule())
+
+#define swait_event_idle(wq, condition)					\
+({									\
+	int __ret = 0;							\
+	if (!(condition))						\
+		__ret = __swait_event_idle(wq, condition);		\
+	__ret;								\
+})
+
+#define __swait_event_idle_timeout(wq, condition, timeout)		\
+	___swait_event(wq, ___wait_cond_timeout(condition),		\
+		       TASK_IDLE, timeout,				\
+		       __ret = schedule_timeout(__ret))
+
+#define swait_event_idle_timeout(wq, condition, timeout)		\
+({									\
+	long __ret = timeout;						\
+	if (!___wait_cond_timeout(condition))				\
+		__ret = __swait_event_idle_timeout(wq,			\
+						   condition, timeout);	\
+	__ret;								\
+})
+
 #endif /* _LINUX_SWAIT_H */
-- 
2.11.0