[PATCH v2] mm/lruvec: trace LRU add drains

[JP Kobryn <jp.kobryn@linux.dev>]

LRU add batches can be drained before they reach capacity. This can be a
source of LRU lock contention, but it is not currently possible to
attribute these drains to callers with existing tracepoints.

Add mm_lru_add_drain to report the CPU and lru_add batch count when an
lru_add batch is drained. This allows tracing to distinguish full drains
from partial drains and attribute them to the calling stack.

Signed-off-by: JP Kobryn <jp.kobryn@linux.dev>
---
v2:
  - removed mm_lru_drain_all tracepoint

v1: https://lore.kernel.org/linux-mm/20260609041156.31127-1-jp.kobryn@linux.dev/

 include/trace/events/pagemap.h | 19 +++++++++++++++++++
 mm/swap.c                      |  5 ++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
index 171524d3526d..fc28745507be 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -77,6 +77,25 @@ TRACE_EVENT(mm_lru_activate,
 	TP_printk("folio=%p pfn=0x%lx", __entry->folio, __entry->pfn)
 );
 
+TRACE_EVENT(mm_lru_add_drain,
+
+	TP_PROTO(int cpu, unsigned int nr),
+
+	TP_ARGS(cpu, nr),
+
+	TP_STRUCT__entry(
+		__field(int,		cpu	)
+		__field(unsigned int,	nr	)
+	),
+
+	TP_fast_assign(
+		__entry->cpu	= cpu;
+		__entry->nr	= nr;
+	),
+
+	TP_printk("cpu=%d nr=%u", __entry->cpu, __entry->nr)
+);
+
 #endif /* _TRACE_PAGEMAP_H */
 
 /* This part must be outside protection */
diff --git a/mm/swap.c b/mm/swap.c
index 588f50d8f1a8..8fd6808bfc6e 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -694,9 +694,12 @@ void lru_add_drain_cpu(int cpu)
 {
 	struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
 	struct folio_batch *fbatch = &fbatches->lru_add;
+	unsigned int nr_folios_add = folio_batch_count(fbatch);
 
-	if (folio_batch_count(fbatch))
+	if (nr_folios_add) {
 		folio_batch_move_lru(fbatch, lru_add);
+		trace_mm_lru_add_drain(cpu, nr_folios_add);
+	}
 
 	fbatch = &fbatches->lru_move_tail;
 	/* Disabling interrupts below acts as a compiler barrier. */
-- 
2.54.0