[PATCH 5.15 131/136] perf/x86/intel/uncore: Fix broken read_counter() for SNB IMC PMU

Greg Kroah-Hartman
2022-08-29T11:38:06Z

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 = {