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>
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 73dd16d..0061a7b 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -168,7 +168,7 @@
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 @@
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 @@
/* 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 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 @@
/* 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 @@
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 @@
/* 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;
