On Fri, Mar 13, 2026 at 10:49:53AM +0100, Simon Schippers wrote: > 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?
It's easier if you do, yes. > And how about the ptr_ring race: > I see there is a seperate discussion for that now. [1] > Should I wait for that? If there's a conflict it will be trivial to resolve, so I'd say no. > 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 > >
