[3/3] wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event()

Commit Message

Mateusz Guzik March 3, 2025, 11:04 p.m. UTC

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

However, even in that case the thread ends up calling into
prepare_to_wait_event() again, suffering a spurious irq + lock trip.

Then it calls into finish_wait() to unlink itself.

Note that in case of a pending signal the work done by
prepare_to_wait_event() gets ignored even without the change.

pre-check the condition 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 <[e-mail]>
---

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, so the now avoided lock trip changes
nothing.

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

Patch

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;									\