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

[Tobias Schaffner]

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 very latency
being measured.

Signed-off-by: Tobias Schaffner <tobias.schaffner@xxxxxxxxxxx>
---
 MAINTAINERS                  |  1 +
 include/linux/rv_edge_stat.h | 45 ++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)
 create mode 100644 include/linux/rv_edge_stat.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 8014b9f8253e..04589dda0873 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -23654,6 +23654,7 @@ L:	linux-trace-kernel@xxxxxxxxxxxxxxx
 S:	Maintained
 F:	Documentation/trace/rv/
 F:	include/linux/rv.h
+F:	include/linux/rv_edge_stat.h
 F:	include/rv/
 F:	kernel/trace/rv/
 F:	tools/testing/selftests/verification/
diff --git a/include/linux/rv_edge_stat.h b/include/linux/rv_edge_stat.h
new file mode 100644
index 000000000000..751de8074dcc
--- /dev/null
+++ b/include/linux/rv_edge_stat.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Per-edge dwell-time statistics for RV monitors.
+ *
+ * Copyright (C) 2026 Siemens AG
+ * Author: Tobias Schaffner <tobias.schaffner@xxxxxxxxxxx>
+ */
+#ifndef _LINUX_RV_EDGE_STAT_H
+#define _LINUX_RV_EDGE_STAT_H
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+#include <asm/local64.h>
+
+/*
+ * Per-CPU counters kept in local64_t so accounting is safe against interrupt
+ * and NMI nesting on the owning CPU without disabling interrupts -- the same
+ * approach the trace ring buffer uses. Only the owning CPU writes.
+ */
+struct rv_edge_stat {
+	local64_t	count;
+	local64_t	sum_ns;
+	local64_t	max_ns;
+};
+
+static __always_inline
+void rv_edge_stat_account(struct rv_edge_stat *s, u64 dwell_ns)
+{
+	s64 max;
+
+	local64_inc(&s->count);
+	local64_add(dwell_ns, &s->sum_ns);
+
+	/* Keep the largest dwell; retry only if a nested update raced us. */
+	max = local64_read(&s->max_ns);
+	while (dwell_ns > (u64)max) {
+		s64 prev = local64_cmpxchg(&s->max_ns, max, dwell_ns);
+
+		if (prev == max)
+			break;
+		max = prev;
+	}
+}
+
+#endif /* _LINUX_RV_EDGE_STAT_H */
-- 
2.43.0