Re: [PATCH 6/6] vhost_net: use RCU callbacks instead of synchronize_rcu()

[Jason Wang]

On Mon, Nov 15, 2021 at 11:32 PM Andrey Ryabinin <arbn@xxxxxxxxxxxxxxx> 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

>
> Signed-off-by: Andrey Ryabinin <arbn@xxxxxxxxxxxxxxx>
> ---
>  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
>