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

[Suzuki K Poulose]

On 10/10/2022 14:55, James Clark wrote:
> 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@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > ---
> >   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.

Given the generic events framework already checks this for any
privileged branch samples (i.e., for both KERNEL and HV), the
individual drivers must not add additional restrictions.

> > +				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.

+1

Suzuki