From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 906C5156F20 for ; Wed, 16 Apr 2025 13:56:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744811815; cv=none; b=FR39ug1lkxDyLu96V2PLxL0WDkEdZUGCfjhOvqjX14t7LiD6obafhytMBL06sNzj+hsmw4BFzx4XHtM+GDCPoEqAEmXyVerTkDoT0cwiRvABbSNWOCyn35Hwk6eAWqubiqeY7VymqxJbnS1xs3LvHmL6XZ+pba9sMlnO+Kdqfzg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1744811815; c=relaxed/simple; bh=9IXqGz7/97dAlu3CPAMjzoFLk15SVIkoVOTKZbEIcpM=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=F0TrnJHV9dhLKJTuD/u/F/2LssCxKZfWjAs+058rMwn5LdvEx8YyWJVyvFE8m+CfBqsuwIrIYKNv3DQJ4UazGN8RxWn60St7OKMSgPZdUBizNi50DrjoMRlQH/hs24vsN3dvpeEmwcEAdGRs5sB+zx16+8t4oRdOLZUtB+ADz88= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=RKAvPT+p; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="RKAvPT+p" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1744811812; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=2zb2lt6s4GxmFeaXQ2LR2QpbEMVI0Udd83Rv96tkVQo=; b=RKAvPT+pU8tYQXwbFI7LikMX8C5w/hWoBwaaL//C5O7DGJcsgGVI4umlPJV40StQxg9HEy vAPlY3gA1BXz6qMvQp5zMw3m1N4sUaow7dCpuJR9XrAMk9hhE3WHTV0U/erSpXrC1SeKmq 9lm+QluL6P+MSt00Dq22YtWhDt/i5FU= Received: from mail-lf1-f71.google.com (mail-lf1-f71.google.com [209.85.167.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-458-YkKPVQ-TOa6fOw_vilhR4Q-1; Wed, 16 Apr 2025 09:56:51 -0400 X-MC-Unique: YkKPVQ-TOa6fOw_vilhR4Q-1 X-Mimecast-MFC-AGG-ID: YkKPVQ-TOa6fOw_vilhR4Q_1744811810 Received: by mail-lf1-f71.google.com with SMTP id 2adb3069b0e04-54b0e3136ddso355139e87.0 for ; Wed, 16 Apr 2025 06:56:50 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1744811809; x=1745416609; h=mime-version:message-id:date:references:in-reply-to:subject:cc:to :from:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=2zb2lt6s4GxmFeaXQ2LR2QpbEMVI0Udd83Rv96tkVQo=; b=WooevIdNcAJvU3DtKkJf4nImOKRDrr1d+JZvbS30RW7ZSJ44cbhqByndMT9geXgZu3 CFWsEg3OjhbfQ/WX1EEws3Z+B/wCqFRZmwFdO9GEKgwr08WIgc7y+vkrZpU6M7VUisUA 3OP/cjvxUurEPdZMfCvVd5kweMAxyOMvY6G7cAGLhOd1J7UAQWwMCwZ89EwOcRoISLsf 3TNj+yHv3BklpvKya1gOXhKtMbmYx182IDYJU0ft7oLgNz3v3Zws5+b/9svW8SV0UhWQ Y7CmXHvLtSU4DCh0edO4XriVyHXgLTyNRH2YlF0UZS2vex++uGr3KSTXrtsv6JBM9LKp SulA== X-Forwarded-Encrypted: i=1; AJvYcCVGIpEtP7U3Whf5gzogCyNJUw+s9qtl9RwoZUNIdOetqsT3bHdRiT75SdSvDBBnJLBh9X6RKHg=@vger.kernel.org X-Gm-Message-State: AOJu0Yxv1pge+a7Cd9XmqV1VxsoPu6qXJedLxduM2ZOC2T2clcIQPBd1 7fn9NtDmZ/nFtt8lOR+E0v04hGBzg07s7HbknfMmYbno79p2BiObwy29NvPssgwht5ekbCRI6J7 +7xk3YimgS1h9+6BToCMQ3PJMXEPjx41bQAckzYllb3/Ef2RgnQOJyQ== X-Gm-Gg: ASbGncvnBAe34FcZ9aNXwGq201wCbwa16hCx49vX3XtGv78crbG/tb1xnXgDKXRfMIW 4E1QjC+GCjJdBlABL3C0dCc1FY/2HrbT/Fqopc2Rc/sCw24PxeqltoIhVTp6caaiZdcYBkWtxjb uow/IeyamR8qLbHdBq1kEpTx6olh0xR+ivjdXsSEUB2yYc9tkgosmBDLT9cgdnhlIAFUK8lm+ni fNMN+Mxat2bc7+SNFMYS1LS+/QhpN6GA4It+s/6kVE+706S0voD96OXBute9qeFZ1Z6eY1HT8eA U0teaM04 X-Received: by 2002:a05:6512:31c1:b0:549:8f14:a839 with SMTP id 2adb3069b0e04-54d64b95247mr621845e87.11.1744811809431; Wed, 16 Apr 2025 06:56:49 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGVX3/8kuJFFd4NzAMo25LJ0Ly9/TC9S+jk7mqofEjD6pbIQtjg+MzGZEF3d3S6W+UYAmTFcg== X-Received: by 2002:a05:6512:31c1:b0:549:8f14:a839 with SMTP id 2adb3069b0e04-54d64b95247mr621837e87.11.1744811808984; Wed, 16 Apr 2025 06:56:48 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([2a0c:4d80:42:443::2]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-54d3d50260csm1733952e87.132.2025.04.16.06.56.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 16 Apr 2025 06:56:47 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 64492199293D; Wed, 16 Apr 2025 15:56:46 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Jesper Dangaard Brouer , netdev@vger.kernel.org, Jakub Kicinski Cc: Jesper Dangaard Brouer , bpf@vger.kernel.org, tom@herbertland.com, Eric Dumazet , "David S. Miller" , Paolo Abeni , dsahern@kernel.org, makita.toshiaki@lab.ntt.co.jp, kernel-team@cloudflare.com, phil@nwl.cc Subject: Re: [PATCH net-next V4 2/2] veth: apply qdisc backpressure on full ptr_ring to reduce TX drops In-Reply-To: <174472470529.274639.17026526070544068280.stgit@firesoul> References: <174472463778.274639.12670590457453196991.stgit@firesoul> <174472470529.274639.17026526070544068280.stgit@firesoul> X-Clacks-Overhead: GNU Terry Pratchett Date: Wed, 16 Apr 2025 15:56:46 +0200 Message-ID: <87tt6oi50h.fsf@toke.dk> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Jesper Dangaard Brouer writes: > 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 > Signed-off-by: Jesper Dangaard Brouer > --- > 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 */ > } > > - 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); > + > + 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 +897,16 @@ 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 = rcu_dereference(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 +955,9 @@ 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); > + netif_tx_wake_queue() does a test_and_clear_bit() and does nothing if the bit is not set; so does this optimisation really make any difference? :) -Toke