On Mon, Nov 15, 2021 at 11:32 PM Andrey Ryabinin [off-list ref] wrote:
Currently vhost_net_release() uses synchronize_rcu() to synchronize
freeing with vhost_zerocopy_callback(). However synchronize_rcu()
is quite costly operation. It take more than 10 seconds
to shutdown qemu launched with couple net devices like this:
-netdev tap,id=tap0,..,vhost=on,queues=80
because we end up calling synchronize_rcu() netdev_count*queues times.
Free vhost net structures in rcu callback instead of using
synchronize_rcu() to fix the problem.
I admit the release code is somehow hard to understand. But I wonder
if the following case can still happen with this:
CPU 0 (vhost_dev_cleanup) CPU1
(vhost_net_zerocopy_callback()->vhost_work_queue())
if (!dev->worker)
dev->worker = NULL
wake_up_process(dev->worker)
If this is true. It seems the fix is to move RCU synchronization stuff
in vhost_net_ubuf_put_and_wait()?
Thanks
quoted hunk ↗ jump to hunk
Signed-off-by: Andrey Ryabinin <redacted>
---
drivers/vhost/net.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 97a209d6a527..0699d30e83d5 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -132,6 +132,7 @@ struct vhost_net {
struct vhost_dev dev;
struct vhost_net_virtqueue vqs[VHOST_NET_VQ_MAX];
struct vhost_poll poll[VHOST_NET_VQ_MAX];
+ struct rcu_head rcu;
/* Number of TX recently submitted.
* Protected by tx vq lock. */
unsigned tx_packets;@@ -1389,6 +1390,18 @@ static void vhost_net_flush(struct vhost_net *n)
}
}
+static void vhost_net_free(struct rcu_head *rcu_head)
+{
+ struct vhost_net *n = container_of(rcu_head, struct vhost_net, rcu);
+
+ kfree(n->vqs[VHOST_NET_VQ_RX].rxq.queue);
+ kfree(n->vqs[VHOST_NET_VQ_TX].xdp);
+ kfree(n->dev.vqs);
+ if (n->page_frag.page)
+ __page_frag_cache_drain(n->page_frag.page, n->refcnt_bias);
+ kvfree(n);
+}
+
static int vhost_net_release(struct inode *inode, struct file *f)
{
struct vhost_net *n = f->private_data;@@ -1404,15 +1417,8 @@ static int vhost_net_release(struct inode *inode, struct file *f)
sockfd_put(tx_sock);
if (rx_sock)
sockfd_put(rx_sock);
- /* Make sure no callbacks are outstanding */
- synchronize_rcu();
- kfree(n->vqs[VHOST_NET_VQ_RX].rxq.queue);
- kfree(n->vqs[VHOST_NET_VQ_TX].xdp);
- kfree(n->dev.vqs);
- if (n->page_frag.page)
- __page_frag_cache_drain(n->page_frag.page, n->refcnt_bias);
- kvfree(n);
+ call_rcu(&n->rcu, vhost_net_free);
return 0;
}
--2.32.0