[PATCH v3 06/13] perf/x86/amd: add AMD branch sampling period adjustment

[Stephane Eranian]

This supplemental patch adds the 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 e294ff5d176e..22f882bc33c6 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 e7c904a9515d..74552845d942 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1257,6 +1257,14 @@ 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)
@@ -1281,6 +1289,10 @@ 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)
-- 
2.34.0.rc2.393.gf8c9666880-goog