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This commit prevents tail-drop when IFF_BACKPRESSURE is set, a qdisc is present and the ptr_ring becomes full. Once the ring reaches capacity after a produce attempt, the netdev queue is stopped instead of dropping subsequent packets. Without the flag, or if no qdisc is present, the previous tail-drop behavior is preserved. IFF_BACKPRESSURE is added to TUN_FEATURES here and not in the patch that defines it, so that TUNSETIFF honours the flag only once the implementation behind it is complete. The unconditional version of this behavior was reverted because it caused a significant throughput drop in an IPv6 multicast testcase on Brett Sheffield's librecast testbed [1]: with 8 iperf3 TCP threads sending, the throughput dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is why the queue stopping is now gated on IFF_BACKPRESSURE. If producing an entry fails anyway due to a race, tun_net_xmit() drops the packet. Such rare races are expected because LLTX is enabled and the transmit path operates without the usual locking. The queue state is only touched while the device is running. The stop itself would be harmless during teardown, as tun_net_close() sets the same bit, but the re-check below it wakes the queue again and must not clear that stop. A later TUNSETIFF can clear the flag again while the device has at most one queue. Past that point tun_set_iff() returns before it writes tun->flags, which is how it already treats every other TUN_FEATURES bit. For the case where the flag does change, tun_set_iff() calls tun_force_wake_queue() for the attached tfiles, so that no queue stays stopped without a consumer that would wake it. The __tun_wake_queue() function of the consumer races with the producer for waking/stopping the netdev queue, which could result in a stalled queue. Therefore, an smp_mb__after_atomic() is introduced that pairs with the smp_mb() of the consumer. It follows the principle of store buffering described in tools/memory-model/Documentation/recipes.txt: - The producer in tun_net_xmit() first sets __QUEUE_STATE_DRV_XOFF, followed by an smp_mb__after_atomic() (= smp_mb()), and then reads the ring with __ptr_ring_check_produce(). - The consumer in __tun_wake_queue() first writes zero to the ring in __ptr_ring_consume(), followed by an smp_mb(), and then reads the queue status with netif_tx_queue_stopped(). => Following the aforementioned principle, it is impossible for the producer to see a full ring (and therefore not wake the queue on the re-check) while the consumer simultaneously fails to see a stopped queue (and therefore also does not wake it). tun_net_xmit() holds only the producer_lock and can not reset cons_cnt, which the consumer_lock protects, so the wake on the re-check leaves stale credit behind. That is accepted as best-effort, the re-check rarely succeeds and the next drain corrects the count. The documentation in tuntap.rst is updated accordingly. Benchmarks: My own benchmarks show a slight regression in raw transmission performance when using two sending threads. Packet loss also occurs only in the two-thread sending case; no packet loss was observed with a single sending thread. Test setup: AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads; Average over 50 runs @ 100,000,000 packets. SRSO and spectre v2 mitigations disabled. Note for tap+vhost-net: XDP drop program active in VM -> ~2.5x faster; slower for tap due to more syscalls (high utilization of entry_SYSRETQ_unsafe_stack in perf) +--------------------------+--------------+----------------+----------+ | 1 thread | Stock | Patched with | diff | | sending | | fq_codel qdisc | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 1.132 Mpps | 1.123 Mpps | -0.8% | | +-------------+--------------+----------------+----------+ | | Lost/s | 3.765 Mpps | 0 pps | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 3.857 Mpps | 3.901 Mpps | +1.1% | | +-------------+--------------+----------------+----------+ | +vhost-net | Lost/s | 0.802 Mpps | 0 pps | | +------------+-------------+--------------+----------------+----------+ +--------------------------+--------------+----------------+----------+ | 2 threads | Stock | Patched with | diff | | sending | | fq_codel qdisc | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 1.115 Mpps | 1.081 Mpps | -3.0% | | +-------------+--------------+----------------+----------+ | | Lost/s | 8.490 Mpps | 391 pps | | +------------+-------------+--------------+----------------+----------+ | TAP | Received | 3.664 Mpps | 3.555 Mpps | -3.0% | | +-------------+--------------+----------------+----------+ | +vhost-net | Lost/s | 5.330 Mpps | 938 pps | | +------------+-------------+--------------+----------------+----------+ [1] https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/ Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de> Link: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/ Link: https://patch.msgid.link/20260803183641.96882-6-simon.schippers@tu-dortmund.de Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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