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

From: Lihua (lihua, ran)
Date: Mon Aug 08 2022 - 08:23:57 EST


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.