Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 19, 2017 at 10:06:21AM -0700, kan.liang@intel.com wrote:
> From: Kan Liang <Kan.liang@intel.com>
> 
> The CPU ref_cycles can only be used by one user at the same time,
> otherwise a "not counted" error will be displaced.
>     [kan]$ sudo perf stat -x, -e ref-cycles,ref-cycles -- sleep 1
>     1203264,,ref-cycles,513112,100.00,,,,
>     <not counted>,,ref-cycles,0,0.00,,,,
> 
> CPU ref_cycles can only be counted by fixed counter 2. It uses
> pseudo-encoding. The GP counter doesn't recognize.
> 
> BUS_CYCLES (0x013c) is another event which is not affected by core
> frequency changes. It has a constant ratio with the CPU ref_cycles.
> BUS_CYCLES could be used as an alternative event for ref_cycles on GP
> counter.
> A hook is implemented in x86_schedule_events. If the fixed counter 2 is
> occupied and a GP counter is assigned, BUS_CYCLES is used to replace
> ref_cycles. A new flag PERF_X86_EVENT_REF_CYCLES_REP in
> hw_perf_event is introduced to indicate the replacement.
> To make the switch transparent, counting and sampling are also specially
> handled.
>  - For counting, it multiplies the result with the constant ratio after
>    reading it.
>  - For sampling with fixed period, the BUS_CYCLES period = ref_cycles
>    period / the constant ratio.
>  - For sampling with fixed frequency, the adaptive frequency algorithm
>    will figure it out on its own. Do nothing.
> 
> The constant ratio is model specific.
> For the model after NEHALEM but before Skylake, the ratio is defined in
> MSR_PLATFORM_INFO.
> For the model after Skylake, it can be get from CPUID.15H.
> For Knights Landing, Goldmont and later, the ratio is always 1.
> 
> The old Silvermont/Airmont, Core2 and Atom machines are not covered by
> the patch. The behavior on those machines will not change.

Maybe I missed it, but *why* are we doing this?