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Date: Mon, 8 Aug 2022 20:23:46 +0800
From: "Lihua (lihua, ran)" <hucool.lihua@...wei.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
"open list:SCHEDULER" <linux-kernel@...r.kernel.org>,
<frederic@...nel.org>
Subject: Re: [Question] Reading /proc/stat has a time backward issue
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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