Messages in this thread | | | From | Stephane Eranian <> | Date | Thu, 25 Aug 2022 11:58:22 -0700 | Subject | Re: [PATCH] perf/x86/intel/uncore: fix broken read_counter() for SNB IMC PMU |
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On Thu, Aug 25, 2022 at 2:26 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Aug 15, 2022 at 03:28:36PM -0700, Stephane Eranian wrote: > > On Thu, Aug 4, 2022 at 6:09 AM Liang, Kan <kan.liang@linux.intel.com> wrote: > > > > > > > > > > > > On 2022-08-03 12:00 p.m., Stephane Eranian wrote: > > > > Existing code was generating bogus counts for the SNB IMC bandwidth counters: > > > > > > > > $ perf stat -a -I 1000 -e uncore_imc/data_reads/,uncore_imc/data_writes/ > > > > 1.000327813 1,024.03 MiB uncore_imc/data_reads/ > > > > 1.000327813 20.73 MiB uncore_imc/data_writes/ > > > > 2.000580153 261,120.00 MiB uncore_imc/data_reads/ > > > > 2.000580153 23.28 MiB uncore_imc/data_writes/ > > > > > > > > The problem was introduced by commit: > > > > 07ce734dd8ad ("perf/x86/intel/uncore: Clean up client IMC") > > > > > > > > Where the read_counter callback was replace to point to the generic > > > > uncore_mmio_read_counter() function. > > > > > > > > The SNB IMC counters are freerunnig 32-bit counters laid out contiguously in > > > > MMIO. But uncore_mmio_read_counter() is using a readq() call to read from > > > > MMIO therefore reading 64-bit from MMIO. Although this is okay for the > > > > uncore_perf_event_update() function because it is shifting the value based > > > > on the actual counter width to compute a delta, it is not okay for the > > > > uncore_pmu_event_start() which is simply reading the counter and therefore > > > > priming the event->prev_count with a bogus value which is responsible for > > > > causing bogus deltas in the perf stat command above. > > > > > > > > The fix is to reintroduce the custom callback for read_counter for the SNB > > > > IMC PMU and use readl() instead of readq(). With the change the output of > > > > perf stat is back to normal: > > > > $ perf stat -a -I 1000 -e uncore_imc/data_reads/,uncore_imc/data_writes/ > > > > 1.000120987 296.94 MiB uncore_imc/data_reads/ > > > > 1.000120987 138.42 MiB uncore_imc/data_writes/ > > > > 2.000403144 175.91 MiB uncore_imc/data_reads/ > > > > 2.000403144 68.50 MiB uncore_imc/data_writes/ > > > > > > > > Signed-off-by: Stephane Eranian <eranian@google.com> > > > > > > Reviewed-by: Kan Liang <kan.liang@linux.intel.com> > > > > > Any further comments? > > Got lost in the holiday pile-up, applied!
Thanks.
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