linux-kernel - Re: [PATCH] sched/clock: use static_branch_likely() check at sched_clock

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Date:   Wed, 27 Nov 2019 10:14:56 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Zhenzhong Duan <zhenzhong.duan@...cle.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, mgorman@...e.de
Subject: Re: [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@...cle.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@...dmis.org>

-- Steve

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