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