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Miller" , Paolo Abeni , Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= , dsahern@kernel.org, makita.toshiaki@lab.ntt.co.jp, kernel-team@cloudflare.com Subject: Re: [PATCH net-next V2 0/2] veth: qdisc backpressure and qdisc check refactor In-Reply-To: <174412623473.3702169.4235683143719614624.stgit@firesoul> (Jesper Dangaard Brouer's message of "Tue, 08 Apr 2025 17:31:13 +0200") References: <174412623473.3702169.4235683143719614624.stgit@firesoul> Date: Fri, 11 Apr 2025 14:59:06 +0200 Message-ID: <87ecxyhn1h.fsf@cloudflare.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain On Tue, Apr 08, 2025 at 05:31 PM +02, Jesper Dangaard Brouer wrote: > This series addresses TX drops observed in production when using veth > devices with threaded NAPI, and refactors a common qdisc check into a > shared helper. > > In threaded NAPI mode, packet drops can occur when the ptr_ring backing > the veth peer fills up. This is typically due to a combination of > scheduling delays and the consumer (NAPI thread) being slower than the > producer. When the ring overflows, packets are dropped in veth_xmit(). > > Patch 1 introduces a backpressure mechanism: when the ring is full, the > driver returns NETDEV_TX_BUSY, signaling the qdisc layer to requeue the > packet. This allows Active Queue Management (AQM) - such as fq or sfq - > to spread traffic more fairly across flows and reduce damage from > elephant flows. > > To minimize invasiveness, this backpressure behavior is only enabled when > a qdisc is attached. If no qdisc is present, the driver retains its > original behavior (dropping packets on a full ring), avoiding behavior > changes for configurations without a qdisc. > > Detecting the presence of a "real" qdisc relies on a check that is > already duplicated across multiple drivers (e.g., veth, vrf). Patch-2 > consolidates this logic into a new helper, qdisc_txq_is_noop(), to avoid > duplication and clarify intent. > > --- > > Jesper Dangaard Brouer (2): > veth: apply qdisc backpressure on full ptr_ring to reduce TX drops > net: sched: generalize check for no-op qdisc > > > drivers/net/veth.c | 49 ++++++++++++++++++++++++++++++++------- > drivers/net/vrf.c | 3 +-- > include/net/sch_generic.h | 7 +++++- > 3 files changed, 48 insertions(+), 11 deletions(-) This setup scenario is currently not covered by the veth selftest [1]. Would be great to extend it so the code gets exercised by the CI. [1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/tools/testing/selftests/net/veth.sh