Re: [PATCH V3 7/7] arm64/perf: Enable branch stack sampling

[James Clark <james.clark@arm.com>]

On 29/09/2022 08:58, Anshuman Khandual wrote:
> Now that all the required pieces are already in place, just enable the perf
> branch stack sampling support on arm64 platform, by removing the gate which
> blocks it in armpmu_event_init().
> 
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  drivers/perf/arm_pmu.c | 32 +++++++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
> index 93b36933124f..2a9b988b53c2 100644
> --- a/drivers/perf/arm_pmu.c
> +++ b/drivers/perf/arm_pmu.c
> @@ -537,9 +537,35 @@ 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))
> -		return -EOPNOTSUPP;
> +	if (has_branch_stack(event)) {
> +		/*
> +		 * BRBE support is absent. Select CONFIG_ARM_BRBE_PMU
> +		 * in the config, before branch stack sampling events
> +		 * can be requested.
> +		 */
> +		if (!IS_ENABLED(CONFIG_ARM_BRBE_PMU)) {
> +			pr_warn_once("BRBE is disabled, select CONFIG_ARM_BRBE_PMU\n");
> +			return -EOPNOTSUPP;
> +		}
> +
> +		if (event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_KERNEL) {
> +			if (!perfmon_capable()) {

I'm still getting different behaviour compared to x86 when using
perf_event_paranoid because of this perfmon_capable() call here.

> +				pr_warn_once("does not have permission for kernel branch filter\n");

Also I was under the impression that this should be more like a
KERN_INFO loglevel rather than a KERN_WARNING. It's more like expected
behavior rather than unexpected behavior and as far as I know anyone who
sees something in dmesg might think something has gone wrong and try to
follow it up. It is quite a useful message but I remember getting a
review like this before and it made sense to me.

> +				return -EPERM;
> +			}
> +		}
> +
> +		/*
> +		 * Branch stack sampling event can not be supported in
> +		 * case either the required driver itself is absent or
> +		 * BRBE buffer, is not supported. Besides checking for
> +		 * the callback prevents a crash in case it's absent.
> +		 */
> +		if (!armpmu->brbe_supported || !armpmu->brbe_supported(event)) {
> +			pr_warn_once("BRBE is not supported\n");
> +			return -EOPNOTSUPP;
> +		}
> +	}
>  
>  	if (armpmu->map_event(event) == -ENOENT)
>  		return -ENOENT;