linux-kernel - [PATCH v7 12/13] perf tools: Improve error handling of AMD Branch Sampling

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Date:   Tue, 22 Mar 2022 15:15:16 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, kim.phillips@....com, acme@...hat.com,
        jolsa@...hat.com, songliubraving@...com, rafael@...nel.org,
        ravi.bangoria@....com, sandipan.das@....com
Subject: [PATCH v7 12/13] perf tools: Improve error handling of AMD Branch Sampling

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@...gle.com>
[Rebased on commit 9fe8895a27a84 ("perf env: Add perf_env__cpuid, perf_env__{nr_}pmu_mappings")]
Signed-off-by: Kim Phillips <kim.phillips@....com>
---
 tools/perf/util/evsel.c | 43 ++++++++++++++++++++++++++++++-----------
 1 file changed, 32 insertions(+), 11 deletions(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index e8ff7a4bd490..03f720a85764 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2858,6 +2858,16 @@ static bool is_amd_ibs(struct evsel *evsel)
 	    || (evsel->pmu_name && !strncmp(evsel->pmu_name, "ibs", 3));
 }
 
+static bool is_amd_brs_event(struct evsel *evsel)
+{
+	return (evsel->core.attr.config & 0xff) == 0xc4;
+}
+
+static bool has_amd_brs(struct evsel *evsel)
+{
+	return (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)
 {
@@ -2947,6 +2957,28 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
 	"We found oprofile daemon running, please stop it and try again.");
 		break;
 	case EINVAL:
+		if (is_amd(arch, cpuid)) {
+			if (is_amd_ibs(evsel)) {
+				if (evsel->core.attr.exclude_kernel)
+					return scnprintf(msg, size,
+	"AMD IBS can't exclude kernel events.  Try running at a higher privilege level.");
+				if (!evsel->core.system_wide)
+					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 (has_amd_brs(evsel)) {
+				if (!is_amd_brs_event(evsel))
+					return scnprintf(msg, size,
+	"AMD Branch Sampling can only be used with the cpu/branch-brs,period=xxxx/ event or equivalent encodings.");
+				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.");
+			}
+		}
+
 		if (evsel->core.attr.sample_type & PERF_SAMPLE_CODE_PAGE_SIZE && perf_missing_features.code_page_size)
 			return scnprintf(msg, size, "Asking for the code page size isn't supported by this kernel.");
 		if (evsel->core.attr.sample_type & PERF_SAMPLE_DATA_PAGE_SIZE && perf_missing_features.data_page_size)
@@ -2963,17 +2995,6 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
 			return scnprintf(msg, size,
 	"Invalid event (%s) in per-thread mode, enable system wide with '-a'.",
 					evsel__name(evsel));
-		if (is_amd(arch, cpuid)) {
-			if (is_amd_ibs(evsel)) {
-				if (evsel->core.attr.exclude_kernel)
-					return scnprintf(msg, size,
-	"AMD IBS can't exclude kernel events.  Try running at a higher privilege level.");
-				if (!evsel->core.system_wide)
-					return scnprintf(msg, size,
-	"AMD IBS may only be available in system-wide/per-cpu mode.  Try using -a, or -C and workload affinity");
-			}
-		}
-
 		break;
 	case ENODATA:
 		return scnprintf(msg, size, "Cannot collect data source with the load latency event alone. "
-- 
2.35.1.894.gb6a874cedc-goog