linux-kernel - Re: [PATCHv3 2/2] powerpc: implement arch_scale_smt

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Date:	Fri, 29 Jan 2010 12:23:05 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Joel Schopp <jschopp@...tin.ibm.com>
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: [PATCHv3 2/2] powerpc: implement arch_scale_smt_power for
 Power7

On Thu, 2010-01-28 at 17:24 -0600, Joel Schopp wrote:
> On Power7 processors running in SMT4 mode with 2, 3, or 4 idle threads 
> there is performance benefit to idling the higher numbered threads in
> the core.  
> 
> This patch implements arch_scale_smt_power to dynamically update smt
> thread power in these idle cases in order to prefer threads 0,1 over
> threads 2,3 within a core.

Almost there :-) Joel, Peter, can you help me figure something out tho ?

On machine that don't have SMT, I would like to avoid calling
arch_scale_smt_power() at all if possible (in addition to not compiling
it in if SMT is not enabled in .config).

Now, I must say I'm utterly confused by how the domains are setup and I
haven't quite managed to sort it out... it looks to me that
SD_SHARE_CPUPOWER is always going to be set on all CPUs when the config
option is set (though each CPU will have its own domain) or am I
misguided ? IE. Is there any sense in having at least a fast exit path
out of arch_scale_smt_power() for non-SMT CPUs ?

Joel, can you look at compiling it out when SMT is not set ? We don't
want to bloat SMP kernels for 32-bit non-SMT embedded platforms.

Oh, and one minor nit:

> Signed-off-by: Joel Schopp <jschopp@...tin.ibm.com>
> ---
> Version 2 addresses style and optimization, same basic functionality
> Version 3 adds a comment, resent due to mailing format error
> Index: linux-2.6.git/arch/powerpc/kernel/smp.c
> ===================================================================
> --- linux-2.6.git.orig/arch/powerpc/kernel/smp.c
> +++ linux-2.6.git/arch/powerpc/kernel/smp.c
> @@ -620,3 +620,59 @@ void __cpu_die(unsigned int cpu)
>  		smp_ops->cpu_die(cpu);
>  }
>  #endif

 ^^^ Please add the /* CONFIG_CPU_HOTPLUG */ (or whatever it is) that's
missing after that #endif :-)

Cheers,
Ben.

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