Re: [PATCH v2 2/3] arm64: perf: Add support for event counting threshold

From: Suzuki K Poulose
Date: Tue Oct 10 2023 - 11:01:06 EST

On 10/10/2023 11:40, James Clark wrote:
FEAT_PMUv3_TH (Armv8.8) permits a PMU counter to increment only on
events whose count meets a specified threshold condition. For example if
PMEVTYPERn.TC (Threshold Control) is set to 0b101 (Greater than or
equal, count), and the threshold is set to 2, then the PMU counter will
now only increment by 1 when an event would have previously incremented
the PMU counter by 2 or more on a single processor cycle.

Two new Perf event config fields, 'threshold' and 'threshold_control'
have been added for controlling the feature:

$ perf stat -e stall_slot/threshold=2,threshold_control=5/

A new capability for reading out the maximum supported threshold value
has also been added:

$ cat /sys/bus/event_source/devices/armv8_pmuv3/caps/threshold_max

0x000000ff

If a threshold higher than threshold_max is provided, then no error is
generated but the threshold is clamped to the max value. If
FEAT_PMUv3_TH isn't implemented or a 32 bit kernel is running, then
threshold_max reads zero, and neither the 'threshold' nor
'threshold_control' parameters will be used.

The threshold is per PMU counter, and there are potentially different
threshold_max values per PMU type on heterogeneous systems.

Bits higher than 32 now need to be written into PMEVTYPER, so
armv8pmu_write_evtype() has to be updated to take an unsigned long value
rather than u32 which gives the correct behavior on both aarch32 and 64.

Signed-off-by: James Clark <james.clark@xxxxxxx>
---
drivers/perf/arm_pmuv3.c | 67 +++++++++++++++++++++++++++++++++-
include/linux/perf/arm_pmuv3.h | 1 +
2 files changed, 67 insertions(+), 1 deletion(-)

diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index 8fcaa26f0f8a..6d669b16a2bc 100644
--- a/drivers/perf/arm_pmuv3.c
+++ b/drivers/perf/arm_pmuv3.c
@@ -15,6 +15,7 @@
#include <clocksource/arm_arch_timer.h>
#include <linux/acpi.h>
+#include <linux/bitfield.h>
#include <linux/clocksource.h>
#include <linux/of.h>
#include <linux/perf/arm_pmu.h>
@@ -294,9 +295,16 @@ static const struct attribute_group armv8_pmuv3_events_attr_group = {
.is_visible = armv8pmu_event_attr_is_visible,
};
+#define TH_LO 2
+#define TH_HI 13
+#define TH_CNTL_LO 14
+#define TH_CNTL_HI 16
+
PMU_FORMAT_ATTR(event, "config:0-15");
PMU_FORMAT_ATTR(long, "config1:0");
PMU_FORMAT_ATTR(rdpmc, "config1:1");
+PMU_FORMAT_ATTR(threshold, "config1:" __stringify(TH_LO) "-" __stringify(TH_HI));
+PMU_FORMAT_ATTR(threshold_control, "config1:" __stringify(TH_CNTL_LO) "-" __stringify(TH_CNTL_HI));


static int sysctl_perf_user_access __read_mostly;
@@ -310,10 +318,22 @@ static inline bool armv8pmu_event_want_user_access(struct perf_event *event)
return event->attr.config1 & 0x2;
}
+static inline u32 armv8pmu_event_threshold(struct perf_event_attr *attr)
+{
+ return FIELD_GET(GENMASK(TH_HI, TH_LO), attr->config1);
+}
+
+static inline u8 armv8pmu_event_threshold_control(struct perf_event_attr *attr)
+{
+ return FIELD_GET(GENMASK(TH_CNTL_HI, TH_CNTL_LO), attr->config1);
+}
+
static struct attribute *armv8_pmuv3_format_attrs[] = {
&format_attr_event.attr,
&format_attr_long.attr,
&format_attr_rdpmc.attr,

+ &format_attr_threshold.attr,
+ &format_attr_threshold_control.attr,

Given this is not supported for !CONFIG_ARM64, does it make sense to remove them for that case, given we already take care of this in
the code using IS_ENABLED(CONFIG_ARM64) ? The nice part is that
the arm32 doesn't see something that is never usable. I understand
the maxthreshold=0 already gives out the hint.

