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

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Message-ID: <20180716133751.GC2494@hirez.programming.kicks-ass.net>
Date:   Mon, 16 Jul 2018 15:37:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     xlpang@...ux.alibaba.com, tglx@...utronix.de, frederic@...nel.org,
        lcapitulino@...hat.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, hpa@...or.com, tj@...nel.org,
        mingo@...nel.org
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/cputime: Ensure accurate utime and stime
 ratio in cputime_adjust()

On Sun, Jul 15, 2018 at 04:36:17PM -0700, tip-bot for Xunlei Pang wrote:
> Commit-ID:  8d4c00dc38a8aa30dae8402955e55e7b34e74bc8
> Gitweb:     https://git.kernel.org/tip/8d4c00dc38a8aa30dae8402955e55e7b34e74bc8
> Author:     Xunlei Pang <xlpang@...ux.alibaba.com>
> AuthorDate: Mon, 9 Jul 2018 22:58:43 +0800
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Mon, 16 Jul 2018 00:28:31 +0200
> 
> sched/cputime: Ensure accurate utime and stime ratio in cputime_adjust()
> 
> 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 get:
> 
>   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.

Ingo, please make this one go away. I still have no idae what the
problem is and I've not had time to reverse engineer the patch.

The previous (v1) Changelog was a pile of incoherent rambling and the
above doesn't explain anything much.

I want a clear description of the problem and a coherent explanation of
the proposed solution without having to reverse engineer the actual
patch.