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Date:   Thu,  9 Sep 2021 00:56:59 -0700
In-Reply-To: <20210909075700.4025355-1-eranian@google.com>
Message-Id: <20210909075700.4025355-13-eranian@google.com>
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References: <20210909075700.4025355-1-eranian@google.com>
Subject: [PATCH v1 12/13] perf tools: improve error handling of AMD Branch Sampling
From:   Stephane Eranian <eranian@google.com>
To:     linux-kernel@vger.kernel.org
Cc:     peterz@infradead.org, acme@redhat.com, jolsa@redhat.com,
        kim.phillips@amd.com, namhyung@kernel.org, irogers@google.com
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Precedence: bulk

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>
---
 tools/perf/util/evsel.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index f203f178fdb9..8945f08f41b2 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2824,6 +2824,14 @@ int evsel__open_strerror(struct evsel *evsel, struct target *target,
 			return scnprintf(msg, size, "The 'aux_output' feature is not supported, update the kernel.");
 		if (is_amd && (evsel->core.attr.precise_ip || !strncmp(evsel->pmu_name, "ibs", 3)) && (evsel->core.attr.exclude_kernel))
 			return scnprintf(msg, size, "AMD IBS can't exclude kernel events.  Try running at a higher privilege level.");
+
+		if (is_amd && ((evsel->core.attr.config & 0xff) == 0xc4) && (evsel->core.attr.sample_type & PERF_SAMPLE_BRANCH_STACK)) {
+			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;
 	case ENODATA:
 		return scnprintf(msg, size, "Cannot collect data source with the load latency event alone. "
-- 
2.33.0.153.gba50c8fa24-goog