linux-kernel - Re: [PATCHv3 0/4] Improve fallback LPJ calculation

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Date:	Fri, 11 Mar 2011 15:26:40 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Phil Carmody <ext-phil.2.carmody@...ia.com>
Cc:	gregkh@...e.de, linux-kernel@...r.kernel.org, sboyd@...eaurora.org
Subject: Re: [PATCHv3 0/4] Improve fallback LPJ calculation

On Thu, 10 Mar 2011 16:48:03 +0200
Phil Carmody <ext-phil.2.carmody@...ia.com> wrote:

> 
> Apologies for picking on you, Andrew, and sending this out of the blue,

Someone has to do it.  This code hasn't really been touched for half a
decade or more.

> but I didn't have much luck with my previous attempt, and I quite like
> this patchset, so thought it was worth trying again.
> (http://lkml.org/lkml/2010/9/28/121)
> 
> The guts of this patchset are in patch 2/4. The motivation for that patch
> is that currently our OMAP calibrates itself using the trial-and-error 
> binary chop fallback that some other architectures no longer need to 
> perform. This is a lengthy process, taking 0.2s in an environment where 
> boot time is of great interest.
> 
> Patch 2/4 has two optimisations. Firstly, it replaces the initial repeated-
> doubling to find the relevant power of 2 with a tight loop that just does 
> as much as it can in a jiffy. Secondly, it doesn't binary chop over an 
> entire power of 2 range, it choses a much smaller range based on how much 
> it squeezed in, and failed to squeeze in, during the first stage. Both 
> are significant optimisations, and bring our calibration down from 23 
> jiffies to 5, and, in the process, often arrive at a more accurate lpj 
> value.

A worthwhile benefit.

> The 'bands' and 'sub-logarithmic' growth may look over-engineered, but 
> they only cost a small level of inaccuracy in the initial guess (for all 
> architectures) in order to avoid the very large inaccuracies that appeared
> during testing (on x86_64 architectures, and presumably others with less 
> metronomic operation). Note that due to the existence of the TSC and 
> other timers, the x86_64 will not typically use this fallback routine, 
> but I wanted to code defensively, able to cope with all kinds of processor 
> behaviours and kernel command line options.
> 
> Patch 3/4 is an additional trap for the nightmare scenario where the
> initial estimate is very inaccurate, possibly due to things like SMIs.
> It simply retries with a larger bound.
> 
> 1/4 is simply cosmetic to prepare for 2/4. 
> 4/4 is simply to assist testing and not intended for integration.
> 
> 
> Changes since initial RFC:
>  - More informational commit messages
>  - Inserted patch 3/4 after discovering that x86_64 had a failure case.

OK, I guess we'll toss it in there and see how it goes.
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