Messages in this thread | | | Date | Mon, 8 Aug 2022 20:23:46 +0800 | Subject | Re: [Question] Reading /proc/stat has a time backward issue | From | "Lihua (lihua, ran)" <> |
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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.
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