Re: [PATCH v3] mm/vmalloc: Use dedicated unbound workqueues for vmap drain

Baoquan He
2026-04-02T00:23:28Z

On 03/31/26 at 10:23pm, Uladzislau Rezki (Sony) wrote:
> drain_vmap_area_work() function can take >10ms to complete
> when there are many accumulated vmap areas in a system with
> high CPU count, causing workqueue watchdog warnings when run
> via schedule_work():
>
> workqueue: drain_vmap_area_work hogged CPU for >10000us
>
> Move the top-level drain work to a dedicated WQ_UNBOUND
> workqueue so the scheduler can run this background work
> on any available CPU, improving responsiveness. Use the
> WQ_MEM_RECLAIM to ensure forward progress under memory
> pressure.
>
> Move purge helpers to separate WQ_UNBOUND | WQ_MEM_RECLAIM
> workqueue. This allows drain_vmap_work to wait for helpers
> completion without creating dependency on the same rescuer
> thread and avoid a potential parent/child deadlock.
>
> Simplify purge helper scheduling by removing cpumask-based
> iteration to iterating directly over vmap nodes checking
> work_queued state.
>
> Cc: stable@vger.kernel.org
> Cc: lirongqing <lirongqing@baidu.com>
> Fixes: 72210662c5a2 ("mm: vmalloc: offload free_vmap_area_lock lock")
> Link: https://lore.kernel.org/all/20260319074307.2325-1-lirongqing@baidu.com/
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
> mm/vmalloc.c | 79 ++++++++++++++++++++++++++++++++++------------------
> 1 file changed, 52 insertions(+), 27 deletions(-)

LGTM,

Reviewed-by: Baoquan He <bhe@redhat.com>

>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 61caa55a4402..0fa1208a910b 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 +1068,8 @@ 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_helpers_wq;
> +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 +2338,16 @@ static void purge_vmap_node(struct work_struct *work)
> reclaim_list_global(&local_list);
> }
>
> +static bool
> +schedule_drain_vmap_work(struct workqueue_struct *wq,
> + struct work_struct *work)
> +{
> + if (wq)
> + return queue_work(wq, work);
> +
> + return false;
> +}
> +
> /*
> * Purges all lazily-freed vmap areas.
> */
> @@ -2342,19 +2355,12 @@ 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 +2380,9 @@ 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 +2390,31 @@ 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(
> + READ_ONCE(drain_vmap_helpers_wq), &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 +2478,8 @@ 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(READ_ONCE(drain_vmap_wq),
> + &drain_vmap_work);
> }
>
> /*
> @@ -5483,3 +5491,20 @@ 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 *drain_wq, *helpers_wq;
> + unsigned int flags = WQ_UNBOUND | WQ_MEM_RECLAIM;
> +
> + drain_wq = alloc_workqueue("vmap_drain", flags, 0);
> + WARN_ON_ONCE(drain_wq == NULL);
> + WRITE_ONCE(drain_vmap_wq, drain_wq);
> +
> + helpers_wq = alloc_workqueue("vmap_drain_helpers", flags, 0);
> + WARN_ON_ONCE(helpers_wq == NULL);
> + WRITE_ONCE(drain_vmap_helpers_wq, helpers_wq);
> +
> + return 0;
> +}
> +early_initcall(vmalloc_init_workqueue);
> --
> 2.47.3
>