[PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU
Anshuman Khandual
anshuman.khandual at arm.com
Thu Jun 8 21:00:59 PDT 2023
On 6/8/23 15:43, Suzuki K Poulose wrote:
> On 06/06/2023 11:34, Anshuman Khandual wrote:
>>
>>
>> On 6/5/23 17:35, Mark Rutland wrote:
>>> On Wed, May 31, 2023 at 09:34:23AM +0530, Anshuman Khandual wrote:
>>>> This enables support for branch stack sampling event in ARMV8 PMU, checking
>>>> has_branch_stack() on the event inside 'struct arm_pmu' callbacks. Although
>>>> these branch stack helpers armv8pmu_branch_XXXXX() are just dummy functions
>>>> for now. While here, this also defines arm_pmu's sched_task() callback with
>>>> armv8pmu_sched_task(), which resets the branch record buffer on a sched_in.
>>>
>>> This generally looks good, but I have a few comments below.
>>>
>>> [...]
>>>
>>>> +static inline bool armv8pmu_branch_valid(struct perf_event *event)
>>>> +{
>>>> + WARN_ON_ONCE(!has_branch_stack(event));
>>>> + return false;
>>>> +}
>>>
>>> IIUC this is for validating the attr, so could we please name this
>>> armv8pmu_branch_attr_valid() ?
>>
>> Sure, will change the name and updated call sites.
>>
>>>
>>> [...]
>>>
>>>> +static int branch_records_alloc(struct arm_pmu *armpmu)
>>>> +{
>>>> + struct pmu_hw_events *events;
>>>> + int cpu;
>>>> +
>>>> + for_each_possible_cpu(cpu) {
>
> Shouldn't this be supported_pmus ? i.e.
> for_each_cpu(cpu, &armpmu->supported_cpus) {
>
>
>>>> + events = per_cpu_ptr(armpmu->hw_events, cpu);
>>>> + events->branches = kzalloc(sizeof(struct branch_records), GFP_KERNEL);
>>>> + if (!events->branches)
>>>> + return -ENOMEM;
>
> Do we need to free the allocated branches already ?
This gets fixed in the next patch via per-cpu allocation. I will
move and fold the code block in here. Updated function will look
like the following.
static int branch_records_alloc(struct arm_pmu *armpmu)
{
struct branch_records __percpu *records;
int cpu;
records = alloc_percpu_gfp(struct branch_records, GFP_KERNEL);
if (!records)
return -ENOMEM;
/*
* FIXME: Memory allocated via records gets completely
* consumed here, never required to be freed up later. Hence
* losing access to on stack 'records' is acceptable.
* Otherwise this alloc handle has to be saved some where.
*/
for_each_possible_cpu(cpu) {
struct pmu_hw_events *events_cpu;
struct branch_records *records_cpu;
events_cpu = per_cpu_ptr(armpmu->hw_events, cpu);
records_cpu = per_cpu_ptr(records, cpu);
events_cpu->branches = records_cpu;
}
return 0;
}
Regarding the cpumask argument in for_each_cpu().
- hw_events is a __percpu pointer in struct arm_pmu
- pmu->hw_events = alloc_percpu_gfp(struct pmu_hw_events, GFP_KERNEL)
- 'records' above is being allocated via alloc_percpu_gfp()
- records = alloc_percpu_gfp(struct branch_records, GFP_KERNEL)
If 'armpmu->supported_cpus' mask gets used instead of possible cpu mask,
would not there be some dangling per-cpu branch_record allocated areas,
that remain unsigned ? Assigning all of them back into hw_events should
be harmless.
>
>>>> + }
>
>
> May be:
> int ret = 0;
>
> for_each_cpu(cpu, &armpmu->supported_cpus) {
> events = per_cpu_ptr(armpmu->hw_events, cpu);
> events->branches = kzalloc(sizeof(struct branch_records), GFP_KERNEL);
>
> if (!events->branches) {
> ret = -ENOMEM;
> break;
> }
> }
>
> if (!ret)
> return 0;
>
> for_each_cpu(cpu, &armpmu->supported_cpus) {
> events = per_cpu_ptr(armpmu->hw_events, cpu);
> if (!events->branches)
> break;
> kfree(events->branches);
> }
> return ret;
>
>>>> + return 0;
>>>
>>> This leaks memory if any allocation fails, and the next patch replaces this
>>> code entirely.
>>
>> Okay.
>>
>>>
>>> Please add this once in a working state. Either use the percpu allocation
>>> trick in the next patch from the start, or have this kzalloc() with a
>>> corresponding kfree() in an error path.
>>
>> I will change branch_records_alloc() as suggested in the next patch's thread
>> and fold those changes here in this patch.
