Messages in this thread | | | Date | Mon, 13 Sep 2021 13:56:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 04/19] cpufreq: amd: introduce a new amd pstate driver to support future processors |
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On Mon, Sep 13, 2021 at 06:54:58PM +0800, Huang Rui wrote: > On Mon, Sep 13, 2021 at 04:56:24PM +0800, Peter Zijlstra wrote:
> > > 1) Full MSR Support > > > If current hardware has the full MSR support, we register "pstate_funcs" > > > callback functions to implement the MSR operations to control the clocks. > > > > What's the WRMSR cost for those? I've not really kept track of the MSR > > costs on AMD platforms, but on Intel it has (luckily) been coming down > > quite a bit. > > Good to know this, I didn't have a chance to give a check. May I know how > did you test this latency? But MSR is new hardware design for this > solution, as designer mentioned, the WRMSR is low-latency register model is > faster than ACPI AML code interpreter. > > > > > > 2) Shared Memory Support > > > If current hardware doesn't have the full MSR support, that means it only > > > provides share memory support. We will leverage APIs in cppc_acpi libs with > > > "cppc_funcs" to implement the target function for the frequency control. > > > > Right, the mailbox thing. How is the performance of this vs MSR accesses? > > I will give a check. If you have a existing test method that can be used, I > can check it quickly.
Oh, I was mostly wondering if using the mailbox as MMIO would be faster than an MSR, but you've already answered that above. Also:
> > > 1. As mentioned above, amd-pstate driver can implement > > > fast_switch/adjust_perf function with full MSR operations that have better > > > performance for schedutil and other governors. > > > > Why couldn't the existing cppc-cpufreq grow this? > > Because fast_switch can adjust the frequency directly in the interrupt > context, if we use the acpi cppc handling with shared memory solution, it > will have a deadlock. So fast switch needs the control with registers > directly like acpi-cpufreq and intel-pstate.
Aah, I see, you're only doing fast_switch support when you have MSRs. That was totally non-obvious.. :/
But then amd_pstate_adjust_perf() could just direct call the pstate methods and we don't need that indirection *at*all*, right?
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