Re: [PATCHv3 2/2] powerpc: implement arch_scale_smt_power for Power7

From: Joel Schopp
Date: Fri Jan 29 2010 - 13:41:51 EST

Benjamin Herrenschmidt wrote:
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.
I can wrap the powerpc definition of arch_scale_smt in an #ifdef, if it's not there the scheduler uses the default, which is the same as it uses if SMT isn't compiled.

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