[PATCH v4] mm/lruvec: trace LRU add drains and drain-all requests

[JP Kobryn]

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.

Add mm_lru_add_drain_all to capture callers of __lru_add_drain_all and
whether they set the force flag for all CPUs. The tracepoint resembles
the signature of the enclosing function, but is needed because of
potential inlining.

Signed-off-by: JP Kobryn <jp.kobryn@xxxxxxxxx>
Reviewed-by: Barry Song <baohua@xxxxxxxxxx>
Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
---
v4:
  - renamed nr_folio_add to nr_folios in lru_add_drain()
  - renamed nr to nr_folios in tracepoint for consistency

v3: https://lore.kernel.org/linux-mm/20260610195220.12403-1-jp.kobryn@xxxxxxxxx/
  - restored and renamed tracepoint in __lru_add_drain_all

v2: https://lore.kernel.org/linux-mm/20260609041156.31127-1-jp.kobryn@xxxxxxxxx/
  - removed mm_lru_drain_all tracepoint

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

 include/trace/events/pagemap.h | 37 ++++++++++++++++++++++++++++++++++
 mm/swap.c                      |  7 ++++++-
 2 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
index 171524d3526d..df6ac4d13dcf 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -77,6 +77,43 @@ 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_folios),
+
+	TP_ARGS(cpu, nr_folios),
+
+	TP_STRUCT__entry(
+		__field(int,		cpu	)
+		__field(unsigned int,	nr_folios	)
+	),
+
+	TP_fast_assign(
+		__entry->cpu	= cpu;
+		__entry->nr_folios	= nr_folios;
+	),
+
+	TP_printk("cpu=%d nr_folios=%u", __entry->cpu, __entry->nr_folios)
+);
+
+TRACE_EVENT(mm_lru_add_drain_all,
+
+	TP_PROTO(bool force_all_cpus),
+
+	TP_ARGS(force_all_cpus),
+
+	TP_STRUCT__entry(
+		__field(bool,	force_all_cpus	)
+	),
+
+	TP_fast_assign(
+		__entry->force_all_cpus	= force_all_cpus;
+	),
+
+	TP_printk("force_all_cpus=%s",
+		__entry->force_all_cpus ? "true" : "false")
+);
+
 #endif /* _TRACE_PAGEMAP_H */
 
 /* This part must be outside protection */
diff --git a/mm/swap.c b/mm/swap.c
index 588f50d8f1a8..b506fa912a93 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 = folio_batch_count(fbatch);
 
-	if (folio_batch_count(fbatch))
+	if (nr_folios) {
 		folio_batch_move_lru(fbatch, lru_add);
+		trace_mm_lru_add_drain(cpu, nr_folios);
+	}
 
 	fbatch = &fbatches->lru_move_tail;
 	/* Disabling interrupts below acts as a compiler barrier. */
@@ -869,6 +872,8 @@ static inline void __lru_add_drain_all(bool force_all_cpus)
 	if (WARN_ON(!mm_percpu_wq))
 		return;
 
+	trace_mm_lru_add_drain_all(force_all_cpus);
+
 	/*
 	 * Guarantee folio_batch counter stores visible by this CPU
 	 * are visible to other CPUs before loading the current drain
-- 
2.54.0