Messages in this thread | | | Date | Thu, 4 Aug 2022 15:44:08 +0800 | Subject | Re: [Question] Reading /proc/stat has a time backward issue | From | "Lihua (lihua, ran)" <> |
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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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