[PATCH 3/6] perf/tool/amd/ibs: Warn about sampling period skew

[Ravi Bangoria]

Samples without an L3 miss are discarded and counter is reset with
random value (between 1-15 for fetch pmu and 1-127 for op pmu) when
IBS L3 miss filtering is enabled. This causes a sampling period skew
but there is no way to reconstruct aggregated sampling period. So
print a warning at perf record if user sets l3missonly=1.

Ex:
  # perf record -c 10000 -C 0 -e ibs_op/l3missonly=1/
  WARNING: Hw internally resets sampling period when L3 Miss Filtering is enabled
  and tagged operation does not cause L3 Miss. This causes sampling period skew.

Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>
---
 tools/perf/arch/x86/util/evsel.c | 31 +++++++++++++++++++++++++++++++
 tools/perf/util/evsel.c          |  7 +++++++
 tools/perf/util/evsel.h          |  1 +
 3 files changed, 39 insertions(+)

diff --git a/tools/perf/arch/x86/util/evsel.c b/tools/perf/arch/x86/util/evsel.c
index ac2899a25b7a..acaabdba5db8 100644
--- a/tools/perf/arch/x86/util/evsel.c
+++ b/tools/perf/arch/x86/util/evsel.c
@@ -4,6 +4,8 @@
 #include "util/evsel.h"
 #include "util/env.h"
 #include "linux/string.h"
+#include "util/pmu.h"
+#include "util/debug.h"
 
 void arch_evsel__set_sample_weight(struct evsel *evsel)
 {
@@ -29,3 +31,32 @@ void arch_evsel__fixup_new_cycles(struct perf_event_attr *attr)
 
 	free(env.cpuid);
 }
+
+void arch_evsel__warn_ambiguity(struct evsel *evsel, struct perf_event_attr *attr)
+{
+	struct perf_env *env = evsel__env(evsel);
+	struct perf_pmu *evsel_pmu = evsel__find_pmu(evsel);
+	struct perf_pmu *ibs_fetch_pmu = perf_pmu__find("ibs_fetch");
+	struct perf_pmu *ibs_op_pmu = perf_pmu__find("ibs_op");
+	static int warned_once;
+
+	if (warned_once || !perf_env__cpuid(env) || !env->cpuid ||
+	    !strstarts(env->cpuid, "AuthenticAMD") || !evsel_pmu)
+		return;
+
+	if (ibs_fetch_pmu && ibs_fetch_pmu->type == evsel_pmu->type) {
+		if (attr->config & (1ULL << 59)) {
+			pr_warning(
+"WARNING: Hw internally resets sampling period when L3 Miss Filtering is enabled\n"
+"and tagged operation does not cause L3 Miss. This causes sampling period skew.\n");
+			warned_once = 1;
+		}
+	} else if (ibs_op_pmu && ibs_op_pmu->type == evsel_pmu->type) {
+		if (attr->config & (1ULL << 16)) {
+			pr_warning(
+"WARNING: Hw internally resets sampling period when L3 Miss Filtering is enabled\n"
+"and tagged operation does not cause L3 Miss. This causes sampling period skew.\n");
+			warned_once = 1;
+		}
+	}
+}
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 2a1729e7aee4..4f8b72d4a521 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1064,6 +1064,11 @@ void __weak arch_evsel__fixup_new_cycles(struct perf_event_attr *attr __maybe_un
 {
 }
 
+void __weak arch_evsel__warn_ambiguity(struct evsel *evsel __maybe_unused,
+				       struct perf_event_attr *attr __maybe_unused)
+{
+}
+
 static void evsel__set_default_freq_period(struct record_opts *opts,
 					   struct perf_event_attr *attr)
 {
@@ -1339,6 +1344,8 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
 	 */
 	if (evsel__is_dummy_event(evsel))
 		evsel__reset_sample_bit(evsel, BRANCH_STACK);
+
+	arch_evsel__warn_ambiguity(evsel, attr);
 }
 
 int evsel__set_filter(struct evsel *evsel, const char *filter)
diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
index 041b42d33bf5..195ae30ec45b 100644
--- a/tools/perf/util/evsel.h
+++ b/tools/perf/util/evsel.h
@@ -281,6 +281,7 @@ void evsel__set_sample_id(struct evsel *evsel, bool use_sample_identifier);
 
 void arch_evsel__set_sample_weight(struct evsel *evsel);
 void arch_evsel__fixup_new_cycles(struct perf_event_attr *attr);
+void arch_evsel__warn_ambiguity(struct evsel *evsel, struct perf_event_attr *attr);
 
 int evsel__set_filter(struct evsel *evsel, const char *filter);
 int evsel__append_tp_filter(struct evsel *evsel, const char *filter);
-- 
2.27.0