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

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Date:	Thu, 16 Feb 2012 00:37:08 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Turner <pjt@...gle.com>
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, 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

> +       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 :-)
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