Re: [RFC PATCH net-next] veth: apply qdisc backpressure on full ptr_ring to reduce TX drops

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 04/04/2025 16.49, Jesper Dangaard Brouer wrote:
> In production, we're seeing TX drops on veth devices when the ptr_ring
> fills up. This can occur when NAPI mode is enabled, though it's
> relatively rare. However, with threaded NAPI - which we use in
> production - the drops become significantly more frequent.
> 
> The underlying issue is that with threaded NAPI, the consumer often runs
> on a different CPU than the producer. This increases the likelihood of
> the ring filling up before the consumer gets scheduled, especially under
> load, leading to drops in veth_xmit() (ndo_start_xmit()).
> 
> This patch introduces backpressure by returning NETDEV_TX_BUSY when the
> ring is full, signaling the qdisc layer to requeue the packet. The txq
> (netdev queue) is stopped in this condition and restarted once
> veth_poll() drains entries from the ring, ensuring coordination between
> NAPI and qdisc.
> 
> Backpressure is only enabled when a qdisc is attached. Without a qdisc,
> the driver retains its original behavior - dropping packets immediately
> when the ring is full. This avoids unexpected behavior changes in setups
> without a configured qdisc.
> 
> With a qdisc in place (e.g. fq, sfq) this allows Active Queue Management
> (AQM) to fairly schedule packets across flows and reduce collateral
> damage from elephant flows.
> 
> A known limitation of this approach is that the full ring sits in front
> of the qdisc layer, effectively forming a FIFO buffer that introduces
> base latency. While AQM still improves fairness and mitigates flow
> dominance, the latency impact is measurable.
> 
> In hardware drivers, this issue is typically addressed using BQL (Byte
> Queue Limits), which tracks in-flight bytes needed based on physical link
> rate. However, for virtual drivers like veth, there is no fixed bandwidth
> constraint - the bottleneck is CPU availability and the scheduler's ability
> to run the NAPI thread. It is unclear how effective BQL would be in this
> context.
> 
> This patch serves as a first step toward addressing TX drops. Future work
> may explore adapting a BQL-like mechanism to better suit virtual devices
> like veth.
> 
> Reported-by: Yan Zhai <yan@cloudflare.com>
> Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
> ---
>   drivers/net/veth.c |   58 +++++++++++++++++++++++++++++++++++++++++++++-------
>   1 file changed, 50 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 7bb53961c0ea..fff2b615781e 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -308,11 +308,10 @@ static void __veth_xdp_flush(struct veth_rq *rq)
>   static int veth_xdp_rx(struct veth_rq *rq, struct sk_buff *skb)
>   {
>   	if (unlikely(ptr_ring_produce(&rq->xdp_ring, skb))) {
> -		dev_kfree_skb_any(skb);
> -		return NET_RX_DROP;
> +		return NETDEV_TX_BUSY; /* signal qdisc layer */
>   	}
>   
> -	return NET_RX_SUCCESS;
> +	return NET_RX_SUCCESS; /* same as NETDEV_TX_OK */
>   }
>   
>   static int veth_forward_skb(struct net_device *dev, struct sk_buff *skb,
> @@ -342,15 +341,26 @@ static bool veth_skb_is_eligible_for_gro(const struct net_device *dev,
>   		 rcv->features & (NETIF_F_GRO_FRAGLIST | NETIF_F_GRO_UDP_FWD));
>   }
>   
> +/* Does specific txq have a real qdisc attached? - see noqueue_init() */
> +static inline bool txq_has_qdisc(struct netdev_queue *txq)
> +{
> +	struct Qdisc *q;
> +
> +	q = rcu_dereference(txq->qdisc);
> +	if (q->enqueue)
> +		return true;
> +	else
> +		return false;
> +}
> +
>   static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
>   {
>   	struct veth_priv *rcv_priv, *priv = netdev_priv(dev);
>   	struct veth_rq *rq = NULL;
> -	int ret = NETDEV_TX_OK;
>   	struct net_device *rcv;
>   	int length = skb->len;
>   	bool use_napi = false;
> -	int rxq;
> +	int ret, rxq;
>   
>   	rcu_read_lock();
>   	rcv = rcu_dereference(priv->peer);
> @@ -373,17 +383,39 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
>   	}
>   
>   	skb_tx_timestamp(skb);
> -	if (likely(veth_forward_skb(rcv, skb, rq, use_napi) == NET_RX_SUCCESS)) {
> +
> +	ret = veth_forward_skb(rcv, skb, rq, use_napi);
> +	switch(ret) {
> +	case NET_RX_SUCCESS: /* same as NETDEV_TX_OK */
>   		if (!use_napi)
>   			dev_sw_netstats_tx_add(dev, 1, length);
>   		else
>   			__veth_xdp_flush(rq);
> -	} else {
> +		break;
> +	case NETDEV_TX_BUSY:
> +		/* If a qdisc is attached to our virtual device, returning
> +		 * NETDEV_TX_BUSY is allowed.
> +		 */
> +		struct netdev_queue *txq = netdev_get_tx_queue(dev, rxq);
> +
> +		if (!txq_has_qdisc(txq)) {
> +			dev_kfree_skb_any(skb);
> +			goto drop;
> +		}
> +		netif_tx_stop_queue(txq); /* Unconditional netif_txq_try_stop */
> +		if (use_napi)
> +			__veth_xdp_flush(rq);
> +

Found a bug here... I need to skb_push back Ethernet header, because
__dev_forward_skb() via eth_type_trans() pulled it off.

Code fix:
		__skb_push(skb, ETH_HLEN)


> +		break;
> +	case NET_RX_DROP: /* same as NET_XMIT_DROP */
>   drop:
>   		atomic64_inc(&priv->dropped);
>   		ret = NET_XMIT_DROP;
> +		break;
> +	default:
> +		net_crit_ratelimited("veth_xmit(%s): Invalid return code(%d)",
> +				     dev->name, ret);
>   	}
> -
>   	rcu_read_unlock();
>   
>   	return ret;
> @@ -874,9 +906,16 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
>   			struct veth_xdp_tx_bq *bq,
>   			struct veth_stats *stats)
>   {
> +	struct veth_priv *priv = netdev_priv(rq->dev);
> +	int queue_idx = rq->xdp_rxq.queue_index;
> +	struct netdev_queue *peer_txq;
> +	struct net_device *peer_dev;
>   	int i, done = 0, n_xdpf = 0;
>   	void *xdpf[VETH_XDP_BATCH];
>   
> +	peer_dev = priv->peer;
> +	peer_txq = netdev_get_tx_queue(peer_dev, queue_idx);
> +
>   	for (i = 0; i < budget; i++) {
>   		void *ptr = __ptr_ring_consume(&rq->xdp_ring);
>   
> @@ -925,6 +964,9 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
>   	rq->stats.vs.xdp_packets += done;
>   	u64_stats_update_end(&rq->stats.syncp);
>   
> +	if (unlikely(netif_tx_queue_stopped(peer_txq)))
> +		netif_tx_wake_queue(peer_txq);
> +
>   	return done;
>   }
>   
> 
>