Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to the actual runtime.

[Frederic Weisbecker <fweisbec@gmail.com>]

On Tue, Jul 07, 2015 at 10:09:13AM +0200, Peter Zijlstra wrote:
> On Tue, Jul 07, 2015 at 09:59:54AM +0200, Peter Zijlstra wrote:
> > > > +	/*
> > > > +	 * Make sure stime doesn't go backwards; this preserves monotonicity
> > > > +	 * for utime because rtime is monotonic.
> > > > +	 *
> > > > +	 *  utime_i+1 = rtime_i+1 - stime_i
> > > > +	 *            = rtime_i+1 - (rtime_i - stime_i)
> > > > +	 *            = (rtime_i+1 - rtime_i) + stime_i
> > > > +	 *            >= stime_i
> > > > +	 */
> 
> Argh, just noticed I messed that up, it should read:
> 
> +       /*
> +        * Make sure stime doesn't go backwards; this preserves monotonicity
> +        * for utime because rtime is monotonic.
> +        *
> +        *  utime_i+1 = rtime_i+1 - stime_i
> +        *            = rtime_i+1 - (rtime_i - utime_i)
> +        *            = (rtime_i+1 - rtime_i) + utime_i
> +        *            >= utime_i
> +        */
> 
> I got some [us] confusion. Typing is hard.
> 
> So we compute: utime = rtime - stime, which we'll denote as:
> 
> 	utime_i+1 = rtime_i+1 - stime_i

But I don't get how you come to that.

Imagine the following rounds:

    utime:2 stime:2 rtime:4 --> prev->utime = 2 prev->stime = 2

    utime:2 stime:6 rtime:8 --> prev->utime = 2 prev->stime = 6

So here if I apply your above formula we have:

     utime_i+1:2 = rtime_i+1:8 - stime_i:2

Which doesn't work, so probably I still misunderstand those _i things...