On Thu, Mar 12, 2026 at 02:06:35PM +0100, 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. 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. > Pktgen benchmarks are provided per commit, with the final commit showing > the overall performance. > > Thanks!
I posted a minor nit on patch 2. Otherwise LGTM: Acked-by: Michael S. Tsirkin <[email protected]> thanks for the work! > [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 > [3] Link: > https://lore.kernel.org/r/174549940981.608169.4363875844729313831.stgit@firesoul > [4] Link: > https://lore.kernel.org/r/176295323282.307447.14790015927673763094.stgit@firesoul > > --- > Changelog: > V8: > - 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 [3] (excluding the recently fixed > variant [4]), 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 > > V6: > https://lore.kernel.org/netdev/[email protected]/ > General: > - Major adjustments to the descriptions. Special thanks to Jon Kohler! > - Fix git bisect by moving most logic into dedicated functions and only > start using them in patch 7. > - Moved the main logic of the coupled producer and consumer into a single > patch to avoid a chicken-and-egg dependency between commits :-) > - Rebased to 6.18-rc5 and ran benchmarks again that now also include lost > packets (previously I missed a 0, so all benchmark results were higher by > factor 10...). > - Also include the benchmark in patch 7. > > Producer: > - Move logic into the new helper tun_ring_produce() > - Added a smp_rmb() paired with the consumer, ensuring freed space of the > consumer is visible > - Assume that ptr_ring is not full when __ptr_ring_full_next() is called > > Consumer: > - Use an unpaired smp_rmb() instead of barrier() to ensure that the > netdev_tx_queue_stopped() call completes before discarding > - Also wake the netdev queue if it was stopped before discarding and then > becomes empty > -> Fixes race with producer as identified by MST in V5 > -> Waking the netdev queues upon resize is not required anymore > - Use __ptr_ring_consume_created_space() instead of messing with ptr_ring > internals > -> Batched consume now just calls > __tun_ring_consume()/__tap_ring_consume() in a loop > - Added an smp_wmb() before waking the netdev queue which is paired with > the smp_rmb() discussed above > > V5: > https://lore.kernel.org/netdev/[email protected]/T/#u > - Stop the netdev queue prior to producing the final fitting ptr_ring entry > -> Ensures the consumer has the latest netdev queue state, making it safe > to wake the queue > -> Resolves an issue in vhost-net where the netdev queue could remain > stopped despite being empty > -> For TUN/TAP, the netdev queue no longer needs to be woken in the > blocking loop > -> Introduces new helpers __ptr_ring_full_next and > __ptr_ring_will_invalidate for this purpose > - vhost-net now uses wrappers of TUN/TAP for ptr_ring consumption rather > than maintaining its own rx_ring pointer > > V4: > https://lore.kernel.org/netdev/[email protected]/T/#u > - Target net-next instead of net > - Changed to patch series instead of single patch > - Changed to new title from old title > "TUN/TAP: Improving throughput and latency by avoiding SKB drops" > - Wake netdev queue with new helpers wake_netdev_queue when there is any > spare capacity in the ptr_ring instead of waiting for it to be empty > - Use tun_file instead of tun_struct in tun_ring_recv as a more consistent > logic > - Use smp_wmb() and smp_rmb() barrier pair, which avoids any packet drops > that happened rarely before > - Use safer logic for vhost-net using RCU read locks to access TUN/TAP data > > V3: > https://lore.kernel.org/netdev/[email protected]/T/#u > - Added support for TAP and TAP+vhost-net. > > V2: > https://lore.kernel.org/netdev/[email protected]/T/#u > - Removed NETDEV_TX_BUSY return case in tun_net_xmit and removed > unnecessary netif_tx_wake_queue in tun_ring_recv. > > V1: > https://lore.kernel.org/netdev/[email protected]/T/#u > --- > > 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 | 91 +++++++++++++++++++++++++++++++++++++--- > drivers/vhost/net.c | 15 +++++-- > include/linux/if_tun.h | 3 ++ > include/linux/ptr_ring.h | 14 ++++++- > 4 files changed, 111 insertions(+), 12 deletions(-) > > -- > 2.43.0
