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SubjectRe: [PATCH -next] sched/cputime: Fix the bug of reading time backward from /proc/stat
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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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