Benjamin Herrenschmidt wrote:

On Thu, 2010-01-28 at 17:24 -0600, Joel Schopp wrote:
  

quoted

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.