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Date:   Thu, 30 Nov 2023 11:51:35 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     James Clark <james.clark@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, suzuki.poulose@....com,
        will@...nel.org, mark.rutland@....com, namhyung@...il.com
Subject: Re: [PATCH v6 2/3] arm64: perf: Add support for event counting
 threshold



On 11/27/23 15:54, James Clark wrote:
> 
> 
> On 27/11/2023 05:32, Anshuman Khandual wrote:
>>
>>
>> On 11/24/23 15:58, 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.
>>>
>>> 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/
>>>
>>> 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.
>>>
>>> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
>>> Signed-off-by: James Clark <james.clark@....com>
>>> ---
>>>  drivers/perf/arm_pmuv3.c       | 84 +++++++++++++++++++++++++++++++++-
>>>  include/linux/perf/arm_pmuv3.h |  1 +
>>>  2 files changed, 84 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
>>> index 1d40d794f5e4..eb1ef84e1dbb 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 +295,20 @@ static const struct attribute_group armv8_pmuv3_events_attr_group = {
>>>  	.is_visible = armv8pmu_event_attr_is_visible,
>>>  };
>>>  
>>> +#define THRESHOLD_LOW		2
>>> +#define THRESHOLD_HIGH		13
>>> +#define THRESHOLD_CNT		14
>>> +#define THRESHOLD_CMP_LO	15
>>> +#define THRESHOLD_CMP_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(THRESHOLD_LOW) "-"
>>> +				      __stringify(THRESHOLD_HIGH));
>>> +PMU_FORMAT_ATTR(threshold_compare, "config1:" __stringify(THRESHOLD_CMP_LO) "-"
>>> +					      __stringify(THRESHOLD_CMP_HI));
>>> +PMU_FORMAT_ATTR(threshold_count, "config1:" __stringify(THRESHOLD_CNT));
>>
>> Small nit - could this be formatted better ? Is not that the column could go
>> upto 100 without setting off checkpatch.pl warning these days ?
> 
> I think it looks perfectly readable to me, is there a specific
> formatting rule that's been broken? And no, it can't be unindented
> without exceeding the 100 char limit.

Fair enough. There is nothing broken in here, otherwise checkpatch.pl
would have warned. I was just wondering if the indentation could have
been avoided. But as you mentioned, it cannot be without going beyond
the 100 char limit.

> 
>>
>>>  
>>>  static int sysctl_perf_user_access __read_mostly;
>>>  
>>> @@ -310,10 +322,33 @@ 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(THRESHOLD_HIGH, THRESHOLD_LOW), attr->config1);
>>> +}
>>> +
>>> +static inline u8 armv8pmu_event_threshold_control(struct perf_event_attr *attr)
>>> +{
>>> +	u8 th_compare = FIELD_GET(GENMASK(THRESHOLD_CMP_HI, THRESHOLD_CMP_LO),
>>> +				  attr->config1);
>>
>> Ditto
>>
> 
> There's no rule saying that you can't indent _before_ 100 chars. Most of
> the code in this file is indented at 80 chars, and consistency is
> usually valued above other things.

Fair enough.

> 
>>> +	u8 th_count = FIELD_GET(BIT(THRESHOLD_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;
>>> +}
>>> +
>>>  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 +400,38 @@ 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.THWIDTH 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,
>>>  	NULL,
>>>  };
>>>  
>>> @@ -552,7 +615,7 @@ 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 +984,10 @@ 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);
>>
>> Ditto
>>
>>> +	struct arm_pmu *cpu_pmu = to_arm_pmu(perf_event->pmu);
>>> +	u32 th, th_max;
>>>  
>>>  	if (attr->exclude_idle)
>>>  		return -EPERM;
>>> @@ -952,6 +1019,21 @@ 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);
>>> +		if (th) {
>>> +			config_base |= FIELD_PREP(ARMV8_PMU_EVTYPE_TH, th);
>>> +			config_base |= FIELD_PREP(ARMV8_PMU_EVTYPE_TC,
>>> +						  armv8pmu_event_threshold_control(attr));
>>
>> Ditto. As mentioned earlier this could have been avoided using a local variable.
>>
> 
> But explained why it's like that on the previous review. It's completely
> down to personal preference whether a local variable is used or not. I
> don't see why this is a review comment, it doesn't affect how the code
> behaves or the readability in any way.
> 
> And what could be avoided? No formatting rules have been broken.

Just an indentation could have been avoided, but again nothing is broken
here to be clear, neither functionality nor the formatting. It was just
a suggestion, which you could very well ignore.

> 
>>> +		}
>>> +	}
>>> +
>>>  	/*
>>>  	 * 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)
>>>  
>>>  /*
>>>   * This code is really good
>>
>> Otherwise LGTM
>>
>> Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>
>>

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