[tip: sched/urgent] sched/clock: Use static_branch_likely() with sched_clock_running

[tip-bot2 for Zhenzhong Duan]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     c5105d764e0214bcc4c6d40d7ba231d01b2e9dda
Gitweb:        https://git.kernel.org/tip/c5105d764e0214bcc4c6d40d7ba231d01b2e9dda
Author:        Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
AuthorDate:    Wed, 27 Nov 2019 16:37:28 +08:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 29 Nov 2019 08:10:54 +01:00

sched/clock: Use static_branch_likely() with sched_clock_running

sched_clock_running is enabled early at bootup stage and never
disabled. So hint that to the compiler by using static_branch_likely()
rather than static_branch_unlikely().

The branch probability mis-annotation was introduced in the original
commit that converted the plain sched_clock_running flag to a static key:

  46457ea464f5 ("sched/clock: Use static key for sched_clock_running")

Steve further notes:

  | Looks like the confusion was the moving of the "!":
  |
  | -       if (unlikely(!sched_clock_running))
  | +       if (!static_branch_unlikely(&sched_clock_running))
  |
  | Where, it was unlikely that !sched_clock_running would be true, but
  | because the "!" was moved outside the "unlikely()" it makes the test
  | "likely()". That is, if we added an intermediate step, it would have
  | been:
  |
  |         if (!likely(sched_clock_running))
  |
  | which would have prevented the mistake that this patch fixes.

  [ mingo: Edited the changelog. ]

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: bsegall@xxxxxxxxxx
Cc: dietmar.eggemann@xxxxxxx
Cc: juri.lelli@xxxxxxxxxx
Cc: mgorman@xxxxxxx
Cc: vincent.guittot@xxxxxxxxxx
Link: https://lkml.kernel.org/r/1574843848-26825-1-git-send-email-zhenzhong.duan@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/clock.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 1152259..12bca64 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -370,7 +370,7 @@ u64 sched_clock_cpu(int cpu)
 	if (sched_clock_stable())
 		return sched_clock() + __sched_clock_offset;
 
-	if (!static_branch_unlikely(&sched_clock_running))
+	if (!static_branch_likely(&sched_clock_running))
 		return sched_clock();
 
 	preempt_disable_notrace();
@@ -393,7 +393,7 @@ void sched_clock_tick(void)
 	if (sched_clock_stable())
 		return;
 
-	if (!static_branch_unlikely(&sched_clock_running))
+	if (!static_branch_likely(&sched_clock_running))
 		return;
 
 	lockdep_assert_irqs_disabled();
@@ -460,7 +460,7 @@ void __init sched_clock_init(void)
 
 u64 sched_clock_cpu(int cpu)
 {
-	if (!static_branch_unlikely(&sched_clock_running))
+	if (!static_branch_likely(&sched_clock_running))
 		return 0;
 
 	return sched_clock();