netdev - Re: [PATCH net-next V2 1/2] veth: apply qdisc backpressure on full ptr

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Message-ID: <20250411124553.GD395307@horms.kernel.org>
Date: Fri, 11 Apr 2025 13:45:53 +0100
From: Simon Horman <horms@...nel.org>
To: Jesper Dangaard Brouer <hawk@...nel.org>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
	bpf@...r.kernel.org, tom@...bertland.com,
	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	dsahern@...nel.org, makita.toshiaki@....ntt.co.jp,
	kernel-team@...udflare.com
Subject: Re: [PATCH net-next V2 1/2] veth: apply qdisc backpressure on full
 ptr_ring to reduce TX drops

On Tue, Apr 08, 2025 at 05:31:19PM +0200, 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@...udflare.com>
> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>

Thanks Jesper,

It's very nice to see backpressure support being added here.

...

> @@ -874,9 +909,16 @@ 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;

I think you need to take into account RCU here.

Sparse says:

  .../veth.c:919:18: warning: incorrect type in assignment (different address spaces)
  .../veth.c:919:18:    expected struct net_device *peer_dev
  .../veth.c:919:18:    got struct net_device [noderef] __rcu *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);
>  

...