Re: [PATCH net-next v3 4/8] virtio-net: xsk zero copy xmit implement wakeup and xmit

[Jason Wang <jasowang@redhat.com>]

在 2021/3/31 下午3:11, Xuan Zhuo 写道:
> When the user calls sendto to consume the data in the xsk tx queue,
> virtnet_xsk_wakeup will be called.
>
> In wakeup, it will try to send a part of the data directly, the quantity
> is operated by the module parameter xsk_budget.


Any reason that we can't use NAPI budget?


>   There are two purposes
> for this realization:
>
> 1. Send part of the data quickly to reduce the transmission delay of the
>     first packet
> 2. Trigger tx interrupt, start napi to consume xsk tx data
>
> All sent xsk packets share the virtio-net header of xsk_hdr. If xsk
> needs to support csum and other functions later, consider assigning xsk
> hdr separately for each sent packet.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
> ---
>   drivers/net/virtio_net.c | 183 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 183 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4e25408a2b37..c8a317a93ef7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -28,9 +28,11 @@ static int napi_weight = NAPI_POLL_WEIGHT;
>   module_param(napi_weight, int, 0444);
>   
>   static bool csum = true, gso = true, napi_tx = true;
> +static int xsk_budget = 32;
>   module_param(csum, bool, 0444);
>   module_param(gso, bool, 0444);
>   module_param(napi_tx, bool, 0644);
> +module_param(xsk_budget, int, 0644);
>   
>   /* FIXME: MTU in config. */
>   #define GOOD_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
> @@ -47,6 +49,8 @@ module_param(napi_tx, bool, 0644);
>   
>   #define VIRTIO_XDP_FLAG	BIT(0)
>   
> +static struct virtio_net_hdr_mrg_rxbuf xsk_hdr;
> +
>   /* RX packet size EWMA. The average packet size is used to determine the packet
>    * buffer size when refilling RX rings. As the entire RX ring may be refilled
>    * at once, the weight is chosen so that the EWMA will be insensitive to short-
> @@ -138,6 +142,9 @@ struct send_queue {
>   	struct {
>   		/* xsk pool */
>   		struct xsk_buff_pool __rcu *pool;
> +
> +		/* save the desc for next xmit, when xmit fail. */
> +		struct xdp_desc last_desc;
>   	} xsk;
>   };
>   
> @@ -2532,6 +2539,179 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>   	return err;
>   }
>   
> +static void virtnet_xsk_check_space(struct send_queue *sq)
> +{


The name is confusing, the function does more than just checking the 
space, it may stop the queue as well.


> +	struct virtnet_info *vi = sq->vq->vdev->priv;
> +	struct net_device *dev = vi->dev;
> +	int qnum = sq - vi->sq;
> +
> +	/* If this sq is not the exclusive queue of the current cpu,
> +	 * then it may be called by start_xmit, so check it running out
> +	 * of space.
> +	 */


So the code can explain itself. We need a better comment to explain why 
we need to differ the case of the raw buffer queue.


> +	if (is_xdp_raw_buffer_queue(vi, qnum))
> +		return;
> +
> +	/* Stop the queue to avoid getting packets that we are
> +	 * then unable to transmit. Then wait the tx interrupt.
> +	 */
> +	if (sq->vq->num_free < 2 + MAX_SKB_FRAGS)
> +		netif_stop_subqueue(dev, qnum);
> +}
> +
> +static int virtnet_xsk_xmit(struct send_queue *sq, struct xsk_buff_pool *pool,
> +			    struct xdp_desc *desc)
> +{
> +	struct virtnet_info *vi;
> +	struct page *page;
> +	void *data;
> +	u32 offset;
> +	u64 addr;
> +	int err;
> +
> +	vi = sq->vq->vdev->priv;
> +	addr = desc->addr;
> +	data = xsk_buff_raw_get_data(pool, addr);
> +	offset = offset_in_page(data);
> +
> +	sg_init_table(sq->sg, 2);
> +	sg_set_buf(sq->sg, &xsk_hdr, vi->hdr_len);
> +	page = xsk_buff_xdp_get_page(pool, addr);
> +	sg_set_page(sq->sg + 1, page, desc->len, offset);
> +
> +	err = virtqueue_add_outbuf(sq->vq, sq->sg, 2, NULL, GFP_ATOMIC);
> +	if (unlikely(err))
> +		sq->xsk.last_desc = *desc;


So I think it's better to make sure we had at least 2 slots to avoid 
handling errors like this? (Especially consider the queue size is not 
necessarily the power of 2 when packed virtqueue is used).


> +
> +	return err;
> +}
> +
> +static int virtnet_xsk_xmit_batch(struct send_queue *sq,
> +				  struct xsk_buff_pool *pool,
> +				  unsigned int budget,
> +				  bool in_napi, int *done)
> +{
> +	struct xdp_desc desc;
> +	int err, packet = 0;
> +	int ret = -EAGAIN;
> +
> +	if (sq->xsk.last_desc.addr) {


Any reason that num_free is not checked here?


