Re: [PATCH -next] sched/cputime: Fix the bug of reading time backward from /proc/stat

[Peter Zijlstra]

On Sat, Aug 13, 2022 at 08:01:02AM +0800, Li Hua wrote:
> The problem that the statistical time goes backward, the value read first is 319, and the value read again is 318. As follows：
> first：
> cat /proc/stat |  grep cpu1
> cpu1    319    0    496    41665    0    0    0    0    0    0
> then：
> cat /proc/stat |  grep cpu1
> cpu1    318    0    497    41674    0    0    0    0    0    0
> 
> Time goes back, which is counterintuitive.
> 
> After debug this, The problem is caused by the implementation of kcpustat_cpu_fetch_vtime. As follows：
> 
>                               CPU0                                                                          CPU1
> First:
> show_stat():
>     ->kcpustat_cpu_fetch()
>         ->kcpustat_cpu_fetch_vtime()
>             ->cpustat[CPUTIME_USER] = kcpustat_cpu(cpu) + vtime->utime + delta;              rq->curr is in user mod
>              ---> When CPU1 rq->curr running on userspace, need add utime and delta
>                                                                                              --->  rq->curr->vtime->utime is less than 1 tick
> Then:
> show_stat():
>     ->kcpustat_cpu_fetch()
>         ->kcpustat_cpu_fetch_vtime()
>             ->cpustat[CPUTIME_USER] = kcpustat_cpu(cpu);                                     rq->curr is in kernel mod
>             ---> When CPU1 rq->curr running on kernel space, just got kcpustat

This is unreadable, what?!?