Re: [PATCH] x86/aperfmperf: use time_is_before_jiffies(a + b) to replace "jiffies - a > b"

From: Peter Zijlstra
Date: Wed Jul 27 2022 - 12:01:06 EST


On Wed, Jul 27, 2022 at 11:14:05AM +0800, Yu Zhe wrote:
> time_is_before_jiffies deals with timer wrapping correctly.

Please explain how the current code does not.

>
> Signed-off-by: Yu Zhe <yuzhe@xxxxxxxxxxxx>
> ---
> arch/x86/kernel/cpu/aperfmperf.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c
> index 1f60a2b27936..22e0bac3fffe 100644
> --- a/arch/x86/kernel/cpu/aperfmperf.c
> +++ b/arch/x86/kernel/cpu/aperfmperf.c
> @@ -423,7 +423,7 @@ unsigned int arch_freq_get_on_cpu(int cpu)
> * Bail on invalid count and when the last update was too long ago,
> * which covers idle and NOHZ full CPUs.
> */
> - if (!mcnt || (jiffies - last) > MAX_SAMPLE_AGE)
> + if (!mcnt || time_is_before_jiffies(last + MAX_SAMPLE_AGE))
> goto fallback;
>
> return div64_u64((cpu_khz * acnt), mcnt);
> --
> 2.11.0
>