Re: [PATCH 0/3] mm: add per-migratetype counts to buddy allocator and optimize pagetypeinfo access

[Barry Song <baohua@kernel.org>]

On Fri, Nov 28, 2025 at 11:11 AM Hongru Zhang <zhanghongru06@gmail.com> wrote:
>
> On mobile devices, some user-space memory management components check
> memory pressure and fragmentation status periodically or via PSI, and
> take actions such as killing processes or performing memory compaction
> based on this information.
>
> Under high load scenarios, reading /proc/pagetypeinfo causes memory
> management components or memory allocation/free paths to be blocked
> for extended periods waiting for the zone lock, leading to the following
> issues:
> 1. Long interrupt-disabled spinlocks - occasionally exceeding 10ms on Qcom
>    8750 platforms, reducing system real-time performance
> 2. Memory management components being blocked for extended periods,
>    preventing rapid acquisition of memory fragmentation information for
>    critical memory management decisions and actions
> 3. Increased latency in memory allocation and free paths due to prolonged
>    zone lock contention

Do you have an idea how long each seq_printf call takes?

Assuming seq_printf is costly, printing while holding
zone->lock may be suboptimal. A further optimization might be:

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 2370c6fb1fcd..f501ca2840a6 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1570,6 +1570,7 @@ static int frag_show(struct seq_file *m, void *arg)
 	return 0;
 }
 
+#if 0
 static void pagetypeinfo_showfree_print(struct seq_file *m,
 					pg_data_t *pgdat, struct zone *zone)
 {
@@ -1611,6 +1612,63 @@ static void pagetypeinfo_showfree_print(struct seq_file *m,
 		seq_putc(m, '\n');
 	}
 }
+#endif
+
+static void pagetypeinfo_showfree_print(struct seq_file *m,
+					pg_data_t *pgdat,
+					struct zone *zone)
+{
+	unsigned long freecounts[MIGRATE_TYPES][NR_PAGE_ORDERS] = { 0 };
+	bool overflow[MIGRATE_TYPES][NR_PAGE_ORDERS] = { 0 };
+	int order, mtype;
+
+	for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
+		for (order = 0; order < NR_PAGE_ORDERS; ++order) {
+			struct free_area *area;
+			struct list_head *curr;
+			unsigned long freecount = 0;
+
+			area = &zone->free_area[order];
+
+			list_for_each(curr, &area->free_list[mtype]) {
+				/*
+				 * Cap the free_list iteration because it might
+				 * be really large and we are under a spinlock
+				 * so a long time spent here could trigger a
+				 * hard lockup detector. Anyway this is a
+				 * debugging tool so knowing there is a handful
+				 * of pages of this order should be more than
+				 * sufficient.
+				 */
+				if (++freecount >= 100000) {
+					overflow[mtype][order] = true;
+					break;
+				}
+			}
+			freecounts[mtype][order] = freecount;
+			spin_unlock_irq(&zone->lock);
+			cond_resched();
+			spin_lock_irq(&zone->lock);
+		}
+	}
+
+	/* printing completely outside the lock */
+	spin_unlock_irq(&zone->lock);
+	for (mtype = 0; mtype < MIGRATE_TYPES; mtype++) {
+		seq_printf(m, "Node %4d, zone %8s, type %12s ",
+			   pgdat->node_id,
+			   zone->name,
+			   migratetype_names[mtype]);
+
+		for (order = 0; order < NR_PAGE_ORDERS; ++order) {
+			seq_printf(m, "%s%6lu ",
+				   overflow[mtype][order] ? ">" : "",
+				   freecounts[mtype][order]);
+		}
+		seq_putc(m, '\n');
+	}
+	spin_lock_irq(&zone->lock);
+}
 
 /* Print out the free pages at each order for each migratetype */
 static void pagetypeinfo_showfree(struct seq_file *m, void *arg)

Thanks
Barry