Messages in this thread | | | From | "Liang, Kan" <> | Subject | Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics | Date | Tue, 28 May 2019 14:23:09 -0400 |
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On 5/28/2019 8:43 AM, Peter Zijlstra wrote: > On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@linux.intel.com wrote: >> The 8bit metrics ratio values lose precision when the measurement period >> gets longer. >> >> To avoid this we always reset the metric value when reading, as we >> already accumulate the count in the perf count value. >> >> For a long period read, low precision is acceptable. >> For a short period read, the register will be reset often enough that it >> is not a problem. > >> The PERF_METRICS may report wrong value if its delta was less than 1/255 >> of SLOTS (Fixed counter 3). >> >> To avoid this, the PERF_METRICS and SLOTS registers have to be reset >> simultaneously. The slots value has to be cached as well. > > That doesn't sound like it is NMI-safe. > > > >> RDPMC >> ========= >> The TopDown can be collected per thread/process. To use TopDown >> through RDPMC in applications on Icelake, the metrics and slots values >> have to be saved/restored during context switching. >> >> Add specific set_period() to specially handle the slots and metrics >> event. Because, >> - The initial value must be 0. >> - Only need to restore the value in context switch. For other cases, >> the counters have been cleared after read. > > So the above claims to explain RDPMC, but doesn't mention that magic > value below at all. In fact, I don't see how the above relates to RDPMC > at all.
Current perf only support per-core Topdown RDPMC. On Icelake, it can be extended to per-thread Topdown RDPMC. It tries to explain the extra work for per-thread topdown RDPMC, e.g. save/restore slots and metrics value in context switch.
> >> @@ -2141,7 +2157,9 @@ static int x86_pmu_event_idx(struct perf_event *event) >> if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED)) >> return 0; >> >> - if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) { >> + if (is_metric_idx(idx)) >> + idx = 1 << 29; > > I can't find this in the SDM RDPMC description. What does it return?
It will return the value of PERF_METRICS. I will add it in the changelog.
Thanks, Kan > >> + else if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) { >> idx -= INTEL_PMC_IDX_FIXED; >> idx |= 1 << 30; >> }
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