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d08a0a88db
commit 710ad98c363a66a0cd8526465426c5c5f8377ee0 upstream.
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:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.