linux-kernel - Re: questions on calc_delta

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Message-ID: <481B79EF.3090200@austin.ibm.com>
Date:	Fri, 02 May 2008 15:30:39 -0500
From:	Joel Schopp <jschopp@...tin.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Anton Blanchard <anton@....ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: questions on calc_delta_mine() in sched.c

>>      if (unlikely(!lw->inv_weight))
>>          lw->inv_weight = (WMULT_CONST-lw->weight/2) / (lw->weight+1);
>>
>>
>> Q1) This code is hit often in scenarios I run, is this really unlikely for 
>> others?
> 
> I think it became a lot more likely recently, perhaps removing that
> unlikely is not such a bad idea.

Especially if you remove the divide in the other locations, then this might 
even be likely.

> 
>> Q2) The rest of the code in sched.c seems to make inv_weight == 
>> WMULT_CONST/weight and I was wondering if you could explain why this 
>> instance is different.
> 
> because the rest of the code is wrong, there are only 2 other sites, and
> I have a patch that removes those div64_64() with =0;
> 
> The idea is to use rounding division: (x + y/2) / y
> but we can't because 'x' is touching the limits of our modulo space,
> hence we do: (x - y/2) / y
> which comes in 1 short, that fixup has been lost along the way.

Ah, that makes more sense, and the fact that it is broken explains why I 
couldn't figure it out.

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