[RFC PATCH] wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event()

[Mateusz Guzik]

In vast majority of cases the condition determining whether the thread
can proceed is true after the first wake up.

However, even in that there is an additional call to prepare_to_wait_event(),
resulting in a spurious irq + lock trip.

Then it calls into finish_wait() to unlink itself.

pre-check the condition instead after waking up instead.

Stats gathared during a kernel build:
bpftrace -e 'kprobe:prepare_to_wait_event,kprobe:finish_wait \
		 { @[probe] = count(); }'

@[kprobe:finish_wait]: 392483
@[kprobe:prepare_to_wait_event]: 778690

As in calls to prepare_to_wait_event() almost double calls to
finish_wait(). This evens out with the patch.

Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---

One may worry about using "condition" twice. However, macros leading up
to this one already do it, so it should be fine.

Also one may wonder about fences -- to my understanding going off and on
CPU guarantees a full fence.

 include/linux/wait.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 2bdc8f47963b..965a19809c7e 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -316,6 +316,9 @@ extern void init_wait_entry(struct wait_queue_entry *wq_entry, int flags);
 		}								\
 										\
 		cmd;								\
+										\
+		if (condition)							\
+			break;							\
 	}									\
 	finish_wait(&wq_head, &__wq_entry);					\
 __out:	__ret;									\
-- 
2.43.0