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From: =?UTF-8?Q?Fredrik_Markstr=C3=B6m?= <fredrik.markstrom@gmail.com>
Date: Tue, 7 Jul 2015 14:10:56 +0200
Message-ID: <CAKdL+dS6kHVsF08vkiXajH7pzoXAA1pe7gtx2JCotvBA24txkQ@mail.gmail.com>
Subject: Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to
 the actual runtime.
To: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, mingo@redhat.com,
        linux-kernel@vger.kernel.org, Rik van Riel <riel@redhat.com>,
        Jason Low <jason.low2@hp.com>
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Just to let you know, I've tested your last patch and it solves all my
original problems (which is should since the code is functionally
equivalent).

/Fredrik

On Tue, Jul 7, 2015 at 10:09 AM, Peter Zijlstra <peterz@infradead.org> 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
>
> since: stime_i + utime_i = rtime_i, we can write: stime_i = rtime_i -
> utime_i and substitute in the above:
>
>                  = rtime_i+1 - (rtime_i - utime_i)
>
> Rearrange terms:
>
>                 = (rtime_i+1 - rtime_i) + utime_i
>
> And since we have: rtime_i+1 >= rtime_i, which we can write like:
> rtime_i+1 - rtime_i >= 0, substitution gives:
>
>                 >= utime_i
>
> And we've proven that the new utime must be equal or greater than the
> previous utime, because rtime is monotonic.


-- 
/Fredrik
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