On 23/11/2023 03:42, Anshuman Khandual wrote:

On 11/13/23 16:55, James Clark wrote:

quoted

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.

Three new Perf event config fields, 'threshold', 'threshold_compare' and
'threshold_count' have been added to control the feature.
threshold_compare maps to the upper two bits of PMEVTYPERn.TC and
threshold_count maps to the first bit of TC. These separate attributes
have been picked rather than enumerating all the possible combinations
of the TC field as in the Arm ARM. The attributes would be used on a
Perf command line like this:

  $ perf stat -e stall_slot/threshold=2,threshold_compare=2/

If threshold_count = 0, then threshold and threshold_compare should just
be ignored ?

No, threshold_count only effects the value which the PMU counts by after
the threshold condition is passed.

  Threshold_count == 1 : Always increment by 1
  Threshold_count == 0 : Increment by whatever the count was on that
                         cycle

Threshold_count never causes anything else to be ignored, only threshold
== 0 does.

quoted

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

Makes sense.

quoted

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.

Makes sense.

quoted

Signed-off-by: James Clark <redacted>
---
 drivers/perf/arm_pmuv3.c       | 79 +++++++++++++++++++++++++++++++++-
 include/linux/perf/arm_pmuv3.h |  1 +
 2 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index 1d40d794f5e4..694d914ffc08 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 +295,18 @@ 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_CNT	14
+#define TH_CMP_LO	15
+#define TH_CMP_HI	16

TH_ prefix sounds too cryptic. I had suggested some clean up earlier

https://lore.kernel.org/all/20231009051753.179355-1-anshuman.khandual@arm.com/ (local)

But for now, something like the following might be bit better instead ?

THRESHOLD_LOW
THRESHOLD_HIGH
THRESHOLD_CNT
THRESHOLD_CMP_LOW
THRESHOLD_CMP_HIGH

Yep I can expand them. I agree they're a bit cryptic but it was making
the lines too long.

quoted

+
 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_compare, "config1:" __stringify(TH_CMP_LO) "-" __stringify(TH_CMP_HI));
+PMU_FORMAT_ATTR(threshold_count, "config1:" __stringify(TH_CNT));
 
 static int sysctl_perf_user_access __read_mostly;
 

@@ -310,10 +320,32 @@ 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)
+{
+	u8 th_compare = FIELD_GET(GENMASK(TH_CMP_HI, TH_CMP_LO), attr->config1);
+	u8 th_count = FIELD_GET(BIT(TH_CNT), attr->config1);
+
+	/*
+	 * The count bit is always the bottom bit of the full control field, and
+	 * the comparison is the upper two bits, but it's not explicitly
+	 * labelled in the Arm ARM. For the Perf interface we split it into two
+	 * fields, so reconstruct it here.
+	 */
+	return (th_compare << 1) | th_count;

If user provides 'th_count = 0', then all these threshold control code can be
skipped as if FEAT_PMUv3_TH was never implemented ?

Same as above, no nothing can be skipped based on th_count.

Also what happens when
threshold = 0 ?

When threshold = 0 then the feature is disabled. I will add that to the
docs and I can add an early exit condition for that when writing to
config_base. Although it wouldn't have any actual effect as the hardware
already ignores the control field when the threshold is 0.

quoted

+}
+
 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_compare.attr,
+	&format_attr_threshold_count.attr,
 	NULL,
 };
 

@@ -365,10 +397,38 @@ 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

PMMIR.THWIDTH ^^^^^^^^ ?

quoted

+	 * impossible to write the threshold in the upper 32 bits of PMEVTYPER.
+	 */

Could aarch32 support FEAT_PMUv3_TH ? if not,

Kind of, if a 64 bit host sets the register then I think thresholding
might work on a 32 bit guest. You'd have to check the manual if you
really want to know, but I don't think it's really relevant as this code
is only about not letting 32 bit guests write to it themselves, because
it wouldn't even compile.

how can the PMMIR.THWIDTH value
here be non-zero ?

It just is, it's written like that in the ARM Arm as far as I read it.

Also wondering if just a non-zero PMMIR.THWIDTH indicates
the presence for FEAT_PMUv3_TH on a given ARM PMU.

Yes.

quoted

+	if (IS_ENABLED(CONFIG_ARM))
+		return 0;

Small nit - would a negative check on CONFIG_ARM64 be better ?

Only if we plan to add a third Arm config type in the future that
doesn't support FEAT_PMUv3_TH but also uses this code? Seems so unlikely
as to not worth considering to me.

Couldn't you make the same point about every CONFIG_ARM in the code?

quoted

+
+	/*
+	 * 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;

Makes sense to return the adjusted value.

Are you saying to make a change here? Or just that it makes sense?

quoted

+}
+
+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,
 	NULL,
 };
 

@@ -552,7 +612,7 @@ 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);
 	unsigned long mask = ARMV8_PMU_EVTYPE_EVENT |

@@ -921,6 +981,10 @@ 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;

@@ -952,6 +1016,19 @@ 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) {

As mentioned above, using negative check on CONFIG_ARM64 in threshold_max()
will complement this condition here, making it clear that these threshold
configurations are applicable only on 64 bit platforms.

quoted

+		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));

Small nit - armv8pmu_event_threshold_control() might also be captured before
into a 'tc' local variable before adjusting the config_base similar to 'th'.

I don't see the improvement, I don't get putting things in variables if
they are only used once and there is no line length issue. The only
reason I probably did it for th was to make the line shorter.

Also better to add a small comment before 'th = min(..., ..) ' regarding the
clamping user input to platform max_threshold.

		th = min(armv8pmu_event_threshold(attr), th_max);
		tc = armv8pmu_event_threshold_control(attr));
		config_base |= FIELD_PREP(ARMV8_PMU_EVTYPE_TH, th);
		config_base |= FIELD_PREP(ARMV8_PMU_EVTYPE_TC, tc);

The comment would just say "clamp to the max" and the code says min(...,
th_max) so it would just be repeating what's already there. I also don't
see the benefit of this one either.

quoted

+	}
+
 	/*
 	 * 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 ddd1fec86739..ccbc0f9a74d8 100644
--- a/include/linux/perf/arm_pmuv3.h
+++ b/include/linux/perf/arm_pmuv3.h

@@ -258,6 +258,7 @@
 #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)

Small nit - may be ARMV8_PMU_TH_WIDTH instead ?

I probably won't change this one. 'THWIDTH' isn't very nice I agree, but
it's from the reference manual, and this enum describes the field. So
you remove people's ability to grep for it, just to make it look a
little bit nicer. It's not worth it.

quoted

 
 /*
  * This code is really good

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