On 3/12/26 14:55, Michael S. Tsirkin wrote: > 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!
Thanks! Should I do a new version for the minor nit? And how about the ptr_ring race: I see there is a seperate discussion for that now. [1] Should I wait for that? Before sending a potential new version I would of course wait for Jason's take. [1] Link: https://lore.kernel.org/netdev/CANn89iJLC9p+N=rqtuj7zuprdpsgcatctzdz1vi9mbzf3at...@mail.gmail.com/ > > >> [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 >
