linux-kernel - [PATCH v4 13/14] perf tools: Improve error handling of AMD Branch Sampling

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Message-Id: <20211210210229.2991238-14-eranian@google.com>
Date:   Fri, 10 Dec 2021 13:02:28 -0800
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, mpe@...erman.id.au,
        maddy@...ux.ibm.com
Subject: [PATCH v4 13/14] perf tools: Improve error handling of AMD Branch Sampling

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@...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 | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index ff9a755b869f..20c970d0862b 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2862,6 +2862,12 @@ 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)
 {
@@ -2972,6 +2978,14 @@ 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;
-- 
2.34.1.173.g76aa8bc2d0-goog