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

[Lihua (lihua, ran)]

ping...

Your suggestions are valuable, I don't have a good way to fix this.

thanks all.

在 2022/8/4 15:44, Lihua (lihua, ran) 写道:
> 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.