Re: [Question] Reading /proc/stat has a time backward issue

[Lihua (lihua, ran)]

ping...

Any good suggestions?

thanks all.

在 2022/7/27 12:02, Lihua (lihua, ran) 写道:
> Hi all,
>
> I found a 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, I found that 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
>
> Because the values ​​of utime、 stime and delta are temporarily written to cpustat. Therefore, there are two problems  read from /proc/stat:
> 1. There may be a regression phenomenon;
> 2. When there are many tasks, the statistics are not accurate enough when utime and stime do not exceed one TICK.
> The time goes back is counterintuitive, and I want to discuss whether there is a good solution without compromising performance.
>
> Thanks a lot.