linux-kernel - Re: Question about /proc/uptime

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Date:	Mon, 30 Dec 2013 16:11:10 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Rob Landley <rob@...dley.net>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: Question about /proc/uptime

On 12/30, Martin Schwidefsky wrote:
<
> On Fri, 27 Dec 2013 15:45:04 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > Add Frederic, I am not sure I understand this correctly.
> >
> > On 12/25, Rob Landley wrote:
> > >
> > > There are two values here, the first is seconds since boot time (which
> > > is just elapsed time; at one point it was ajusted for suspend and that
> > > was revered as confusing).
> >
> > Hmm, uptime_proc_show() still uses get_monotonic_boottime(), this should
> > include suspend time?
>
> The first value of /proc/uptime is the amount of time the system has been
> running, the sum of the suspend time is not included.

Hmm. It uses get_monotonic_boottime() and this helper adds
timekeeper->total_sleep_time to the returned value? Even the comment says

	* This is similar to CLOCK_MONTONIC/ktime_get_ts, but also
	* includes the time spent in suspend.

> timekeeping_resume()
> is supposed to take care of that.

Not sure I understand... except that timekeeping_resume() does
__timekeeping_inject_sleeptime().

> > > The second value isn't documented but looking at fs/proc/uptime.c I
> > > think it's idle time?
> >
> > And this cpustat[CPUTIME_IDLE] doesn't match get_idle_time(), I guess
> > it only counts ticks.
>
> The second value of /proc/uptime is the sum of the idle time of all cpus.
> The value should roughly match the sum over get_idle_time() for all cpus.
> The difference is that get_idle_time() uses arch_idle_time to add the
> time for the currently running idle period as well (at least for s390).

Only if defined(arch_idle_time) ?

Otherwise get_idle_time() uses get_cpu_idle_time_us() and
CONFIG_NO_HZ_COMMON makes a difference.

> That does make a difference if the cpus stay idle for long periods of
> time.

Yes.

Oleg.

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