From: "Lihua (lihua, ran)" <hucool.lihua@huawei.com>
To: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
"open list:SCHEDULER" <linux-kernel@vger.kernel.org>
Subject: Re: [Question] Reading /proc/stat has a time backward issue
Date: Thu, 4 Aug 2022 15:44:08 +0800 [thread overview]
Message-ID: <1f1f625a-148d-0398-f840-1f9b4e964189@huawei.com> (raw)
In-Reply-To: <85d5087b-450c-351f-270d-c61303cf3187@huawei.com>
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.
next prev parent reply other threads:[~2022-08-04 7:44 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-27 4:02 [Question] Reading /proc/stat has a time backward issue Lihua (lihua, ran)
2022-08-04 7:44 ` Lihua (lihua, ran) [this message]
2022-08-08 12:23 ` Lihua (lihua, ran)
2022-08-11 12:49 ` Frederic Weisbecker
2022-08-13 18:58 ` Li Hua
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