linux-kernel - Re: [PATCH 2/2] powerpc: implement arch_scale_smt

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Date:	Wed, 20 Jan 2010 16:09:30 -0600
From:	Joel Schopp <jschopp@...tin.ibm.com>
To:	Michael Neuling <mikey@...ling.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, ego@...ibm.com
Subject: Re: [PATCH 2/2] powerpc: implement arch_scale_smt_power for Power7


>> +	if (cpu_has_feature(CPU_FTRS_POWER7) && weight == 4) {
>>     
>
> I think we should avoid using cpu_has_feature like this.  It's better to
> create a new feature and add it to POWER7 in the cputable, then check
> for that here.
>
> The way that it is now, I think any CPU that has superset of the POWER7
> features, will be true here.  This is not what we want.
>   
Any ideas for what to call this feature?  ASYM_SMT4 ?
>
>> +	smt_gain /= weight;
>>     
>
> This results in a PPC div, when most of the time it's going to be a
> power of two divide.  You've optimised the divides a few lines above
> this, but not this one.  Some consistency would be good.
>
>   
I can turn that into a conditional branch (case statement) with a shift 
for the common 1,2,4 cases which should cover all procs available today 
falling back to a divide for any theoretical future processors that do 
other numbers of threads.
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