On Fri, May 19, 2017 at 10:06:21AM -0700, kan.li...@intel.com wrote: > From: Kan Liang <kan.li...@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?
