[PATCH v3] mm/lruvec: trace LRU add drains and drain-all requests

[PATCH v3] 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@linux.dev>
---
 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..ff3da07ccb40 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),
+
+	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)
+);
+
+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..e14b7612f896 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. */
@@ -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

Re: [PATCH v3] mm/lruvec: trace LRU add drains and drain-all requests

[Barry Song]

On Thu, Jun 11, 2026 at 3:53 AM JP Kobryn <jp.kobryn@linux.dev> wrote:
>
> 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@linux.dev>

Reviewed-by: Barry Song <baohua@kernel.org>

Some minor nits:

[...]

> +       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);
> +       }

Would "nr_folios" work here, given the surrounding lru_add context?
Alternatively, nr_folios_added might make the meaning a little clearer.

Best Regards
Barry

Re: [PATCH v3] mm/lruvec: trace LRU add drains and drain-all requests

[Shakeel Butt]

On Wed, Jun 10, 2026 at 12:52:20PM -0700, JP Kobryn wrote:
> 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@linux.dev>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>