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

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Date:   Wed, 27 Jul 2022 18:00:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yu Zhe <yuzhe@...china.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        liqiong@...china.com
Subject: Re: [PATCH] x86/aperfmperf: use time_is_before_jiffies(a + b) to
 replace "jiffies - a > b"

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@...china.com>
> ---
>  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
>