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

[Suzuki K Poulose <suzuki.poulose@arm.com>]

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@arm.com>
> ---
>   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