From 6815506e8b73b2cf5a4e8a334039e6f2c0d36c42 Mon Sep 17 00:00:00 2001
From: Daniel Borkmann <daniel@iogearbox.net>
Date: Thu, 6 Jan 2022 01:46:06 +0100
Subject: [PATCH 35/37] veth: Do not record rx queue hint in veth_xmit
Git-commit: 710ad98c363a66a0cd8526465426c5c5f8377ee0
Patch-mainline: v5.17-rc1
References: git-fixes
Laurent reported that they have seen a significant amount of TCP retransmissions
at high throughput from applications residing in network namespaces talking to
the outside world via veths. The drops were seen on the qdisc layer (fq_codel,
as per systemd default) of the phys device such as ena or virtio_net due to all
traffic hitting a _single_ TX queue _despite_ multi-queue device. (Note that the
setup was _not_ using XDP on veths as the issue is generic.)
More specifically, after edbea9220251 ("veth: Store queue_mapping independently
of XDP prog presence") which made it all the way back to v4.19.184+,
skb_record_rx_queue() would set skb->queue_mapping to 1 (given 1 RX and 1 TX
queue by default for veths) instead of leaving at 0.
This is eventually retained and callbacks like ena_select_queue() will also pick
single queue via netdev_core_pick_tx()'s ndo_select_queue() once all the traffic
is forwarded to that device via upper stack or other means. Similarly, for others
not implementing ndo_select_queue() if XPS is disabled, netdev_pick_tx() might
call into the skb_tx_hash() and check for prior skb_rx_queue_recorded() as well.
In general, it is a _bad_ idea for virtual devices like veth to mess around with
queue selection [by default]. Given dev->real_num_tx_queues is by default 1,
the skb->queue_mapping was left untouched, and so prior to edbea9220251 the
netdev_core_pick_tx() could do its job upon __dev_queue_xmit() on the phys device.
Unbreak this and restore prior behavior by removing the skb_record_rx_queue()
from veth_xmit() altogether.
If the veth peer has an XDP program attached, then it would return the first RX
queue index in xdp_md->rx_queue_index (unless configured in non-default manner).
However, this is still better than breaking the generic case.
Fixes: edbea9220251 ("veth: Store queue_mapping independently of XDP prog presence")
Fixes: 638264dc9022 ("veth: Support per queue XDP ring")
Reported-by: Laurent Bernaille <laurent.bernaille@datadoghq.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Cc: Toshiaki Makita <toshiaki.makita1@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Toshiaki Makita <toshiaki.makita1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Denis Kirjanov <denis.kirjanov@suse.com>
---
drivers/net/veth.c | 1 -
1 file changed, 1 deletion(-)
diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index bd520a9d5409..c2161fb86f3c 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -341,7 +341,6 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
*/
use_napi = rcu_access_pointer(rq->napi) &&
veth_skb_is_eligible_for_gro(dev, rcv, skb);
- skb_record_rx_queue(skb, rxq);
}
skb_tx_timestamp(skb);
--
2.16.4