[PATCH v2 1/3] rv: add per-edge dwell-time statistics primitive

[Tobias Schaffner <tobias.schaffner@siemens.com>]

Add a small primitive that records, per automaton edge, how long the
monitor dwelled before taking it with a count, a sum and a maximum.

The counters are per-CPU and lock-free, so a monitor's hot path can
update them without disabling interrupts and without perturbing the latency
being measured. Keep the state and its entry timestamp in one word so
nested transitions cannot charge a dwell to the wrong edge.

Signed-off-by: Tobias Schaffner <tobias.schaffner@siemens.com>
---
 include/rv/edge_stat.h | 69 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 include/rv/edge_stat.h

diff --git a/include/rv/edge_stat.h b/include/rv/edge_stat.h
new file mode 100644
index 000000000000..c5f04fd34aee
--- /dev/null
+++ b/include/rv/edge_stat.h
@@ -0,0 +1,69 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Per-edge dwell-time statistics for RV monitors.
+ *
+ * Copyright (C) 2026 Siemens AG
+ * Author: Tobias Schaffner <tobias.schaffner@siemens.com>
+ */
+#ifndef _RV_EDGE_STAT_H
+#define _RV_EDGE_STAT_H
+
+#include <linux/atomic.h>
+#include <linux/bug.h>
+#include <linux/compiler.h>
+#include <linux/rv.h>
+#include <linux/types.h>
+#include <asm/local64.h>
+
+#ifdef CONFIG_RV_EDGE_STAT
+
+/*
+ * Per-CPU counters kept in local64_t so accounting is safe against interrupt
+ * and NMI nesting on the owning CPU without disabling interrupts. Only the
+ * owning CPU writes.
+ */
+struct rv_edge_stat {
+	local64_t	count;
+	local64_t	sum_ns;
+	local64_t	max_ns;
+};
+
+/*
+ * State and entry timestamp share one word so a transition commits both with
+ * one cmpxchg. Comparing the timestamp also detects nested transitions that
+ * return to the same state.
+ */
+#define RV_STATE_BITS		8
+#define RV_STATE_MASK		GENMASK(RV_STATE_BITS - 1, 0)
+#define RV_TS_MASK		GENMASK(BITS_PER_LONG - RV_STATE_BITS - 1, 0)
+
+#define da_state_of(w)		((unsigned int)((w) & RV_STATE_MASK))
+#define da_ts_of(w)		((u64)(w) >> RV_STATE_BITS)
+#define da_state_pack(s, ts)	(((((da_state_t)(ts)) & RV_TS_MASK) << RV_STATE_BITS) | \
+				 ((da_state_t)(s) & RV_STATE_MASK))
+
+static __always_inline
+void rv_edge_stat_account(struct rv_edge_stat *s, u64 dwell_ns)
+{
+	s64 max;
+	int i;
+
+	local64_inc(&s->count);
+	local64_add(dwell_ns, &s->sum_ns);
+
+	/* Keep the largest dwell; bound retries if a nested update races us. */
+	max = local64_read(&s->max_ns);
+	for (i = 0; dwell_ns > (u64)max; i++) {
+		if (i == MAX_DA_RETRY_RACING_EVENTS) {
+			WARN_ONCE(1, "rv: edge-stat max update exceeded %d retries\n",
+				  MAX_DA_RETRY_RACING_EVENTS);
+			break;
+		}
+		if (local64_try_cmpxchg(&s->max_ns, &max, dwell_ns))
+			break;
+	}
+}
+
+#endif /* CONFIG_RV_EDGE_STAT */
+
+#endif /* _RV_EDGE_STAT_H */
-- 
2.43.0