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

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