Re: [PATCH net-next] veth: set the real number of queues before registering the devices

[Wang Zhan <wang.zhan@smartx.com>]

On Fri, 16 Oct 2026 14:51:05 +0800 Qiliang Yuan wrote:
> Since commit 9d3684c24a52 ("veth: create by default nr_possible_cpus
> queues"), each veth device is allocated with one rx and one tx queue
> per possible CPU, and veth_newlink() reduces the real number of queues
> to one unless the user asked for a number.
>
> Both devices are already registered at that point, so
> netif_set_real_num_tx_queues() and netif_set_real_num_rx_queues()
> remove the sysfs objects of every extra queue that register_netdevice()
> has just created, each one emitting an add and a remove uevent, and the
> tx side also waits for an expedited RCU grace period. This happens with
> RTNL held, twice per pair, and grows with the number of possible CPUs:
> with 240 of them a veth pair emits about 1900 uevents, and udev
> processing them slows down later sysfs work such as deleting devices.
>
> Set the number of queues before registering each device. For a device
> that is not registered yet this only updates the counters, so the extra
> queues never appear in sysfs, and the error paths that undid a
> registration are no longer needed.
>
> Creating and then deleting 500 veth pairs, one ip -batch each, in a 16
> vCPU guest, median of 3 runs, per pair:
>
>                                    before      after
>   16 possible CPUs    create      1644 us     520 us
>                       delete     11822 us    9750 us
>   240 possible CPUs   create     17253 us    1339 us
>                       delete     17082 us    9941 us
>                       uevents        1918          6
>
> Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>

Looks fine to me. virtio-net uses this ordering too. I checked that
veth's ndo_init() leaves the real queue counts alone, and I don't see
any missing cleanup after the move.

I ran some basic veth tests under QEMU TCG; no regressions found.

Reviewed-by: Wang Zhan <wang.zhan@smartx.com>
Tested-by: Wang Zhan <wang.zhan@smartx.com>

BTW, Li RongQing also posted a net fix changing veth_init_queues(peer, tb)
to use tbp, so there may be a conflict.

https://lore.kernel.org/netdev/20261014013620.2497-1-lirongqing@baidu.com/