On Sun, May 10, 2026 at 05:15:25PM +0200, Simon Schippers wrote: > This patch series deals with tun/tap & vhost-net which drop incoming > SKBs whenever their internal ptr_ring buffer is full. Instead, with this > patch series, the associated netdev queue is stopped - but only when a > qdisc is attached. If no qdisc is present the existing behavior is > preserved. The XDP transmit path is not affected. This patch series > touches tun/tap and vhost-net, as they share common logic and must be > updated together. Modifying only one of them would break the other. > > By applying proper backpressure, this change allows the connected qdisc to > operate correctly, as reported in [1], and significantly improves > performance in real-world scenarios, as demonstrated in our paper [2]. For > example, we observed a 36% TCP throughput improvement for an OpenVPN > connection between Germany and the USA. > > Synthetic pktgen benchmarks indicate a slight regression, and packet > loss is reduced to near zero. Pktgen benchmarks are provided per commit, > with the final commit showing the overall performance.
at v12, time to merge this. Acked-by: Michael S. Tsirkin <[email protected]> > Thanks! > > [1] Link: > https://unix.stackexchange.com/questions/762935/traffic-shaping-ineffective-on-tun-device > [2] Link: > https://cni.etit.tu-dortmund.de/storages/cni-etit/r/Research/Publications/2025/Gebauer_2025_VTCFall/Gebauer_VTCFall2025_AuthorsVersion.pdf > > --- > Changelog: > v12: > Patch 1: > - Revert tun_queue_purge() to plain ptr_ring_consume() and instead > explicitly wake the queue in __tun_detach() for the ntfile taking > over the queue slot (if its ring is empty). > - Inlined tun_reset_cons_cnt(), because only tun_attach() uses it. > > - Patches 2-4 and cover letter unchanged. > - Compiled and short pktgen test. > > v11: > - Renamed __ptr_ring_produce_peek() to __ptr_ring_check_produce() > (Sashiko) > - Add return code -EINVAL to __ptr_ring_check_produce() which lets > tun_net_xmit() stop the queue only on -ENOSPC. (MST) > - Resolve race on tfile->queue_index by locking tx_ring.consumer_lock > in __tun_detach(). (Sashiko) > - Wake the queue in tun_queue_resize() to avoid possible stalls. > - Other minor adjustments & reran the benchmarks. > > v10: > https://lore.kernel.org/netdev/[email protected]/ > - Changed the term "Transmitted" to "Received" in the benchmarks, > as correctly pointed out by MST, and reran the benchmarks. > > Addressed the Sashiko AI review: > - Avoid a data race on tfile->cons_cnt by always locking. > - Correctly count the number of consumed packets for vhost-net. > - Corrected a typo in the commit message of commit 3. > - Added a missing barrier on the consumer side. > --> The barriers now follow the "store buffering" principle. > - No longer return NETDEV_TX_BUSY at all, because it is unsafe. > --> Result: There are still a few drops with multiple senders, which > would be avoided by disabling LLTX. > > V9: > https://lore.kernel.org/netdev/[email protected]/ > - Addressed minor nit by MST in patches 1 and 2. > - Rebased patch 3 because of commit d748047 > ("ptr_ring: disable KCSAN warnings"). > - Documented the pair of the smp_mb__after_atomic() in tun_net_xmit() > with tun_ring_consume(). > --> It simply pairs with the test_and_clear_bit() inside of > netif_wake_subqueue(). > - Use 1 ptr_ring consumer spinlock instead of 2. > - Ran pktgen benchmarks with pg_set SHARED for 50 iterations on > latest kernel > --> No significant performance difference noticed > > V8: > https://lore.kernel.org/netdev/[email protected]/ > - Drop code changes in drivers/net/tap.c; The code there deals with > ipvtap/macvtap which are unrelated to the goal of this patch series > and I did not realize that before > -> Greatly simplified logic, 4 instead of 9 commits > -> No more duplicated logics and distinction in vhost required > - Only wake after the queue stopped and half of the ring was consumed > as suggested by MST > -> Performance improvements for TAP, but still slightly slower > - Better benchmarking with pinned threads, XDP drop program for > tap+vhost-net and disabling CPU mitigations (and newer Ryzen 5 5600X > processor) as suggested by Jason Wang > > V7: > https://lore.kernel.org/netdev/[email protected]/ > - Switch to an approach similar to veth (excluding the recently fixed > variant), as suggested by MST, with minor adjustments discussed in V6 > - Rename the cover-letter title > - Add multithreaded pktgen and iperf3 benchmarks, as suggested by Jason > Wang > - Rework __ptr_ring_consume_created_space() so it can also be used after > batched consume > > ... > > --- > > Simon Schippers (4): > tun/tap: add ptr_ring consume helper with netdev queue wakeup > vhost-net: wake queue of tun/tap after ptr_ring consume > ptr_ring: move free-space check into separate helper > tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present > > drivers/net/tun.c | 109 ++++++++++++++++++++++++++++++++++++--- > drivers/vhost/net.c | 21 +++++--- > include/linux/if_tun.h | 3 ++ > include/linux/ptr_ring.h | 20 ++++++- > 4 files changed, 139 insertions(+), 14 deletions(-) > > -- > 2.43.0
