linux-kernel - Re: [PATCH v2] sched/cputime: Ensure accurate utime and stime ratio in cputime

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Date:   Mon, 16 Jul 2018 15:35:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Tejun Heo <tj@...nel.org>, baoyou.xie@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/cputime: Ensure accurate utime and stime ratio
 in cputime_adjust()

On Mon, Jul 09, 2018 at 10:58:43PM +0800, Xunlei Pang wrote:
> If users access "/proc/pid/stat", the utime and stime ratio in the
> current SAMPLE period are excepted, but currently cputime_adjust()
> always calculates with the ratio of the WHOLE lifetime of the process.
> 
> This results in inaccurate utime and stime in "/proc/pid/stat". For
> example,  a process runs for a while with "50% usr, 0% sys", then
> followed by "100% sys". For later while, the following is excepted:
>   0.0 usr,  100.0 sys
> but we got:
>   10.0 usr,  90.0 sys
> 
> This patch uses the accurate ratio in cputime_adjust() to address the
> issue. A new task_cputime type field is added in prev_cputime to record
> previous task_cputime so that we can get the elapsed times as the accurate
> ratio.

This still does not explain anything...