Re: [PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU
From: Suzuki K Poulose
Date: Thu Jun 08 2023 - 06:13:48 EST
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 ?
+ }
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 ?
>> | 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);
>> | if (ret)
>> | return ret;
>> |
>> | ret = branch_records_alloc(cpu_pmu);
>> | if (ret)
>> | armv8pmu_private_free(cpu_pmu);
>> |
>> | return ret;
>> | }
Suzuki