Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance
Simon Schippers <[email protected]> Thu, 2 Jul 2026 09:24:59 +0200
| Newsgroups | dev.linux.lists.regressions,org.kernel.vger.linux-kernel,org.kernel.vger.netdev |
|---|---|
| Message-ID | <[email protected]> |
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! > > 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/ >