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Subject: Re: [PATCH v1 12/13] perf tools: improve error handling of AMD Branch
 Sampling
To:     Stephane Eranian <eranian@google.com>, linux-kernel@vger.kernel.org
Cc:     peterz@infradead.org, acme@redhat.com, jolsa@redhat.com,
        namhyung@kernel.org, irogers@google.com
References: <20210909075700.4025355-1-eranian@google.com>
 <20210909075700.4025355-13-eranian@google.com>
From:   Kim Phillips <kim.phillips@amd.com>
Message-ID: <3fa173c8-2f36-adf5-c55b-ff356c49715f@amd.com>
Date:   Mon, 4 Oct 2021 16:57:37 -0500
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On 9/9/21 2:56 AM, Stephane Eranian wrote:
> 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>
> ---

Hi Stephane,

I've rewritten this patch based on Arnaldo's comments to the previous 
(11/13) patch.  The new version attached depends on this 2-patch series:

https://lore.kernel.org/lkml/20211004214114.188477-1-kim.phillips@amd.com/T/#mc4c9c582e3816ab31af6d0187e6803de1a98ac84

Thanks,

Kim

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From a4cbab762719b30bddec2e278cf8b8eb82e83865 Mon Sep 17 00:00:00 2001
From: Stephane Eranian <eranian@google.com>
Date: Thu, 9 Sep 2021 00:56:59 -0700
Subject: [PATCH] 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@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 f8a9cbd99314..e1f5eff07355 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2753,6 +2753,12 @@ 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)
 {
@@ -2863,6 +2869,14 @@ 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;
-- 
2.31.1


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