>>
>>>
>>>> }
>>>> static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>>>> @@ -1145,12 +1162,24 @@ static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>>>> };
>>>> int ret;
>>>> + ret = armv8pmu_private_alloc(cpu_pmu);
>>>> + if (ret)
>>>> + return ret;
>>>> +
>>>> ret = smp_call_function_any(&cpu_pmu->supported_cpus,
>>>> __armv8pmu_probe_pmu,
>>>> &probe, 1);
>>>> if (ret)
>>>> return ret;
>>>> + if (arm_pmu_branch_stack_supported(cpu_pmu)) {
>>>> + ret = branch_records_alloc(cpu_pmu);
>>>> + if (ret)
>>>> + return ret;
>>>> + } else {
>>>> + armv8pmu_private_free(cpu_pmu);
>>>> + }
>>>
>>> I see from the next patch that "private" is four ints, so please just add that
>>> to struct arm_pmu under an ifdef CONFIG_ARM64_BRBE. That'll simplify this, and
>>> if we end up needing more space in future we can consider factoring it out.
>>
>> struct arm_pmu {
>> ........................................
>> /* Implementation specific attributes */
>> void *private;
>> }
>>
>> private pointer here creates an abstraction for given pmu implementation
>> to hide attribute details without making it known to core arm pmu layer.
>> Although adding ifdef CONFIG_ARM64_BRBE solves the problem as mentioned
>> above, it does break that abstraction. Currently arm_pmu layer is aware
>> about 'branch records' but not about BRBE in particular which the driver
>> adds later on. I suggest we should not break that abstraction.
>>
>> Instead a global 'static struct brbe_hw_attr' in drivers/perf/arm_brbe.c
>> can be initialized into arm_pmu->private during armv8pmu_branch_probe(),
>> which will also solve the allocation-free problem. Also similar helpers
>> armv8pmu_task_ctx_alloc()/free() could be defined to manage task context
>> cache i.e arm_pmu->pmu.task_ctx_cache independently.
>>
>> But now armv8pmu_task_ctx_alloc() can be called after pmu probe confirms
>> to have arm_pmu->has_branch_stack.
>>
>>>
>>>> +
>>>> return probe.present ? 0 : -ENODEV;
>>>> }
>>>
>>> It also seems odd to ceck probe.present *after* checking
>>> arm_pmu_branch_stack_supported().
>>
>> I will reorganize as suggested below.
>>
>>>
>>> With the allocation removed I think this can be written more clearly as:
>>>
>>> | static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>>> | {
>>> | struct armv8pmu_probe_info probe = {
>>> | .pmu = cpu_pmu,
>>> | .present = false,
>>> | };
>>> | int ret;
>>> |
>>> | ret = smp_call_function_any(&cpu_pmu->supported_cpus,
>>> | __armv8pmu_probe_pmu,
>>> | &probe, 1);
>>> | if (ret)
>>> | return ret; > |
>>> | if (!probe.present)
>>> | return -ENODEV;
>>> |
>>> | if (arm_pmu_branch_stack_supported(cpu_pmu))
>>> | ret = branch_records_alloc(cpu_pmu);
>>> |
>>> | return ret;
>>> | }
>
> Could we not simplify this as below and keep the abstraction, since we
> already have it ?
No, there is an allocation dependency before the smp call context.
>
>>> | static int armv8pmu_probe_pmu(struct arm_pmu *cpu_pmu)
>>> | {
>>> | struct armv8pmu_probe_info probe = {
>>> | .pmu = cpu_pmu,
>>> | .present = false,
>>> | };
>>> | int ret;
>>> |
>>> | ret = smp_call_function_any(&cpu_pmu->supported_cpus,
>>> | __armv8pmu_probe_pmu,
>>> | &probe, 1);
>>> | if (ret)
>>> | return ret;
>>> | if (!probe.present)
>>> | return -ENODEV;
>>> |
>>> | if (!arm_pmu_branch_stack_supported(cpu_pmu))
>>> | return 0;
>>> |
>>> | ret = armv8pmu_private_alloc(cpu_pmu);
This needs to be allocated before each supported PMU gets probed via
__armv8pmu_probe_pmu() inside smp_call_function_any() callbacks that
unfortunately cannot do memory allocation.
>>> | if (ret)
>>> | return ret;
>>> |
>>> | ret = branch_records_alloc(cpu_pmu);
>>> | if (ret)
>>> | armv8pmu_private_free(cpu_pmu);
>>> |
>>> | return ret;
>>> | }
Changing the abstraction will cause too much code churn, this late in
the development phase, which should be avoided IMHO.
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