Re: [PATCH V7 3/6] arm64/perf: Add branch stack support in struct arm_pmu

Anshuman Khandual
2023-01-13T04:16:39Z

On 1/12/23 19:24, Mark Rutland wrote:
> On Thu, Jan 05, 2023 at 08:40:36AM +0530, Anshuman Khandual wrote:
>> This updates 'struct arm_pmu' for branch stack sampling support later. This
>> adds a new 'features' element in the structure to track supported features,
>> and another 'private' element to encapsulate implementation attributes on a
>> given 'struct arm_pmu'. These updates here will help in tracking any branch
>> stack sampling support, which is being added later. This also adds a helper
>> arm_pmu_branch_stack_supported().
>>
>> This also enables perf branch stack sampling event on all 'struct arm pmu',
>> supporting the feature but after removing the current gate that blocks such
>> events unconditionally in armpmu_event_init(). Instead a quick probe can be
>> initiated via arm_pmu_branch_stack_supported() to ascertain the support.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> drivers/perf/arm_pmu.c | 3 +--
>> include/linux/perf/arm_pmu.h | 9 +++++++++
>> 2 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
>> index 14a3ed3bdb0b..a85b2d67022e 100644
>> --- a/drivers/perf/arm_pmu.c
>> +++ b/drivers/perf/arm_pmu.c
>> @@ -510,8 +510,7 @@ static int armpmu_event_init(struct perf_event *event)
>> !cpumask_test_cpu(event->cpu, &armpmu->supported_cpus))
>> return -ENOENT;
>>
>> - /* does not support taken branch sampling */
>> - if (has_branch_stack(event))
>> + if (has_branch_stack(event) && !arm_pmu_branch_stack_supported(armpmu))
>> return -EOPNOTSUPP;
>>
>> return __hw_perf_event_init(event);
>> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
>> index 2a9d07cee927..64e1b2594025 100644
>> --- a/include/linux/perf/arm_pmu.h
>> +++ b/include/linux/perf/arm_pmu.h
>> @@ -80,11 +80,14 @@ enum armpmu_attr_groups {
>> ARMPMU_NR_ATTR_GROUPS
>> };
>>
>> +#define ARM_PMU_BRANCH_STACK BIT(0)
>> +
>> struct arm_pmu {
>> struct pmu pmu;
>> cpumask_t supported_cpus;
>> char *name;
>> int pmuver;
>> + int features;
>> irqreturn_t (*handle_irq)(struct arm_pmu *pmu);
>> void (*enable)(struct perf_event *event);
>> void (*disable)(struct perf_event *event);
>
> Hmm, we already have the secure_access field separately. How about we fold that
> in and go with:
>
> unsigned int secure_access : 1,
> has_branch_stack : 1;

Something like this would work, but should we use __u32 instead of unsigned int
to ensure 32 bit width ?

- bool secure_access; /* 32-bit ARM only */
+ unsigned int secure_access : 1, /* 32-bit ARM only */
+ has_branch_stack: 1,
+ reserved : 31;

>
> ... that way we have one way to manage flags, we don't need to allocate the
> bits, and the bulk of the existing code for secure_access can stay as-is.

Right, the changed code also builds on arm32 without any code change.

>
>> @@ -119,8 +122,14 @@ struct arm_pmu {
>>
>> /* Only to be used by ACPI probing code */
>> unsigned long acpi_cpuid;
>> + void *private;
>
> Does this need to be on the end of struct arm_pmu, or can it be placed earlier?

This additional 'private' attribute structure sticking out from struct arm_pmu
should be at the end. But is there any benefit moving this earlier ?

>
> The line spacing makes it look like the ACPI comment applies to 'private',
> which isn't the case.

Sure, will add the following comment, and a space in between.

diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
index f60f7e01acae..c0a090ff7991 100644
--- a/include/linux/perf/arm_pmu.h
+++ b/include/linux/perf/arm_pmu.h
@@ -130,6 +130,8 @@ struct arm_pmu {

/* Only to be used by ACPI probing code */
unsigned long acpi_cpuid;
+
+ /* Implementation specific attributes */
void *private;
};

>
>> };
>>
>> +static inline bool arm_pmu_branch_stack_supported(struct arm_pmu *armpmu)
>> +{
>> + return armpmu->features & ARM_PMU_BRANCH_STACK;
>> +}
>
> With the above, this would become:
>
> static inline bool arm_pmu_branch_stack_supported(struct arm_pmu *armpmu)
> {
> return armpmu->has_branch_stack;
> }

Right, will change this helper as required.