[PATCH v1 06/13] perf/x86/amd: add AMD branch sampling period adjustment

[Stephane Eranian <eranian@google.com>]

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 on Fam19h Zen3), 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@google.com>
---
 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 66a073f21bb7..e1fb0e5ceaa3 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1375,6 +1375,13 @@ 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 12fb1e68f188..a275553e78b9 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1254,6 +1254,14 @@ 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)
@@ -1271,6 +1279,10 @@ static inline void amd_brs_drain(void)
 	return 0;
 }
 
+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)
-- 
2.33.0.153.gba50c8fa24-goog