Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance

[Michael S. Tsirkin]

On Thu, Jul 02, 2026 at 09:24:59AM +0200, Simon Schippers wrote:
> On 7/1/26 22:56, Michael S. Tsirkin wrote:
> > On Wed, Jul 01, 2026 at 09:16:48PM +0200, Brett Sheffield wrote:
> >> TL;DR - Commit 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 causes
> >> significant performance regressions with TAP interfaces and multithreaded
> >> network code. Please revert.
> >>
> >>
> >> Librecast is an IPv6 multicast library. One of the tests (0055) fails under
> >> Linux 7.2-rc1. The test performs data synchronization over IPv6 multicast using a TAP
> >> interface. This test has run successfully on every stable, LTS and mainline RC
> >> released in the past year. Every kernel with my Tested-by has run this test.
> >>
> >> There have been a bunch of changes to MLDv2 so I started bisecting there, but
> >> the culprit is actually 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 "tun/tap &
> >> vhost-net: avoid ptr_ring tail-drop when a qdisc is present"
> >>
> >> Reverting this commit fixes the test.
> >>
> >> To eliminate my code and any multicast weirdness, I ran tests with iperf3
> >> comparing the same host running 7.2-rc1 both with and without 1d6e569b7d0
> >> reverted.
> 
> Thank you very much for your bisect!
> 
> As the author, I am sorry for that regression!
> 
> > 
> > Thanks a lot for the bisect! Reverting is not out of question, but
> > just before we do, it is worth analyzing the situation.
> > 
> > Could you pls tell us
> > - do you see packet drops?
> 
> iperf3 shows no TCP retransmissions, so there were never packet drops
> when the patch was enabled.
> It is the number after the sender data rate (example: threads 1,
> reverted has 368 retransmissions/drops).
> 
> > - does it help to increase the tun queue size?
> 
> I agree, this would be great to know.
> 
> However, even then we must act. I am considering IFF_BACKPRESSURE
> as a feature flag, defaulting to off. It would just enable/disable
> the stopping logic in tun_net_xmit() and the waking logic
> in __tun_wake_queue(). If disabled, it would result in the same logic
> as before.
> 
> I could provide such a patch as [net] material.
> 
> Thanks again!

Or BQL? I quickly wrote a prototype of that and it seems to work well -
could you help test maybe?


