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[80.230.68.31]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47a9e4d7801sm25370218f8f.16.2026.07.06.06.23.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Jul 2026 06:23:44 -0700 (PDT) Date: Mon, 6 Jul 2026 09:23:40 -0400 From: "Michael S. Tsirkin" To: Brett A C Sheffield Cc: Simon Schippers , Willem de Bruijn , Jason Wang , "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, Simon Horman , Jonathan Corbet , Shuah Khan , Andrew Lunn , Tim Gebauer , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH net v2] tun/tap & vhost-net: make qdisc backpressure opt-in via IFF_BACKPRESSURE Message-ID: <20260706091706-mutt-send-email-mst@kernel.org> References: <20260706094242.115992-1-simon.schippers@tu-dortmund.de> Precedence: bulk X-Mailing-List: netdev@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: On Mon, Jul 06, 2026 at 12:11:15PM +0200, Brett A C Sheffield wrote: > On 2026-07-06 11:42, 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 shows a > > significant performance drop in a IPv6 multicast testcase. The regression > > can be pinpointed when multiple iperf3 UDP threads are sending. For 8 > > threads the performance dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is > > the reason why this patch makes the qdisc backpressure behavior opt-in. > > Your v1 commit message was correct. The iperf3 tests were TCP, not UDP. > > The original failing test that alerted me to the problem was IPv6 multicast > (UDP), but the reproducer tests I provided stats for in the regression report > were TCP "To eliminate my code and any multicast weirdness" and also to verify > that this also affected TCP. > > Sorry for the confusion. The command lines used are in the regression report. > > I've tested the v2 patch (with IPv6 multicast), and verified the > previously failing test passes. > > Tested-by: Brett A C Sheffield > > Cheers, Just to clarify, it's more of a work-around, not a fix. It's not really great to have a flag that says "change something opaque in the internals of the device, it affects performance in some way, we can't predict how". So maybe we really should revert for now, and work on something more coherent for the next linux. > > Brett > -- > Brett Sheffield (he/him) > Librecast - Decentralising the Internet with Multicast > https://librecast.net/ > https://blog.brettsheffield.com/