From: Mateusz Guzik Date: Mon, 3 Mar 2025 23:04:09 +0000 (+0100) Subject: wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event() X-Git-Tag: v6.15-rc1~251^2~2^2 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=84654c7f47307692d47ea914d01287c8c54b3532;p=thirdparty%2Flinux.git wait: avoid spurious calls to prepare_to_wait_event() in ___wait_event() 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 Link: https://lore.kernel.org/r/20250303230409.452687-4-mjguzik@gmail.com Signed-off-by: Christian Brauner --- diff --git a/include/linux/wait.h b/include/linux/wait.h index 6d90ad9744087..3503fe822e38b 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; \