linux-kernel - [Question] Reading /proc/stat has a time backward issue

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Message-ID: <85d5087b-450c-351f-270d-c61303cf3187@huawei.com>
Date:   Wed, 27 Jul 2022 12:02:51 +0800
From:   "Lihua (lihua, ran)" <hucool.lihua@...wei.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "open list:SCHEDULER" <linux-kernel@...r.kernel.org>
Subject: [Question] Reading /proc/stat has a time backward issue

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.