time_is_before_jiffies deals with timer wrapping correctly.

Signed-off-by: Yu Zhe <yuzhe@nfschina.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

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@nfschina.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
>

在 2022年07月28日 00:00, Peter Zijlstra 写道:

> 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.

1. If the timer wrap changes in the future you won't have to alter your code.

2. unsigned long ut;

    ut = ULONG_MAX + 4;

    printf("time_after(ut, ULONG_MAX), ut:%d, %d --> %d\n", ut,
              time_after(ut, ULONG_MAX), (ULONG_MAX - ut) < 0);

     

    In this case, time_after returns true, it's correct.

>> Signed-off-by: Yu Zhe <yuzhe@nfschina.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
>>
>

On Thu, Jul 28, 2022 at 10:55:51AM +0800, Yu Zhe wrote:
> 在 2022年07月28日 00:00, Peter Zijlstra 写道:
> 
> > 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.
> 
> 1. If the timer wrap changes in the future you won't have to alter your code.
> 
> 2. unsigned long ut;
> 
>    ut = ULONG_MAX + 4;
> 
>    printf("time_after(ut, ULONG_MAX), ut:%d, %d --> %d\n", ut,
>              time_after(ut, ULONG_MAX), (ULONG_MAX - ut) < 0);
> 
> 
>    In this case, time_after returns true, it's correct.

Now try the same with the existing code...