> > 
> > Thanks a lot!
> > 
> > 
> >> CPU: AMD Ryzen 9 9950X
> >>
> >> [ host ] - [ bridge ] - [ tap ] - [ guest (qemu) ]
> >>
> >> Running matching kernels on host and guest, I started iperf3 in server mode on
> >> the guest and tested from the host so traffic passes through the tap interface.
> >>
> >> iperf3 -s -V                 # server
> >> iperf3 -c guest -P nthreads  # client
> >>
> >> 7.2.0-rc1 (threads 1):
> >>
> >> [  5]   0.00-10.00  sec  20.2 GBytes  17.4 Gbits/sec    0            sender
> >> [  5]   0.00-10.00  sec  2.00 GBytes  1.72 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 1, reverted):
> >>
> >> [  5]   0.00-10.00  sec  15.3 GBytes  13.1 Gbits/sec  368            sender
> >> [  5]   0.00-10.00  sec  2.00 GBytes  1.72 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 2):
> >>
> >> [SUM]   0.00-10.00  sec  10.9 GBytes  9.33 Gbits/sec    0             sender
> >> [SUM]   0.00-10.00  sec  4.00 GBytes  3.43 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 2, reverted):
> >>
> >> [SUM]   0.00-10.00  sec  15.9 GBytes  13.7 Gbits/sec  1567             sender
> >> [SUM]   0.00-10.00  sec  4.00 GBytes  3.43 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 4):
> >>
> >> [SUM]   0.00-10.00  sec  10.9 GBytes  9.33 Gbits/sec    0             sender
> >> [SUM]   0.00-10.00  sec  8.00 GBytes  6.87 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 4, reverted):
> >>
> >> [SUM]   0.00-10.00  sec  16.5 GBytes  14.1 Gbits/sec  6701             sender
> >> [SUM]   0.00-10.00  sec  8.00 GBytes  6.87 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 8):
> >>
> >> [SUM]   0.00-10.00  sec  10.7 GBytes  9.15 Gbits/sec    0             sender
> >> [SUM]   0.00-10.01  sec  10.6 GBytes  9.13 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 8, reverted):
> >>
> >> [SUM]   0.00-10.00  sec  16.2 GBytes  14.0 Gbits/sec  19319             sender
> >> [SUM]   0.00-10.00  sec  15.7 GBytes  13.5 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 16):
> >>
> >> [SUM]   0.00-10.00  sec  10.9 GBytes  9.35 Gbits/sec    0             sender
> >> [SUM]   0.00-10.01  sec  10.9 GBytes  9.32 Gbits/sec                  receiver
> >>
> >> 7.2.0-rc1 (threads 16, reverted):
> >>
> >> [SUM]   0.00-10.00  sec  14.4 GBytes  12.4 Gbits/sec  43593             sender
> >> [SUM]   0.00-10.00  sec  14.4 GBytes  12.4 Gbits/sec                  receiver
> >>
> >>
> >> As you can see, the new code works for single threaded, but for all other cases
> >> there's a significant performance drop. I see this trade-off is mentioned in the
> >> commit, but the performance drop off is much worse than suggested with the
> >> current patch.
> >>
> >> In our multicast use case data is sent by multiple threads to multiple groups
> >> simultaneously, this just breaks things to the extent that a <2 second test
> >> times out after 5 minutes.
> >>
> >>
> >> git bisect start
> >> # status: waiting for both good and bad commits
> >> # bad: [dc59e4fea9d83f03bad6bddf3fa2e52491777482] Linux 7.2-rc1
> >> git bisect bad dc59e4fea9d83f03bad6bddf3fa2e52491777482
> >> # status: waiting for good commit(s), bad commit known
> >> # good: [36bdc0e815b4e8a05b9028d8ef8a25e1ead35cc1] net: usb: asix: ax88772: re-add usbnet_link_change() in phylink callbacks
> >> git bisect good 36bdc0e815b4e8a05b9028d8ef8a25e1ead35cc1
> >> # good: [db314398f618a3a23315f73c87f7d318eaf06c1b] Merge branch 'net-bridge-mcast-support-exponential-field-encoding'
> >> git bisect good db314398f618a3a23315f73c87f7d318eaf06c1b
> >> # bad: [079a028d6327e68cfa5d38b36123637b321c19a7] string: Remove strncpy() from the kernel
> >> git bisect bad 079a028d6327e68cfa5d38b36123637b321c19a7
> >> # bad: [f396f4005180928cd9e15e352a6512865d3bc908] Bluetooth: btmtk: fix URB leak in alloc_mtk_intr_urb error path
> >> git bisect bad f396f4005180928cd9e15e352a6512865d3bc908
> >> # bad: [ec1806a730a1c0b3d68a7f9afe81514fb0dd7991] netfilter: x_tables: disable 32bit compat interface in user namespaces
> >> git bisect bad ec1806a730a1c0b3d68a7f9afe81514fb0dd7991
> >> # good: [50c2d91c5dfa0e465826ec1f8dbad9cdc254bd85] mptcp: do not drop partial packets
> >> git bisect good 50c2d91c5dfa0e465826ec1f8dbad9cdc254bd85
> >> # good: [68993ced0f618e36cf33388f1e50223e5e6e78cc] Merge tag 'net-7.1-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net
> >> git bisect good 68993ced0f618e36cf33388f1e50223e5e6e78cc
> >> # good: [34c78dff59a25110a4ce50c208e42a91490fe615] Merge branch 'net-use-ip_outnoroutes-drop-reason'
> >> git bisect good 34c78dff59a25110a4ce50c208e42a91490fe615
> >> # bad: [9587ed8137fb83d93f84b858337412f4500b21e9] Merge branch 'gve-add-support-for-ptp-gettimex64'
> >> git bisect bad 9587ed8137fb83d93f84b858337412f4500b21e9
> >> # bad: [83ea7fd73b11dd8cbf4416507a5eac3890b49fb0] net: dsa: microchip: remove unused phylink_mac_link_up() callback
> >> git bisect bad 83ea7fd73b11dd8cbf4416507a5eac3890b49fb0
> >> # bad: [f0de88303d5e7e04a1224bc7a00512b5a1c4fe7a] net: make is_skb_wmem() available to modules
> >> git bisect bad f0de88303d5e7e04a1224bc7a00512b5a1c4fe7a
> >> # bad: [c411baa463e85a779a7e68a00ba6298770b58c4c] netconsole: move push_ipv6() from netpoll
> >> git bisect bad c411baa463e85a779a7e68a00ba6298770b58c4c
> >> # good: [fba362c17d9d9211fc51f272156bb84fc23bdf98] ptr_ring: move free-space check into separate helper
> >> git bisect good fba362c17d9d9211fc51f272156bb84fc23bdf98
> >> # bad: [d0273dbe8be1640e597552f81faf1d6c9997d3e3] ipvlan: use netif_receive_skb() in ipvlan_process_multicast()
> >> git bisect bad d0273dbe8be1640e597552f81faf1d6c9997d3e3
> >> # bad: [3803065cd6b0630d4161d86aa04e2d1db0f3a0b5] Merge branch 'tun-tap-vhost-net-apply-qdisc-backpressure-on-full-ptr_ring-to-reduce-tx-drops'
> >> git bisect bad 3803065cd6b0630d4161d86aa04e2d1db0f3a0b5
> >> # bad: [1d6e569b7d0c0b2736636749e4be0a27f3cefcb3] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present
> >> git bisect bad 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3
> >> # first bad commit: [1d6e569b7d0c0b2736636749e4be0a27f3cefcb3] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present
> >>
> >> -- 
> >> Brett Sheffield (he/him)
> >> Librecast - Decentralising the Internet with Multicast
> >> https://librecast.net/
> >> https://blog.brettsheffield.com/
> >