linux-kernel - Re: [PATCH 8/9] per-cgroup boot time

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Date:	Tue, 20 Sep 2011 15:04:20 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, xemul@...allels.com,
	paul@...lmenage.org, lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	mingo@...e.hu, jbottomley@...allels.com
Subject: Re: [PATCH 8/9] per-cgroup boot time

On Tue, 2011-09-20 at 09:37 -0300, Glauber Costa wrote:

> >> +	getboottime(&boottime);
> >> +	ts = timespec_add(boottime, ca->start_time);
> >> +	jif = ts.tv_sec;
> >>
> >>   	for_each_possible_cpu(i) {
> >>   		cpustat = per_cpu_ptr(ca->cpustat, i);
> >
> >
> > I'm confused, what does it do? You take a boot time timestamp at cgroup
> > creation, add that to all boot-time readings and print the result. How
> > does that make sense? Subtracting the start_time, maybe, that would make
> > the cgroup creation time 0, adding, not so much.
> 
> Boot time represent at which times the machine was booted. In this 
> context, at which time the container/cgroup was created. So it have to 
> be an addition.... don't really understand your question

I think we're all properly confused now :-)

getboottime() gives a time since boot, right? 

You take stamp at cgroup creation: say 50s after boot.

Then on usage you take a new getboottime() reading (which per definition
is > 50s) and add your 50s that you read previous. This results in the
cgroup having 50s of boot-time _MORE_ than the machine. Say you read at
123s, you then add your 50s timestamp, resulting in 173s to report.

If instead you did a subtraction: 123-50=73, you would have reported the
time since cgroup creation.

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