Re: [PATCH v6 11/12] perf tools: Improve error handling of AMD Branch Sampling

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Feb 08, 2022 at 01:16:36PM -0800, Stephane Eranian escreveu:
> Improve the error message printed by perf when perf_event_open() fails on
> AMD Zen3 when using the branch sampling feature. In the case of EINVAL, there
> are two main reasons: frequency mode or period is smaller than the depth of
> the branch sampling buffer (16). The patch checks the parameters of the call
> and tries to print a relevant message to explain the error:
> 
> $ perf record -b -e cpu/branch-brs/ -c 10 ls
> Error:
> AMD Branch Sampling does not support sampling period smaller than what is reported in /sys/devices/cpu/caps/branches.
> 
> $ perf record -b -e cpu/branch-brs/ ls
> Error:
> AMD Branch Sampling does not support frequency mode sampling, must pass a fixed sampling period via -c option or cpu/branch-brs,period=xxxx/.
> 
> Signed-off-by: Stephane Eranian <eranian@google.com>
> [Rebased on commit 9fe8895a27a84 ("perf env: Add perf_env__cpuid, perf_env__{nr_}pmu_mappings")]
> Signed-off-by: Kim Phillips <kim.phillips@amd.com>
> ---
>  tools/perf/util/evsel.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index d42f63a484df..7311e7b4d34d 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -2857,6 +2857,12 @@ static bool is_amd_ibs(struct evsel *evsel)
>  	return evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name, "ibs", 3);
>  }
>  
> +static bool is_amd_brs(struct evsel *evsel)
> +{
> +	return ((evsel->core.attr.config & 0xff) == 0xc4) &&
> +	       (evsel->core.attr.sample_type & PERF_SAMPLE_BRANCH_STACK);
> +}
> +

Well, this assumes we're on x86_64, right? Shouldn't we have some extra
condition using perf_env routines to check we're on x86_64.

Did a quick check and powerpc also supports PERF_SAMPLE_BRANCH_STACK

⬢[acme@toolbox perf]$ find arch/ -name "*.c" | xargs grep PERF_SAMPLE_BRANCH_STACK
arch/powerpc/perf/core-book3s.c:		if (event->attr.sample_type & PERF_SAMPLE_BRANCH_STACK) {
arch/x86/events/intel/ds.c:	if (sample_type & PERF_SAMPLE_BRANCH_STACK) {
arch/x86/events/intel/ds.c:					PERF_SAMPLE_BRANCH_STACK |
arch/x86/events/intel/lbr.c: * in PERF_SAMPLE_BRANCH_STACK sample may vary.
arch/x86/kvm/vmx/pmu_intel.c:	 * - set 'sample_type = PERF_SAMPLE_BRANCH_STACK' and
arch/x86/kvm/vmx/pmu_intel.c:		.sample_type = PERF_SAMPLE_BRANCH_STACK,
⬢[acme@toolbox perf]$

arch/powerpc/perf/core-book3s.c:

                if (event->attr.sample_type & PERF_SAMPLE_BRANCH_STACK) {
                        struct cpu_hw_events *cpuhw;
                        cpuhw = this_cpu_ptr(&cpu_hw_events);
                        power_pmu_bhrb_read(event, cpuhw);
                        data.br_stack = &cpuhw->bhrb_stack;
                }

>  int evsel__open_strerror(struct evsel *evsel, struct target *target,
>  			 int err, char *msg, size_t size)
>  {
> @@ -2971,6 +2977,14 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
>  					return scnprintf(msg, size,
>  	"AMD IBS may only be available in system-wide/per-cpu mode.  Try using -a, or -C and workload affinity");
>  			}
> +			if (is_amd_brs(evsel)) {
> +				if (evsel->core.attr.freq)
> +					return scnprintf(msg, size,
> +	"AMD Branch Sampling does not support frequency mode sampling, must pass a fixed sampling period via -c option or cpu/branch-brs,period=xxxx/.");
> +				/* another reason is that the period is too small */
> +				return scnprintf(msg, size,
> +	"AMD Branch Sampling does not support sampling period smaller than what is reported in /sys/devices/cpu/caps/branches.");
> +			}
>  		}
>  
>  		break;
> -- 
> 2.35.0.263.gb82422642f-goog

-- 

- Arnaldo