[PATCH v6 06/12] perf/x86/amd: add AMD branch sampling period adjustment

[Stephane Eranian]

Add code to adjust the sampling event period when used with the Branch
Sampling feature (BRS). Given the depth of the BRS (16), the period is
reduced by that depth such that in the best case scenario, BRS saturates at
the desired sampling period. In practice, though, the processor may execute
more branches. Given a desired period P and a depth D, the kernel programs
the actual period at P - D. After P occurrences of the sampling event, the
counter overflows. It then may take X branches (skid) before the NMI is
caught and held by the hardware and BRS activates. Then, after D branches,
BRS saturates and the NMI is delivered.  With no skid, the effective period
would be (P - D) + D = P. In practice, however, it will likely be (P - D) +
X + D. There is no way to eliminate X or predict X.

Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
---
 arch/x86/events/core.c       |  7 +++++++
 arch/x86/events/perf_event.h | 12 ++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index c2a890caeb0a..ed285f640efe 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1374,6 +1374,13 @@ int x86_perf_event_set_period(struct perf_event *event)
 	    x86_pmu.set_topdown_event_period)
 		return x86_pmu.set_topdown_event_period(event);
 
+	/*
+	 * decrease period by the depth of the BRS feature to get
+	 * the last N taken branches and approximate the desired period
+	 */
+	if (has_branch_stack(event))
+		period = amd_brs_adjust_period(period);
+
 	/*
 	 * If we are way outside a reasonable range then just skip forward:
 	 */
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 3485a4cf0241..25b037b571e4 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1263,6 +1263,14 @@ static inline bool amd_brs_active(void)
 	return cpuc->brs_active;
 }
 
+static inline s64 amd_brs_adjust_period(s64 period)
+{
+	if (period > x86_pmu.lbr_nr)
+		return period - x86_pmu.lbr_nr;
+
+	return period;
+}
+
 #else /* CONFIG_CPU_SUP_AMD */
 
 static inline int amd_pmu_init(void)
@@ -1287,6 +1295,10 @@ static inline void amd_brs_disable_all(void)
 {
 }
 
+static inline s64 amd_brs_adjust_period(s64 period)
+{
+	return period;
+}
 #endif /* CONFIG_CPU_SUP_AMD */
 
 static inline int is_pebs_pt(struct perf_event *event)
-- 
2.35.0.263.gb82422642f-goog