Re: [PATCH net-next V4 2/2] veth: apply qdisc backpressure on full ptr_ring to reduce TX drops

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

On 16/04/2025 14.44, Toshiaki Makita wrote:
> On 2025/04/15 22:45, 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.
> 
> Thank you for the patch.
> 
>> Reported-by: Yan Zhai <yan@cloudflare.com>
>> Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
>> ---
>>   drivers/net/veth.c |   49 
>> +++++++++++++++++++++++++++++++++++++++++--------
>>   1 file changed, 41 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
>> index 7bb53961c0ea..a419d5e198d8 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 */
>>       }
> 
> You don't need this braces any more?
> 
> if (...)
>      return ...;
> 

Correct, fixed for V5.

>> -    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,
>> @@ -346,11 +345,11 @@ 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 netdev_queue *txq;
>>       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 +372,41 @@ 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.
>> +         */
>> +        txq = netdev_get_tx_queue(dev, rxq);
>> +
>> +        if (qdisc_txq_has_no_queue(txq)) {
>> +            dev_kfree_skb_any(skb);
>> +            goto drop;
>> +        }
>> +        netif_tx_stop_queue(txq);
>> +        /* Restore Eth hdr pulled by dev_forward_skb/eth_type_trans */
>> +        __skb_push(skb, ETH_HLEN);
>> +        if (use_napi)
>> +            __veth_xdp_flush(rq);
> 
> You did not add a packet to the ring.
> No need for flush here?

IMHO we do need a flush here.
This is related to the netif_tx_stop_queue(txq) call, that stops the
TXQ, and that need to be started again by NAPI side.

This is need to handle a very unlikely race, but if the race happens
then it can cause the TXQ to stay stopped (blocking all traffic).

Given we arrive at NETDEV_TX_BUSY, when ptr_ring is full, it is very
likely that someone else have called flush and NAPI veth_poll is
running.  Thus, the extra flush will likely be a no-op as
rx_notify_masked is true.

The race is that before calling netif_tx_stop_queue(txq) the other CPU
running NAPI veth_poll manages to NAPI complete and empty the ptr_ring.
In this case, the flush will avoid race, as it will have an effect as
rx_notify_masked will be false.

Looking closer at code: There is still a possible race, in veth_poll,
after calling veth_xdp_rcv() and until rq->rx_notify_masked is set to
false (via smp_store_mb).  If netif_tx_stop_queue(txq) is executed in
this window, then we still have the race, where TXQ stays stopped
forever. (There is a optional call to xdp_do_flush that can increase
race window).

I'll add something in V5 that also handles the second race window.

--Jesper