Re: [PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU

On Tue, Jun 06, 2023 at 04:04:25PM +0530, Anshuman Khandual wrote:

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

quoted

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

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.

I understand the rationale, but I think it's simpler for now to break that
abstraction. We can always refactor it later.

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. 

IIUC that's not going to work for big.LITTLE systems where the BRBE support
varies, as we need this data per arm_pmu.

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.

I think those are different, and should be kept.

Thanks,
Mark.

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