linux-kernel - Re: [RFC PATCH 01/14] sched: track the runnable average on a per-task entitiy basis

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Message-ID: <CAPM31RLZGdL-KqyQo0cWAAS-O3W+=dBzBD3G2NRmNTS+sGbgXA@mail.gmail.com>
Date:	Fri, 17 Feb 2012 03:43:11 -0800
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, Venki Pallipadi <venki@...gle.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Mike Galbraith <efault@....de>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 01/14] sched: track the runnable average on a per-task
 entitiy basis

On Wed, Feb 15, 2012 at 3:37 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Wed, 2012-02-01 at 17:38 -0800, Paul Turner wrote:
>> +       /*
>> +        * Use 1024us as the unit of measurement since it's a reasonable
>> +        * approximation of 1ms and fast to compute.
>> +        */
>> +       delta >>= 10;
>
> ns -> us ?, text talks about ms, slightly confusing

Yes, comment was incorrect; I'd noticed this also on a re-read -- it's fixed.

>
>> +       if (!delta)
>> +               return 0;
>> +       sa->last_runnable_update = now;
>> +
>> +       delta_w = sa->runnable_avg_period % 1024;
>
> so delta_w is the chunk of this p we already accounted.
>
>> +       if (delta + delta_w >= 1024) {
>
> if delta pushes us over 1024*1024 ns (~1ms) we roll a window.
>
>> +               /* period roll-over */
>> +               decayed = 1;
>> +
>> +               delta_w = 1024 - delta_w;
>
> The distance we need to reach the next window.
>
>> +               BUG_ON(delta_w > delta);
>
> somehow reading this code took forever, this suggests clarification,
> either through better variable names or more comments. Could also mean
> I'm a moron and should get more sleep or so :-)

No... this bit is definitely fiddly, I definitely got it wrong more
than once writing it down.  And then got it again wrong later when
"optimizing" :-).

It deserves more comments, I will add.
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