[PATCH RFC 2/2] mm/mglru: add tracepoint for inc_max_seq

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

Aging in MGLRU advances max_seq via inc_max_seq(), creating a new
youngest generation. There is currently no tracepoint on this path, so
the moment a new generation is created, and how the min_seq of each type
trails behind it, cannot be observed as it happens.

Add mm_mglru_inc_max_seq, emitted right after max_seq is bumped, with
the memcg id and the new max_seq alongside the anon and file min_seq.
Paired with the mm_mglru_isolate_folios tracepoint it makes the full
aging-to-eviction window observable per memcg.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 include/trace/events/vmscan.h | 30 ++++++++++++++++++++++++++++++
 mm/vmscan.c                   |  4 ++++
 2 files changed, 34 insertions(+)

diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
index a0e3cf75294b..ab93c2f4b318 100644
--- a/include/trace/events/vmscan.h
+++ b/include/trace/events/vmscan.h
@@ -439,6 +439,36 @@ TRACE_EVENT(mm_mglru_isolate_folios,
 		__entry->max_seq)
 );
 
+TRACE_EVENT(mm_mglru_inc_max_seq,
+
+	TP_PROTO(u64 memcg_id,
+		unsigned long max_seq,
+		unsigned long anon_min_seq,
+		unsigned long file_min_seq),
+
+	TP_ARGS(memcg_id, max_seq, anon_min_seq, file_min_seq),
+
+	TP_STRUCT__entry(
+		__field(u64, memcg_id)
+		__field(unsigned long, max_seq)
+		__field(unsigned long, anon_min_seq)
+		__field(unsigned long, file_min_seq)
+	),
+
+	TP_fast_assign(
+		__entry->memcg_id = memcg_id;
+		__entry->max_seq = max_seq;
+		__entry->anon_min_seq = anon_min_seq;
+		__entry->file_min_seq = file_min_seq;
+	),
+
+	TP_printk("memcg_id=%llu max_seq=%lu anon_min_seq=%lu file_min_seq=%lu",
+		__entry->memcg_id,
+		__entry->max_seq,
+		__entry->anon_min_seq,
+		__entry->file_min_seq)
+);
+
 TRACE_EVENT(mm_vmscan_write_folio,
 
 	TP_PROTO(struct folio *folio),
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 771fe6827939..3c806a57d113 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4065,6 +4065,10 @@ static bool inc_max_seq(struct lruvec *lruvec, unsigned long seq, int swappiness
 	WRITE_ONCE(lrugen->timestamps[next], jiffies);
 	/* make sure preceding modifications appear */
 	smp_store_release(&lrugen->max_seq, lrugen->max_seq + 1);
+	trace_mm_mglru_inc_max_seq(mem_cgroup_id(lruvec_memcg(lruvec)),
+				   lrugen->max_seq,
+				   lrugen->min_seq[LRU_GEN_ANON],
+				   lrugen->min_seq[LRU_GEN_FILE]);
 unlock:
 	lruvec_unlock_irq(lruvec);
 
-- 
2.34.1