Rest looks fine to me.

NULL,
};
@@ -365,10 +385,38 @@ static ssize_t bus_width_show(struct device *dev, struct device_attribute *attr,
static DEVICE_ATTR_RO(bus_width);
+static u32 threshold_max(struct arm_pmu *cpu_pmu)
+{
+ /*
+ * PMMIR.WIDTH is readable and non-zero on aarch32, but it would be
+ * impossible to write the threshold in the upper 32 bits of PMEVTYPER.
+ */
+ if (IS_ENABLED(CONFIG_ARM))
+ return 0;
+
+ /*
+ * The largest value that can be written to PMEVTYPER<n>_EL0.TH is
+ * (2 ^ PMMIR.THWIDTH) - 1.
+ */
+ return (1 << FIELD_GET(ARMV8_PMU_THWIDTH, cpu_pmu->reg_pmmir)) - 1;
+}
+
+static ssize_t threshold_max_show(struct device *dev,
+ struct device_attribute *attr, char *page)
+{
+ struct pmu *pmu = dev_get_drvdata(dev);
+ struct arm_pmu *cpu_pmu = container_of(pmu, struct arm_pmu, pmu);
+
+ return sysfs_emit(page, "0x%08x\n", threshold_max(cpu_pmu));
+}
+
+static DEVICE_ATTR_RO(threshold_max);
+
static struct attribute *armv8_pmuv3_caps_attrs[] = {
&dev_attr_slots.attr,
&dev_attr_bus_slots.attr,
&dev_attr_bus_width.attr,
+ &dev_attr_threshold_max.attr,

Similarly here and could avoid the IS_ENABLED(CONFIG_ARM) above.

Suzuki


NULL,
};
@@ -552,7 +600,7 @@ static void armv8pmu_write_counter(struct perf_event *event, u64 value)
armv8pmu_write_hw_counter(event, value);
}
-static inline void armv8pmu_write_evtype(int idx, u32 val)
+static inline void armv8pmu_write_evtype(int idx, unsigned long val)
{
u32 counter = ARMV8_IDX_TO_COUNTER(idx);
@@ -914,6 +962,10 @@ static int armv8pmu_set_event_filter(struct hw_perf_event *event,
struct perf_event_attr *attr)
{
unsigned long config_base = 0;
+ struct perf_event *perf_event = container_of(attr, struct perf_event,
+ attr);
+ struct arm_pmu *cpu_pmu = to_arm_pmu(perf_event->pmu);
+ u32 th, th_max;
if (attr->exclude_idle)
return -EPERM;
@@ -945,6 +997,19 @@ static int armv8pmu_set_event_filter(struct hw_perf_event *event,
if (attr->exclude_user)
config_base |= ARMV8_PMU_EXCLUDE_EL0;
+ /*
+ * Insert event counting threshold (FEAT_PMUv3_TH) values. If
+ * FEAT_PMUv3_TH isn't implemented, then THWIDTH (threshold_max) will be
+ * 0 and no values will be written.
+ */
+ th_max = threshold_max(cpu_pmu);
+ if (IS_ENABLED(CONFIG_ARM64) && th_max) {
+ th = min(armv8pmu_event_threshold(attr), th_max);
+ config_base |= FIELD_PREP(ARMV8_PMU_EVTYPE_TH, th);
+ config_base |= FIELD_PREP(ARMV8_PMU_EVTYPE_TC,
+ armv8pmu_event_threshold_control(attr));
+ }
+
/*
* Install the filter into config_base as this is used to
* construct the event type.
diff --git a/include/linux/perf/arm_pmuv3.h b/include/linux/perf/arm_pmuv3.h
index ec3a01502e7c..753f8dbd9d10 100644
--- a/include/linux/perf/arm_pmuv3.h
+++ b/include/linux/perf/arm_pmuv3.h
@@ -255,6 +255,7 @@
#define ARMV8_PMU_BUS_SLOTS_MASK 0xff
#define ARMV8_PMU_BUS_WIDTH_SHIFT 16
#define ARMV8_PMU_BUS_WIDTH_MASK 0xf
+#define ARMV8_PMU_THWIDTH GENMASK(23, 20)
/*
* This code is really good