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[94.66.118.61]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-496c44afaecsm112946795e9.2.2026.07.28.12.22.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Jul 2026 12:22:30 -0700 (PDT) Date: Tue, 28 Jul 2026 15:22:26 -0400 From: "Michael S. Tsirkin" To: Simon Schippers Cc: netdev@vger.kernel.org, Willem de Bruijn , Jason Wang , Andrew Lunn , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Eugenio =?iso-8859-1?Q?P=E9rez?= , Simon Horman , Tim Gebauer , Brett Sheffield , kvm@vger.kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH net 0/4] Revert "tun/tap & vhost-net: apply qdisc backpressure on full ptr_ring to reduce TX drops" Message-ID: <20260728152204-mutt-send-email-mst@kernel.org> References: <20260728092240.250257-1-simon.schippers@tu-dortmund.de> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260728092240.250257-1-simon.schippers@tu-dortmund.de> On Tue, Jul 28, 2026 at 11:22:36AM +0200, Simon Schippers wrote: > Commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop when > a qdisc is present") did not show a relevant performance regression in my > testing, but on Brett Sheffield's librecast testbed it causes a > significant throughput drop in an IPv6 multicast testcase. The regression > can be pinpointed to multiple iperf3 TCP threads sending: for 8 threads > the throughput dropped from 13.5 Gbit/s to 9.13 Gbit/s. > > Therefore this series reverts the qdisc backpressure work. > > Making the backpressure opt-in via a new IFF_BACKPRESSURE flag was > proposed in [1], but a new IFF_* flag needs more review scrutiny than is > available at the moment, so a revert was requested instead. The opt-in > will be resubmitted for net-next later. Acked-by: Michael S. Tsirkin Thanks! > [1] Link: https://lore.kernel.org/netdev/20260709095511.168235-1-simon.schippers@tu-dortmund.de/ > > Reported-by: Brett Sheffield > Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/ > > Simon Schippers (4): > Revert "tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is > present" > Revert "ptr_ring: move free-space check into separate helper" > Revert "vhost-net: wake queue of tun/tap after ptr_ring consume" > Revert "tun/tap: add ptr_ring consume helper with netdev queue wakeup" > > drivers/net/tun.c | 109 +++------------------------------------ > drivers/vhost/net.c | 21 +++----- > include/linux/if_tun.h | 3 -- > include/linux/ptr_ring.h | 20 +------ > 4 files changed, 14 insertions(+), 139 deletions(-) > > > base-commit: b14361aca6350ff7907b0e9903c7b94dc7d5d4a0 > -- > 2.43.0