linux-kernel - Re: [PATCH v2] fs/proc/uptime.c: fix idle time reporting in /proc/uptime

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Message-ID: <CANn89iJV+VUgS5mBmtN6An83pRS4LU61efw=VLBvM1_FGYCN5A@mail.gmail.com>
Date:   Fri, 27 Aug 2021 14:40:50 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Josh Don <joshdon@...gle.com>
Cc:     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>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Luigi Rizzo <lrizzo@...gle.com>
Subject: Re: [PATCH v2] fs/proc/uptime.c: fix idle time reporting in /proc/uptime

On Fri, Aug 27, 2021 at 9:55 AM Josh Don <joshdon@...gle.com> wrote:
>
> /proc/uptime reports idle time by reading the CPUTIME_IDLE field from
> the per-cpu kcpustats. However, on NO_HZ systems, idle time is not
> continually updated on idle cpus, leading this value to appear
> incorrectly small.
>
> /proc/stat performs an accounting update when reading idle time; we can
> use the same approach for uptime.
>
> With this patch, /proc/stat and /proc/uptime now agree on idle time.
> Additionally, the following shows idle time tick up consistently on an
> idle machine:
> (while true; do cat /proc/uptime; sleep 1; done) | awk '{print $2-prev; prev=$2}'
>
> Reported-by: Luigi Rizzo <lrizzo@...gle.com>
> Signed-off-by: Josh Don <joshdon@...gle.com>
> ---
>

SGTM thanks !

Reviewed-by: Eric Dumazet <edumazet@...gle.com>