[RFC PATCH 3/7] mm/damon/perf: add debugfs statistics interface

[Kunwu Chan <kunwu.chan@gmail.com>]

From: Lian Wang <lianux.mm@gmail.com>

Expose the aggregated per-CPU counters and the global event-stage in a
debug-only debugfs perf_stats file.  The format is explicitly unstable;
tracepoints are the stable diagnostic interface.

Co-developed-by: Kunwu Chan <kunwu.chan@gmail.com>
Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
Signed-off-by: Lian Wang <lianux.mm@gmail.com>
---
 mm/damon/perf/Makefile  |   2 +-
 mm/damon/perf/debugfs.c | 142 ++++++++++++++++++++++++++++++++++++++++
 mm/damon/perf/stats.c   |   2 +-
 3 files changed, 144 insertions(+), 2 deletions(-)
 create mode 100644 mm/damon/perf/debugfs.c

diff --git a/mm/damon/perf/Makefile b/mm/damon/perf/Makefile
index 5c46d3da7ef8..150cbaa875fa 100644
--- a/mm/damon/perf/Makefile
+++ b/mm/damon/perf/Makefile
@@ -2,4 +2,4 @@
 
 # Observability: per-CPU counters, tracepoints, debugfs perf_stats
 obj-$(CONFIG_DAMON_PERF_OBSERVE)	+= damon-perf.o
-damon-perf-objs			:= stats.o
+damon-perf-objs			:= stats.o debugfs.o
diff --git a/mm/damon/perf/debugfs.c b/mm/damon/perf/debugfs.c
new file mode 100644
index 000000000000..c54dd7644ac3
--- /dev/null
+++ b/mm/damon/perf/debugfs.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * DAMON Perf Observability — debugfs Interface
+ *
+ * Exposes one file under /sys/kernel/debug/damon/:
+ *
+ *   perf_stats — per-CPU pipeline statistics in tabular form
+ *
+ * DEBUG ONLY — format may change without notice; do not parse in
+ * scripts.  For stable diagnostics, use the tracepoints under
+ *   /sys/kernel/debug/tracing/events/damon/
+ */
+
+#include <linux/cpu.h>
+#include <linux/cpumask.h>
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+
+#include "perf.h"
+
+static struct dentry *damon_debugfs_dir;
+
+/*
+ * perf_stats
+ */
+
+static const char *state_name(int s)
+{
+	switch (s) {
+	case DAMON_PERF_STATE_UNINIT:	return "UNINIT";
+	case DAMON_PERF_STATE_CREATED:	return "CREATED";
+	case DAMON_PERF_STATE_BOUND:	return "BOUND";
+	case DAMON_PERF_STATE_ENABLED:	return "ENABLED";
+	case DAMON_PERF_STATE_RUNNING:	return "RUNNING";
+	case DAMON_PERF_STATE_ERROR:	return "ERROR";
+	default:			return "?";
+	}
+}
+
+static int perf_stats_show(struct seq_file *m, void *v)
+{
+	struct damon_perf_stats agg, st;
+	int cpu;
+	bool first = true;
+
+	damon_perf_stats_aggregate(&agg);
+
+	seq_puts(m, "    --------------  ----------\n");
+	seq_puts(m, "    Counter          Value\n");
+	seq_puts(m, "    --------------  ----------\n");
+
+#define STAT_ROW(label, field) \
+	seq_printf(m, "    %-12s   %8llu\n", label, agg.field)
+
+	STAT_ROW("callback",    callback);
+	STAT_ROW("valid",       sample_valid);
+	STAT_ROW("null",        sample_null);
+	STAT_ROW("addr_zero",   sample_addr_zero);
+	STAT_ROW("kernel",      sample_kernel);
+	STAT_ROW("inv_phys",    sample_invalid_phys);
+	STAT_ROW("enqueue",     enqueue);
+	STAT_ROW("dequeue",     dequeue);
+	STAT_ROW("overflow",    overflow);
+	STAT_ROW("ring_peak",   ring_peak);
+	STAT_ROW("match",       match);
+	STAT_ROW("miss_tgid",   miss_tgid);
+	STAT_ROW("miss_region", miss_region);
+	STAT_ROW("miss_bound",  miss_boundary);
+	STAT_ROW("update",      update);
+
+#undef STAT_ROW
+
+	seq_puts(m, "    --------------  ----------\n\n");
+
+	/* Per-CPU breakdown */
+	cpus_read_lock();
+	for_each_online_cpu(cpu) {
+		damon_perf_stats_snapshot(cpu, &st);
+
+		/* Skip truly idle CPUs */
+		if (st.cpu_state == DAMON_PERF_STATE_UNINIT &&
+		    !st.callback && !st.enqueue && !st.dequeue)
+			continue;
+
+		if (first) {
+			seq_puts(m, "  Per-CPU (non-zero / non-UNINIT):\n");
+			first = false;
+		}
+
+		seq_printf(m, "  CPU%02d: st=%-7s cb=%llu enq=%llu deq=%llu ovf=%llu match=%llu tgid=%llu noreg=%llu bound=%llu upd=%llu\n",
+			cpu, state_name(st.cpu_state),
+			st.callback, st.enqueue, st.dequeue,
+			st.overflow, st.match,
+			st.miss_tgid, st.miss_region, st.miss_boundary,
+			st.update);
+	}
+	cpus_read_unlock();
+
+	return 0;
+}
+
+static int perf_stats_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, perf_stats_show, NULL);
+}
+
+static const struct file_operations perf_stats_fops = {
+	.open		= perf_stats_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+/*
+ * Init / teardown
+ */
+
+int damon_perf_debugfs_init(void)
+{
+	if (!debugfs_initialized())
+		return -ENODEV;
+
+	/*
+	 * Create the "damon" directory first.  debugfs_create_dir() mounts
+	 * debugfs via simple_pin_fs() before touching debugfs_mount, so it
+	 * is safe to call during initcall time -- unlike debugfs_lookup(),
+	 * which dereferences debugfs_mount unconditionally and crashes with
+	 * a NULL mount.  If the directory already exists (created by another
+	 * DAMON interface) debugfs_create_dir() returns -EEXIST; fall back
+	 * to debugfs_lookup(), which is now safe because the mount exists.
+	 */
+	damon_debugfs_dir = debugfs_create_dir("damon", NULL);
+	if (damon_debugfs_dir == ERR_PTR(-EEXIST))
+		damon_debugfs_dir = debugfs_lookup("damon", NULL);
+	if (IS_ERR(damon_debugfs_dir))
+		return PTR_ERR(damon_debugfs_dir);
+
+	debugfs_create_file("perf_stats", 0400, damon_debugfs_dir,
+			NULL, &perf_stats_fops);
+
+	return 0;
+}
diff --git a/mm/damon/perf/stats.c b/mm/damon/perf/stats.c
index a869f115bb26..ae5b0037a31d 100644
--- a/mm/damon/perf/stats.c
+++ b/mm/damon/perf/stats.c
@@ -291,5 +291,5 @@ void damon_perf_stats_aggregate(struct damon_perf_stats *dst)
 
 int damon_perf_framework_init(void)
 {
-	return 0;
+	return damon_perf_debugfs_init();
 }
-- 
2.43.0