mm/page_alloc: add trace event for per-zone lowmem reserve setup

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.

Link: https://lkml.kernel.org/r/20250308034606.2036033-3-liumartin@google.com
Signed-off-by: Martin Liu <liumartin@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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.
  */

mm/page_alloc.c

@@ -5943,6 +5943,8 @@ 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]);
 			}
 		}
 	}