[PATCH] mm/vmalloc: Use dedicated unbound workqueue for vmap purge/drain

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

The drain_vmap_area_work() function can take >10ms to complete
when there are many accumulated vmap areas in a system with a
high CPU count, causing workqueue watchdog warnings when run
via schedule_work():

[ 2069.796205] workqueue: drain_vmap_area_work hogged CPU for >10000us 4 times, consider switching to WQ_UNBOUND
[ 2192.823225] workqueue: drain_vmap_area_work hogged CPU for >10000us 5 times, consider switching to WQ_UNBOUND

Switch to a dedicated WQ_UNBOUND workqueue to allow the scheduler to
run this background task on any available CPU, improving responsiveness.
Use WQ_MEM_RECLAIM to ensure forward progress under memory pressure.

Also simplify purge helper scheduling by removing cpumask-based
iteration in favour to iterating directly over vmap nodes with
pending work.

Cc: lirongqing <lirongqing@baidu.com>
Link: https://lore.kernel.org/all/20260319074307.2325-1-lirongqing@baidu.com/
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 mm/vmalloc.c | 63 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 39 insertions(+), 24 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 61caa55a4402..7c1ab4a57409 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -949,6 +949,7 @@ static struct vmap_node {
 	struct list_head purge_list;
 	struct work_struct purge_work;
 	unsigned long nr_purged;
+	bool work_queued;
 } single;
 
 /*
@@ -1067,6 +1068,7 @@ static void reclaim_and_purge_vmap_areas(void);
 static BLOCKING_NOTIFIER_HEAD(vmap_notify_list);
 static void drain_vmap_area_work(struct work_struct *work);
 static DECLARE_WORK(drain_vmap_work, drain_vmap_area_work);
+static struct workqueue_struct *drain_vmap_wq;
 
 static __cacheline_aligned_in_smp atomic_long_t nr_vmalloc_pages;
 static __cacheline_aligned_in_smp atomic_long_t vmap_lazy_nr;
@@ -2335,6 +2337,19 @@ static void purge_vmap_node(struct work_struct *work)
 	reclaim_list_global(&local_list);
 }
 
+static bool
+schedule_drain_vmap_work(struct work_struct *work)
+{
+	struct workqueue_struct *wq = READ_ONCE(drain_vmap_wq);
+
+	if (wq) {
+		queue_work(wq, work);
+		return true;
+	}
+
+	return false;
+}
+
 /*
  * Purges all lazily-freed vmap areas.
  */
@@ -2342,19 +2357,12 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end,
 		bool full_pool_decay)
 {
 	unsigned long nr_purged_areas = 0;
+	unsigned int nr_purge_nodes = 0;
 	unsigned int nr_purge_helpers;
-	static cpumask_t purge_nodes;
-	unsigned int nr_purge_nodes;
 	struct vmap_node *vn;
-	int i;
 
 	lockdep_assert_held(&vmap_purge_lock);
 
-	/*
-	 * Use cpumask to mark which node has to be processed.
-	 */
-	purge_nodes = CPU_MASK_NONE;
-
 	for_each_vmap_node(vn) {
 		INIT_LIST_HEAD(&vn->purge_list);
 		vn->skip_populate = full_pool_decay;
@@ -2374,10 +2382,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end,
 		end = max(end, list_last_entry(&vn->purge_list,
 			struct vmap_area, list)->va_end);
 
-		cpumask_set_cpu(node_to_id(vn), &purge_nodes);
+		nr_purge_nodes++;
 	}
 
-	nr_purge_nodes = cpumask_weight(&purge_nodes);
 	if (nr_purge_nodes > 0) {
 		flush_tlb_kernel_range(start, end);
 
@@ -2385,29 +2392,25 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end,
 		nr_purge_helpers = atomic_long_read(&vmap_lazy_nr) / lazy_max_pages();
 		nr_purge_helpers = clamp(nr_purge_helpers, 1U, nr_purge_nodes) - 1;
 
-		for_each_cpu(i, &purge_nodes) {
-			vn = &vmap_nodes[i];
+		for_each_vmap_node(vn) {
+			vn->work_queued = false;
+
+			if (list_empty(&vn->purge_list))
+				continue;
 
 			if (nr_purge_helpers > 0) {
 				INIT_WORK(&vn->purge_work, purge_vmap_node);
-
-				if (cpumask_test_cpu(i, cpu_online_mask))
-					schedule_work_on(i, &vn->purge_work);
-				else
-					schedule_work(&vn->purge_work);
-
+				vn->work_queued = schedule_drain_vmap_work(&vn->purge_work);
 				nr_purge_helpers--;
 			} else {
-				vn->purge_work.func = NULL;
 				purge_vmap_node(&vn->purge_work);
 				nr_purged_areas += vn->nr_purged;
 			}
 		}
 
-		for_each_cpu(i, &purge_nodes) {
-			vn = &vmap_nodes[i];
-
-			if (vn->purge_work.func) {
+		/* Wait for completion if queued any. */
+		for_each_vmap_node(vn) {
+			if (vn->work_queued) {
 				flush_work(&vn->purge_work);
 				nr_purged_areas += vn->nr_purged;
 			}
@@ -2471,7 +2474,7 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 
 	/* After this point, we may free va at any time */
 	if (unlikely(nr_lazy > nr_lazy_max))
-		schedule_work(&drain_vmap_work);
+		schedule_drain_vmap_work(&drain_vmap_work);
 }
 
 /*
@@ -5483,3 +5486,15 @@ void __init vmalloc_init(void)
 	vmap_node_shrinker->scan_objects = vmap_node_shrink_scan;
 	shrinker_register(vmap_node_shrinker);
 }
+
+static int __init vmalloc_init_workqueue(void)
+{
+	struct workqueue_struct *wq;
+
+	wq = alloc_workqueue("vmap_drain", WQ_UNBOUND | WQ_MEM_RECLAIM, 0);
+	WARN_ON(wq == NULL);
+	WRITE_ONCE(drain_vmap_wq, wq);
+
+	return 0;
+}
+early_initcall(vmalloc_init_workqueue);
-- 
2.47.3