vhost: lockless enqueuing

We use spinlock to synchronize the work list now which may cause
unnecessary contentions. So this patch switch to use llist to remove
this contention. Pktgen tests shows about 5% improvement:

Before:
~1300000 pps
After:
~1370000 pps

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

2 files changed, 29 insertions, 30 deletions

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 73dd16d0f587..0061a7bd85f6 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -168,7 +168,7 @@ static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
 
 void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
 {
-        INIT_LIST_HEAD(&work->node);
+        clear_bit(VHOST_WORK_QUEUED, &work->flags);
         work->fn = fn;
         init_waitqueue_head(&work->done);
 }
@@ -246,15 +246,16 @@ EXPORT_SYMBOL_GPL(vhost_poll_flush);
 
 void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
 {
-        unsigned long flags;
+        if (!dev->worker)
+                return;
 
-        spin_lock_irqsave(&dev->work_lock, flags);
-        if (list_empty(&work->node)) {
-                list_add_tail(&work->node, &dev->work_list);
-                spin_unlock_irqrestore(&dev->work_lock, flags);
+        if (!test_and_set_bit(VHOST_WORK_QUEUED, &work->flags)) {
+                /* We can only add the work to the list after we're
+                 * sure it was not in the list.
+                 */
+                smp_mb();
+                llist_add(&work->node, &dev->work_list);
                 wake_up_process(dev->worker);
-        } else {
-                spin_unlock_irqrestore(&dev->work_lock, flags);
         }
 }
 EXPORT_SYMBOL_GPL(vhost_work_queue);
@@ -262,7 +263,7 @@ EXPORT_SYMBOL_GPL(vhost_work_queue);
 /* A lockless hint for busy polling code to exit the loop */
 bool vhost_has_work(struct vhost_dev *dev)
 {
-        return !list_empty(&dev->work_list);
+        return !llist_empty(&dev->work_list);
 }
 EXPORT_SYMBOL_GPL(vhost_has_work);
 
@@ -305,7 +306,8 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 static int vhost_worker(void *data)
 {
         struct vhost_dev *dev = data;
-        struct vhost_work *work = NULL;
+        struct vhost_work *work, *work_next;
+        struct llist_node *node;
         mm_segment_t oldfs = get_fs();
 
         set_fs(USER_DS);
@@ -315,29 +317,25 @@ static int vhost_worker(void *data)
                 /* mb paired w/ kthread_stop */
                 set_current_state(TASK_INTERRUPTIBLE);
 
-                spin_lock_irq(&dev->work_lock);
-
                 if (kthread_should_stop()) {
-                        spin_unlock_irq(&dev->work_lock);
                         __set_current_state(TASK_RUNNING);
                         break;
                 }
-                if (!list_empty(&dev->work_list)) {
-                        work = list_first_entry(&dev->work_list,
-                                                struct vhost_work, node);
-                        list_del_init(&work->node);
-                } else
-                        work = NULL;
-                spin_unlock_irq(&dev->work_lock);
 
-                if (work) {
+                node = llist_del_all(&dev->work_list);
+                if (!node)
+                        schedule();
+
+                node = llist_reverse_order(node);
+                /* make sure flag is seen after deletion */
+                smp_wmb();
+                llist_for_each_entry_safe(work, work_next, node, node) {
+                        clear_bit(VHOST_WORK_QUEUED, &work->flags);
                         __set_current_state(TASK_RUNNING);
                         work->fn(work);
                         if (need_resched())
                                 schedule();
-                } else
-                        schedule();
-
+                }
         }
         unuse_mm(dev->mm);
         set_fs(oldfs);
@@ -398,9 +396,9 @@ void vhost_dev_init(struct vhost_dev *dev,
         dev->log_file = NULL;
         dev->memory = NULL;
         dev->mm = NULL;
-        spin_lock_init(&dev->work_lock);
-        INIT_LIST_HEAD(&dev->work_list);
         dev->worker = NULL;
+        init_llist_head(&dev->work_list);
+
 
         for (i = 0; i < dev->nvqs; ++i) {
                 vq = dev->vqs[i];
@@ -566,7 +564,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
         /* No one will access memory at this point */
         kvfree(dev->memory);
         dev->memory = NULL;
-        WARN_ON(!list_empty(&dev->work_list));
+        WARN_ON(!llist_empty(&dev->work_list));
         if (dev->worker) {
                 kthread_stop(dev->worker);
                 dev->worker = NULL;
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index d36d8beb3351..6690e645d2f8 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -15,13 +15,15 @@
 struct vhost_work;
 typedef void (*vhost_work_fn_t)(struct vhost_work *work);
 
+#define VHOST_WORK_QUEUED 1
 struct vhost_work {
-        struct list_head          node;
+        struct llist_node         node;
         vhost_work_fn_t           fn;
         wait_queue_head_t         done;
         int                       flushing;
         unsigned                  queue_seq;
         unsigned                  done_seq;
+        unsigned long             flags;
 };
 
 /* Poll a file (eventfd or socket) */
@@ -126,8 +128,7 @@ struct vhost_dev {
         int nvqs;
         struct file *log_file;
         struct eventfd_ctx *log_ctx;
-        spinlock_t work_lock;
-        struct list_head work_list;
+        struct llist_head work_list;
         struct task_struct *worker;
 };