On 6/5/23 17:35, Mark Rutland wrote:

On Wed, May 31, 2023 at 09:34:23AM +0530, Anshuman Khandual wrote:

quoted

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.

[...]

quoted

+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.

[...]

quoted

+static int branch_records_alloc(struct arm_pmu *armpmu)
+{
+	struct pmu_hw_events *events;
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		events = per_cpu_ptr(armpmu->hw_events, cpu);
+		events->branches = kzalloc(sizeof(struct branch_records), GFP_KERNEL);
+		if (!events->branches)
+			return -ENOMEM;
+	}
+	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.

quoted

 }
 
 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.

quoted

+
 	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; 
| }

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel