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

[Yu Zhe]

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@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