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

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