[PATCH] sched/clock: use static_branch_likely() check at sched_clock_running

[PATCH] sched/clock: use static_branch_likely() check at sched_clock_running

[Zhenzhong Duan]

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

Fixes: 46457ea464f5 ("sched/clock: Use static key for sched_clock_running")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
---
 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();
-- 
1.8.3.1

Re: [PATCH] sched/clock: use static_branch_likely() check at sched_clock_running

[Steven Rostedt]

On Wed, 27 Nov 2019 16:37:28 +0800
Zhenzhong Duan <zhenzhong.duan@oracle.com> wrote:

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

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.

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve

> 
> Fixes: 46457ea464f5 ("sched/clock: Use static key for sched_clock_running")
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
> ---
>  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();

[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@oracle.com>
AuthorDate:    Wed, 27 Nov 2019 16:37:28 +08:00
Committer:     Ingo Molnar <mingo@kernel.org>
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@oracle.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bsegall@google.com
Cc: dietmar.eggemann@arm.com
Cc: juri.lelli@redhat.com
Cc: mgorman@suse.de
Cc: vincent.guittot@linaro.org
Link: https://lkml.kernel.org/r/1574843848-26825-1-git-send-email-zhenzhong.duan@oracle.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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();