Re: [PATCH v7 24/26] virtio_net: support rx/tx queue reset

Jason Wang
2022-03-09T09:14:59Z

在 2022/3/8 下午8:35, Xuan Zhuo 写道:
> This patch implements the reset function of the rx, tx queues.
>
> Based on this function, it is possible to modify the ring num of the
> queue. And quickly recycle the buffer in the queue.
>
> In the process of the queue disable, in theory, as long as virtio
> supports queue reset, there will be no exceptions.
>
> However, in the process of the queue enable, there may be exceptions due to
> memory allocation. In this case, vq is not available, but we still have
> to execute napi_enable(). Because napi_disable is similar to a lock,
> napi_enable must be called after calling napi_disable.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
> drivers/net/virtio_net.c | 107 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 107 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 409a8e180918..ffff323dcef0 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -251,6 +251,11 @@ struct padded_vnet_hdr {
> char padding[4];
> };
>
> +static void virtnet_sq_free_unused_bufs(struct virtnet_info *vi,
> + struct send_queue *sq);
> +static void virtnet_rq_free_unused_bufs(struct virtnet_info *vi,
> + struct receive_queue *rq);
> +
> static bool is_xdp_frame(void *ptr)
> {
> return (unsigned long)ptr & VIRTIO_XDP_FLAG;
> @@ -1369,6 +1374,9 @@ static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
> {
> napi_enable(napi);
>
> + if (vq->reset)
> + return;
> +

Let's WARN_ONCE() here?

> /* If all buffers were filled by other side before we napi_enabled, we
> * won't get another interrupt, so process any outstanding packets now.
> * Call local_bh_enable after to trigger softIRQ processing.
> @@ -1413,6 +1421,10 @@ static void refill_work(struct work_struct *work)
> struct receive_queue *rq = &vi->rq[i];
>
> napi_disable(&rq->napi);
> + if (rq->vq->reset) {
> + virtnet_napi_enable(rq->vq, &rq->napi);
> + continue;
> + }

This seems racy and it's a hint that we need sync with the refill work
during reset like what we did in virtnet_close():

/* Make sure refill_work doesn't re-enable napi! */
cancel_delayed_work_sync(&vi->refill);

> still_empty = !try_fill_recv(vi, rq, GFP_KERNEL);
> virtnet_napi_enable(rq->vq, &rq->napi);
>
> @@ -1523,6 +1535,9 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
> if (!sq->napi.weight || is_xdp_raw_buffer_queue(vi, index))
> return;
>
> + if (sq->vq->reset)
> + return;

It looks to me we'd better either WARN or just remove this. Since it
looks like a workaround for the un-synchronized NAPI somehow.

> +
> if (__netif_tx_trylock(txq)) {
> do {
> virtqueue_disable_cb(sq->vq);
> @@ -1769,6 +1784,98 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> return NETDEV_TX_OK;
> }
>
> +static int virtnet_rx_vq_reset(struct virtnet_info *vi,
> + struct receive_queue *rq, u32 ring_num)

It's better to rename this as virtnet_rx_resize().

> +{
> + int err;
> +
> + /* stop napi */
> + napi_disable(&rq->napi);
> +

Here, as discussed above, we need synchronize with the refill work.

> + /* reset the queue */
> + err = virtio_reset_vq(rq->vq);
> + if (err)
> + goto err;

Btw, most comment of this function seems useless since code already
explain themselves.

> +
> + /* free bufs */
> + virtnet_rq_free_unused_bufs(vi, rq);
> +
> + /* reset vring. */
> + err = virtqueue_reset_vring(rq->vq, ring_num);
> + if (err)
> + goto err;
> +
> + /* enable reset queue */
> + err = virtio_enable_resetq(rq->vq);
> + if (err)
> + goto err;
> +
> + /* fill recv */
> + if (!try_fill_recv(vi, rq, GFP_KERNEL))
> + schedule_delayed_work(&vi->refill, 0);
> +
> + /* enable napi */
> + virtnet_napi_enable(rq->vq, &rq->napi);
> + return 0;
> +
> +err:
> + netdev_err(vi->dev,
> + "reset rx reset vq fail: rx queue index: %ld err: %d\n",
> + rq - vi->rq, err);
> + virtnet_napi_enable(rq->vq, &rq->napi);
> + return err;
> +}
> +
> +static int virtnet_tx_vq_reset(struct virtnet_info *vi,
> + struct send_queue *sq, u32 ring_num)
> +{

It looks to me it's better to rename this as "virtnet_rx_resize()"

> + struct netdev_queue *txq;
> + int err, qindex;
> +
> + qindex = sq - vi->sq;
> +
> + txq = netdev_get_tx_queue(vi->dev, qindex);
> + __netif_tx_lock_bh(txq);
> +
> + /* stop tx queue and napi */
> + netif_stop_subqueue(vi->dev, qindex);
> + virtnet_napi_tx_disable(&sq->napi);

There's no need to hold tx lock for napi disable.

Thanks

> +
> + __netif_tx_unlock_bh(txq);
> +
> + /* reset the queue */
> + err = virtio_reset_vq(sq->vq);
> + if (err) {
> + netif_start_subqueue(vi->dev, qindex);
> + goto err;
> + }
> +
> + /* free bufs */
> + virtnet_sq_free_unused_bufs(vi, sq);
> +
> + /* reset vring. */
> + err = virtqueue_reset_vring(sq->vq, ring_num);
> + if (err)
> + goto err;
> +
> + /* enable reset queue */
> + err = virtio_enable_resetq(sq->vq);
> + if (err)
> + goto err;
> +
> + /* start tx queue and napi */
> + netif_start_subqueue(vi->dev, qindex);
> + virtnet_napi_tx_enable(vi, sq->vq, &sq->napi);
> + return 0;
> +
> +err:
> + netdev_err(vi->dev,
> + "reset tx reset vq fail: tx queue index: %ld err: %d\n",
> + sq - vi->sq, err);
> + virtnet_napi_tx_enable(vi, sq->vq, &sq->napi);
> + return err;
> +}
> +
> /*
> * Send command via the control virtqueue and check status. Commands
> * supported by the hypervisor, as indicated by feature bits, should