Messages in this thread | | | Date | Mon, 5 Sep 2022 11:47:41 +0800 | Subject | Re: [PATCH -next] sched/cputime: Fix the bug of reading time backward from /proc/stat | From | zhengzucheng <> |
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Assume that a CPU time“ A” is read from /proc/stat, and after a while, a CPU time “B” is read. If T = B – A < 0, T is identified as a large number as an unsigned integer. As a result, the CPU usage calculated by this way will be abnormally high. It seems to be a problem to be fixed.
original link: https://lore.kernel.org/lkml/20220813000102.42051-1-hucool.lihua@huawei.com/
在 2022/8/15 16:15, Peter Zijlstra 写道: > On Sat, Aug 13, 2022 at 08:01:02AM +0800, Li Hua wrote: >> The 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, 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 > This is unreadable, what?!? > .
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