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

[Yu Zhe <yuzhe@nfschina.com>]

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
>>
>