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Mon, 30 Mar 2026 10:27:23 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 30 Mar 2026 19:27:21 +0200 To: "Uladzislau Rezki (Sony)" Cc: linux-mm@kvack.org, Andrew Morton , Baoquan He , LKML , lirongqing Subject: Re: [PATCH] mm/vmalloc: Use dedicated unbound workqueue for vmap purge/drain Message-ID: References: <20260330160552.485430-1-urezki@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260330160552.485430-1-urezki@gmail.com> On Mon, Mar 30, 2026 at 06:05:52PM +0200, Uladzislau Rezki (Sony) wrote: > 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 > Link: https://lore.kernel.org/all/20260319074307.2325-1-lirongqing@baidu.com/ > Signed-off-by: Uladzislau Rezki (Sony) > --- > 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 > I will send v2 to prevent progress lose during boot. -- Uladzislau Rezki