Re: 答复: [????] Re: [PATCH v2] mm/vmalloc: use dedicated unbound workqueue for vmap area draining

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Mar 19, 2026 at 10:05:42AM +0000, Li,Rongqing(ACG CCN) wrote:
> 
> 
> > On Thu, Mar 19, 2026 at 03:43:07AM -0400, lirongqing wrote:
> > > From: Li RongQing <lirongqing@baidu.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.
> > >
> > > Create vmap_drain_wq in vmalloc_init_late() which is called after
> > > workqueue_init_early() in start_kernel() to avoid boot-time crashes.
> > >
> > > Suggested-by: Uladzislau Rezki <urezki@gmail.com>
> > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > ---
> > > Diff with v1: create dedicated unbound workqueue
> > >
> > >  include/linux/vmalloc.h |  2 ++
> > >  init/main.c             |  1 +
> > >  mm/vmalloc.c            | 14 +++++++++++++-
> > >  3 files changed, 16 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h index
> > > e8e94f9..c028603 100644
> > > --- a/include/linux/vmalloc.h
> > > +++ b/include/linux/vmalloc.h
> > > @@ -301,11 +301,13 @@ static inline void set_vm_flush_reset_perms(void
> > *addr)
> > >  	if (vm)
> > >  		vm->flags |= VM_FLUSH_RESET_PERMS;
> > >  }
> > > +void __init vmalloc_init_late(void);
> > >  #else  /* !CONFIG_MMU */
> > >  #define VMALLOC_TOTAL 0UL
> > >
> > >  static inline unsigned long vmalloc_nr_pages(void) { return 0; }
> > > static inline void set_vm_flush_reset_perms(void *addr) {}
> > > +static inline void __init vmalloc_init_late(void) {}
> > >  #endif /* CONFIG_MMU */
> > >
> > >  #if defined(CONFIG_MMU) && defined(CONFIG_SMP) diff --git
> > > a/init/main.c b/init/main.c index 1cb395d..50b497f 100644
> > > --- a/init/main.c
> > > +++ b/init/main.c
> > > @@ -1099,6 +1099,7 @@ void start_kernel(void)
> > >  	 * workqueue_init().
> > >  	 */
> > >  	workqueue_init_early();
> > > +	vmalloc_init_late();
> > >
> > No, no. We should not patch main.c for such purpose :)
> > 
> > >  	rcu_init();
> > >  	kvfree_rcu_init();
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c index 61caa55..a52ccd4 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -1067,6 +1067,7 @@ 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 *vmap_drain_wq;
> > >
> > >  static __cacheline_aligned_in_smp atomic_long_t nr_vmalloc_pages;
> > > static __cacheline_aligned_in_smp atomic_long_t vmap_lazy_nr; @@
> > > -2471,7 +2472,7 @@ 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);
> > > +		queue_work(vmap_drain_wq, &drain_vmap_work);
> > >  }
> > >
> > >  /*
> > > @@ -5422,6 +5423,17 @@ vmap_node_shrink_scan(struct shrinker
> > *shrink, struct shrink_control *sc)
> > >  	return SHRINK_STOP;
> > >  }
> > >
> > > +void __init vmalloc_init_late(void)
> > > +{
> > > +	vmap_drain_wq = alloc_workqueue("vmap_drain",
> > > +						WQ_UNBOUND | WQ_MEM_RECLAIM, 0);
> > > +	if (!vmap_drain_wq) {
> > > +		pr_warn("vmap_drain_wq creation failed, using
> > system_unbound_wq\n");
> > > +		vmap_drain_wq = system_unbound_wq;
> > > +	}
> > > +
> > > +}
> > > +
> > >  void __init vmalloc_init(void)
> > >  {
> > >  	struct shrinker *vmap_node_shrinker;
> > > --
> > > 2.9.4
> > >
> > Why can't you add this into the vmalloc_ini()?
> > 
> 
> If alloc_workqueue() is added into vmalloc_ini(), system will crash and fail to boot, sine allocate workqueue depends on workqueue_init_early()
> 
> Maybe this commit 3347fa092821("workqueue: make workqueue available early during boot") shows the reason
> 
That is true.

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 61caa55a4402..81e1e74346d5 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1067,6 +1067,7 @@ 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;
@@ -2437,6 +2438,17 @@ static void drain_vmap_area_work(struct work_struct *work)
 	mutex_unlock(&vmap_purge_lock);
 }
 
+static void
+schedule_drain_vmap_work(unsigned long nr_lazy, unsigned long nr_lazy_max)
+{
+	if (unlikely(nr_lazy > nr_lazy_max)) {
+		struct workqueue_struct *wq = READ_ONCE(drain_vmap_wq);
+
+		if (wq)
+			queue_work(wq, &drain_vmap_work);
+	}
+}
+
 /*
  * Free a vmap area, caller ensuring that the area has been unmapped,
  * unlinked and flush_cache_vunmap had been called for the correct
@@ -2470,8 +2482,7 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 	trace_free_vmap_area_noflush(va_start, nr_lazy, nr_lazy_max);
 
 	/* 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(nr_lazy, nr_lazy_max);
 }
 
 /*
@@ -5483,3 +5494,15 @@ 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);
<snip>

--
Uladzislau Rezki