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

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Date:   Wed, 17 Aug 2022 08:44:45 +0800
From:   Li Hua <hucool.lihua@...wei.com>
To:     <peterz@...radead.org>
CC:     <bristot@...hat.com>, <bsegall@...gle.com>,
        <dietmar.eggemann@....com>, <hucool.lihua@...wei.com>,
        <juri.lelli@...hat.com>, <linux-kernel@...r.kernel.org>,
        <mgorman@...e.de>, <mingo@...hat.com>, <rostedt@...dmis.org>,
        <stable@...r.kernel.org>, <vincent.guittot@...aro.org>,
        <vschneid@...hat.com>
Subject: Re: [PATCH -next] sched/cputime: Fix the bug of reading time backward from /proc/stat

It's unreadable, I'm sorry about that.

The CPU statistics time read from /proc/stat should only be incremented. The bug
I found is that the value read latest is smaller than the former.

The root cause of the problem is that the "vtime->utime" and "delta" are temporarily
added to the stack and show to the user. The value of 'vtime->utime + delta' depends
on which task the CPU is executing. As bellow:
show_stat -> kcpustat_cpu_fetch -> kcpustat_cpu_fetch_vtime -> cpustat[CPUTIME_USER] += vtime->utime + delta