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

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Message-ID: <20140102092823.2434c96f@mschwide>
Date:	Thu, 2 Jan 2014 09:28:23 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Rob Landley <rob@...dley.net>
Cc:	Oleg Nesterov <oleg@...hat.com>, mtk.manpages@...il.com,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: Question about /proc/uptime

On Wed, 01 Jan 2014 15:21:00 -0600
Rob Landley <rob@...dley.net> wrote:

> On 01/01/14 06:41, Martin Schwidefsky wrote:
> > On Tue, 31 Dec 2013 22:17:39 -0600
> > Rob Landley <rob@...dley.net> wrote:
> >
> >> On 12/30/13 09:26, Martin Schwidefsky wrote:
> >>> On Mon, 30 Dec 2013 16:11:10 +0100
> >>> Oleg Nesterov <oleg@...hat.com> wrote:
> >>>> Not sure I understand... except that timekeeping_resume() does
> >>>> __timekeeping_inject_sleeptime().
> >>>
> >>> Hmm, you are right. The sleeptime is added to the monotonic boottime.
> >>> So the first value of /proc/uptime is the wall-time since boot.
> >>> And the second value is combined idle time over all cpus.
> >>
> >> Is there an obvious way to query the non-suspend uptime from userspace?
> >
> > clock_gettime with CLOCK_MONOTONIC gives you the uptime minus without the
> > suspend time.
> 
> Given that the clock_gettime man page says:
> 
>    CLOCK_MONOTONIC
>      Clock that cannot be set and  represents  monotonic  time  since
>      some unspecified starting point.
> 
> Can I rely on it _continuing_ to do so in future, and if so should the 
> man page be clarified?

Good point. CLOCK_MONOTONIC is implemented in a specific way now and I
doubt that this will change any time soon, but you have no guarantee that
it will keep the property 'monotonic-time = uptime - suspend-time'.
You can not use the values in /proc/stat either as you can set any CPU
offline, which means that no CPU line contains all ticks since boot.

Without the CLOCK_MONOTONIC option I do not see a way how to get the
non-suspend uptime from user space.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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