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Subject[PATCH v5 12/13] perf tools: Improve error handling of AMD Branch Sampling
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This patch improves 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 a9d506ec3ab9..808c1916a2aa 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);
+}
+
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.rc0.227.g00780c9af4-goog
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