[PATCH v2 2/3] mm/page_alloc: Add trace event for per-zone lowmem reserve setup

[Martin Liu <liumartin@google.com>]

This commit introduces the `mm_setup_per_zone_lowmem_reserve` trace
event,which provides detailed insights into the kernel's per-zone lowmem
reserve configuration.

The trace event provides precise timestamps, allowing developers to

1. Correlate lowmem reserve changes with specific kernel events and
able to diagnose unexpected kswapd or direct reclaim behavior
triggered by dynamic changes in lowmem reserve.

2. know memory allocation failures that occur due to insufficient lowmem
reserve, by precisely correlating allocation attempts with reserve
adjustments.

Signed-off-by: Martin Liu <liumartin@google.com>
---
 include/trace/events/kmem.h | 27 +++++++++++++++++++++++++++
 mm/page_alloc.c             |  2 ++
 2 files changed, 29 insertions(+)

diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index 5fd392dae503..9623e68d4d26 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -375,6 +375,33 @@ TRACE_EVENT(mm_setup_per_zone_wmarks,
 		  __entry->watermark_promo)
 );
 
+TRACE_EVENT(mm_setup_per_zone_lowmem_reserve,
+
+	TP_PROTO(struct zone *zone, struct zone *upper_zone, long lowmem_reserve),
+
+	TP_ARGS(zone, upper_zone, lowmem_reserve),
+
+	TP_STRUCT__entry(
+		__field(int, node_id)
+		__string(name, zone->name)
+		__string(upper_name, upper_zone->name)
+		__field(long, lowmem_reserve)
+	),
+
+	TP_fast_assign(
+		__entry->node_id = zone->zone_pgdat->node_id;
+		__assign_str(name);
+		__assign_str(upper_name);
+		__entry->lowmem_reserve = lowmem_reserve;
+	),
+
+	TP_printk("node_id=%d zone name=%s upper_zone name=%s lowmem_reserve_pages=%ld",
+		  __entry->node_id,
+		  __get_str(name),
+		  __get_str(upper_name),
+		  __entry->lowmem_reserve)
+);
+
 /*
  * Required for uniquely and securely identifying mm in rss_stat tracepoint.
  */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 50893061db66..e472b1275166 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5857,6 +5857,8 @@ static void setup_per_zone_lowmem_reserve(void)
 					zone->lowmem_reserve[j] = 0;
 				else
 					zone->lowmem_reserve[j] = managed_pages / ratio;
+				trace_mm_setup_per_zone_lowmem_reserve(zone, upper_zone,
+								       zone->lowmem_reserve[j]);
 			}
 		}
 	}
-- 
2.49.0.rc0.332.g42c0ae87b1-goog