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

From: Peter Zijlstra
Date: Tue Sep 20 2011 - 09:04:49 EST


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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