[PATCH v2] mm/vmalloc: Use dedicated unbound workqueue for vmap purge/drain

[Uladzislau Rezki (Sony)]

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.

If queuing work to the dedicated workqueue is not possible(during
early boot), fall back to processing locally to avoid losing progress.

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

Cc: lirongqing <lirongqing@xxxxxxxxx>
Link: https://lore.kernel.org/all/20260319074307.2325-1-lirongqing@xxxxxxxxx/
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
 mm/vmalloc.c | 74 +++++++++++++++++++++++++++++++++-------------------
 1 file changed, 47 insertions(+), 27 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 61caa55a4402..6bc2523bf75b 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 +2392,30 @@ 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);
+				vn->work_queued = schedule_drain_vmap_work(&vn->purge_work);
 
-				if (cpumask_test_cpu(i, cpu_online_mask))
-					schedule_work_on(i, &vn->purge_work);
-				else
-					schedule_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;
+				if (vn->work_queued) {
+					nr_purge_helpers--;
+					continue;
+				}
 			}
-		}
 
-		for_each_cpu(i, &purge_nodes) {
-			vn = &vmap_nodes[i];
+			/* Sync path. Process locally. */
+			purge_vmap_node(&vn->purge_work);
+			nr_purged_areas += vn->nr_purged;
+		}
 
-			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 +2479,7 @@ 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 +5491,15 @@ 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