> +		err = virtnet_xsk_xmit(sq, pool, &sq->xsk.last_desc);
> +		if (unlikely(err))
> +			return -EBUSY;
> +
> +		++packet;
> +		--budget;
> +		sq->xsk.last_desc.addr = 0;
> +	}
> +
> +	while (budget-- > 0) {
> +		if (sq->vq->num_free < 2 + MAX_SKB_FRAGS) {
> +			ret = -EBUSY;
> +			break;
> +		}
> +
> +		if (!xsk_tx_peek_desc(pool, &desc)) {
> +			/* done */
> +			ret = 0;
> +			break;
> +		}
> +
> +		err = virtnet_xsk_xmit(sq, pool, &desc);
> +		if (unlikely(err)) {
> +			ret = -EBUSY;
> +			break;
> +		}
> +
> +		++packet;
> +	}
> +
> +	if (packet) {
> +		if (virtqueue_kick_prepare(sq->vq) &&
> +		    virtqueue_notify(sq->vq)) {
> +			u64_stats_update_begin(&sq->stats.syncp);
> +			sq->stats.kicks += 1;
> +			u64_stats_update_end(&sq->stats.syncp);
> +		}
> +
> +		*done = packet;
> +
> +		xsk_tx_release(pool);
> +	}
> +
> +	return ret;
> +}
> +
> +static int virtnet_xsk_run(struct send_queue *sq, struct xsk_buff_pool *pool,
> +			   int budget, bool in_napi)
> +{
> +	int done = 0;
> +	int err;
> +
> +	free_old_xmit_skbs(sq, in_napi);
> +
> +	err = virtnet_xsk_xmit_batch(sq, pool, budget, in_napi, &done);
> +	/* -EAGAIN: done == budget
> +	 * -EBUSY: done < budget
> +	 *  0    : done < budget
> +	 */


Please move them to the comment above virtnet_xsk_xmit_batch().

And it looks to me there's no care for -EAGAIN, any reason for sticking 
a dedicated variable like that?


> +	if (err == -EBUSY) {
> +		free_old_xmit_skbs(sq, in_napi);
> +
> +		/* If the space is enough, let napi run again. */
> +		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> +			done = budget;


So I don't see how this can work in the case of event index where the 
notification needs to be enabled explicitly.


> +	}
> +
> +	virtnet_xsk_check_space(sq);
> +
> +	return done;
> +}
> +
> +static int virtnet_xsk_wakeup(struct net_device *dev, u32 qid, u32 flag)
> +{
> +	struct virtnet_info *vi = netdev_priv(dev);
> +	struct xsk_buff_pool *pool;
> +	struct netdev_queue *txq;
> +	struct send_queue *sq;
> +
> +	if (!netif_running(dev))
> +		return -ENETDOWN;
> +
> +	if (qid >= vi->curr_queue_pairs)
> +		return -EINVAL;
> +
> +	sq = &vi->sq[qid];
> +
> +	rcu_read_lock();
> +
> +	pool = rcu_dereference(sq->xsk.pool);
> +	if (!pool)
> +		goto end;
> +
> +	if (napi_if_scheduled_mark_missed(&sq->napi))
> +		goto end;
> +
> +	txq = netdev_get_tx_queue(dev, qid);
> +
> +	__netif_tx_lock_bh(txq);
> +
> +	/* Send part of the packet directly to reduce the delay in sending the
> +	 * packet, and this can actively trigger the tx interrupts.
> +	 *
> +	 * If no packet is sent out, the ring of the device is full. In this
> +	 * case, we will still get a tx interrupt response. Then we will deal
> +	 * with the subsequent packet sending work.
> +	 */
> +	virtnet_xsk_run(sq, pool, xsk_budget, false);


So the return value is ignored, this means there's no way to report we 
exhaust the budget. Is this intended?

Thanks


> +
> +	__netif_tx_unlock_bh(txq);
> +
> +end:
> +	rcu_read_unlock();
> +	return 0;
> +}
> +
>   static int virtnet_xsk_pool_enable(struct net_device *dev,
>   				   struct xsk_buff_pool *pool,
>   				   u16 qid)
> @@ -2553,6 +2733,8 @@ static int virtnet_xsk_pool_enable(struct net_device *dev,
>   	if (rcu_dereference(sq->xsk.pool))
>   		goto end;
>   
> +	memset(&sq->xsk, 0, sizeof(sq->xsk));
> +
>   	/* Here is already protected by rtnl_lock, so rcu_assign_pointer is
>   	 * safe.
>   	 */
> @@ -2656,6 +2838,7 @@ static const struct net_device_ops virtnet_netdev = {
>   	.ndo_vlan_rx_kill_vid = virtnet_vlan_rx_kill_vid,
>   	.ndo_bpf		= virtnet_xdp,
>   	.ndo_xdp_xmit		= virtnet_xdp_xmit,
> +	.ndo_xsk_wakeup         = virtnet_xsk_wakeup,
>   	.ndo_features_check	= passthru_features_check,
>   	.ndo_get_phys_port_name	= virtnet_get_phys_port_name,
>   	.ndo_set_features	= virtnet_set_features,