Messages in this thread | | | Date | Wed, 27 Jul 2022 12:02:51 +0800 | From | "Lihua (lihua, ran)" <> | Subject | [Question] Reading /proc/stat has a time backward issue |
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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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