linux-kernel - Re: [PATCHv2 2/2] powerpc: implement arch_scale_smt

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Date:	Thu, 28 Jan 2010 16:39:15 -0600
From:	Joel Schopp <jschopp@...tin.ibm.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, ego@...ibm.com,
	linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 2/2] powerpc: implement arch_scale_smt_power for Power7


>
> What about an early exit if !cpu_has_feature(CPU_FTR_SMT) ? That would
> de-facto compile it out for 32-bit CPU platforms that don't support SMT
> at all and avoid some overhead on POWER3,4,970...
>   
If the SD_SHARE_CPUPOWER flag isn't set for the sched domain this 
function isn't called.  So an extra check here is wasteful.
>   
>> +	unsigned long weight = cpumask_weight(sibling_map);
>> +	unsigned long smt_gain = sd->smt_gain;
>> +
>> +	if (cpu_has_feature(CPU_FTR_ASYNC_SMT4) && weight == 4) {
>>     
>
> So that will only handle the case where all 4 threads are online right ?
> There is no provision for the case where the user play tricks like
> offlining thread, in which case it will stop trying to "push down"
> processes right ? Not a big deal per-se I suppose, just something to be
> aware of.
>   
I've tested it with manually offlined threads and it behaves as I'd like 
it to.
> Also, can you add a comment as to why this is done in the code itself ?
> above the if (cpu_has_feature(...)) statement.
>   
OK. v3 coming soon with the comment.
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