From: Stephane Eranian eranian@google.com
commit 11745ecfe8fea4b4a4c322967a7605d2ecbd5080 upstream.
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/
Fixes: 07ce734dd8ad ("perf/x86/intel/uncore: Clean up client IMC") Signed-off-by: Stephane Eranian eranian@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Kan Liang kan.liang@linux.intel.com Link: https://lore.kernel.org/r/20220803160031.1379788-1-eranian@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/events/intel/uncore_snb.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-)
--- a/arch/x86/events/intel/uncore_snb.c +++ b/arch/x86/events/intel/uncore_snb.c @@ -788,6 +788,22 @@ int snb_pci2phy_map_init(int devid) return 0; }
+static u64 snb_uncore_imc_read_counter(struct intel_uncore_box *box, struct perf_event *event) +{ + struct hw_perf_event *hwc = &event->hw; + + /* + * SNB IMC counters are 32-bit and are laid out back to back + * in MMIO space. Therefore we must use a 32-bit accessor function + * using readq() from uncore_mmio_read_counter() causes problems + * because it is reading 64-bit at a time. This is okay for the + * uncore_perf_event_update() function because it drops the upper + * 32-bits but not okay for plain uncore_read_counter() as invoked + * in uncore_pmu_event_start(). + */ + return (u64)readl(box->io_addr + hwc->event_base); +} + static struct pmu snb_uncore_imc_pmu = { .task_ctx_nr = perf_invalid_context, .event_init = snb_uncore_imc_event_init, @@ -807,7 +823,7 @@ static struct intel_uncore_ops snb_uncor .disable_event = snb_uncore_imc_disable_event, .enable_event = snb_uncore_imc_enable_event, .hw_config = snb_uncore_imc_hw_config, - .read_counter = uncore_mmio_read_counter, + .read_counter = snb_uncore_imc_read_counter, };
static struct intel_uncore_type snb_uncore_imc = {