From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bregans-1.gladserv.net (bregans-1.gladserv.net [185.128.211.58]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DE0012E8B82; Thu, 2 Jul 2026 11:08:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.128.211.58 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782990485; cv=none; b=A7+l78aLHAqWKbB5iHkDmrX2nT3t8Wf3Ju0vCxfU7QZpBgZkWEyOrcKDtMp78ZohrjAf6PlcwNpl6tlZdplYXa9tBxYcdOSA6QTQ2sIbUMKobpdGEBzjkv8EWJHrHDheb9IkF2iDdFL3OdwLaI/CIHF+U+MtBWGRxUvI7AgS0Cc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782990485; c=relaxed/simple; bh=NtwljrLY6p2ZueS5jksY4cMrkYyHa7GgXF7EIKbYhZk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=aFnPrg1Za2yanCf6b4pa+xzbYkB6LNx2PIKrBjPaa7hFjbCNwXNKJoYwgPiqOoCyXWZcHEw3rs++4Nr3eVExTkpHjqrmqMSXBB8axCVJVM1S3bXJ4BHMyi1mCBIkHcuhv4W5c05Bn8+UmFnfM6tl+7mrDD6UG/ruzMDXVS/zYBc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=librecast.net; spf=pass smtp.mailfrom=librecast.net; arc=none smtp.client-ip=185.128.211.58 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=librecast.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=librecast.net Date: Thu, 2 Jul 2026 13:07:47 +0200 From: Brett Sheffield To: Simon Schippers Cc: "Michael S. Tsirkin" , regressions@lists.linux.dev, netdev@vger.kernel.org, Jakub Kicinski , Tim Gebauer , Willem de Bruijn , Jason Wang , Andrew Lunn , "David S. Miller" , Eric Dumazet , Paolo Abeni , linux-kernel@vger.kernel.org Subject: Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance Message-ID: References: <20260701165359-mutt-send-email-mst@kernel.org> 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 2026-07-02 09:24, Simon Schippers wrote: > On 7/1/26 22:56, Michael S. Tsirkin wrote: > > On Wed, Jul 01, 2026 at 09:16:48PM +0200, Brett Sheffield wrote: > >> TL;DR - Commit 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 causes > >> significant performance regressions with TAP interfaces and multithreaded > >> network code. Please revert. > >> > >> > >> Librecast is an IPv6 multicast library. One of the tests (0055) fails under > >> Linux 7.2-rc1. The test performs data synchronization over IPv6 multicast using a TAP > >> interface. This test has run successfully on every stable, LTS and mainline RC > >> released in the past year. Every kernel with my Tested-by has run this test. > >> > >> There have been a bunch of changes to MLDv2 so I started bisecting there, but > >> the culprit is actually 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 "tun/tap & > >> vhost-net: avoid ptr_ring tail-drop when a qdisc is present" > >> > >> Reverting this commit fixes the test. > >> > >> To eliminate my code and any multicast weirdness, I ran tests with iperf3 > >> comparing the same host running 7.2-rc1 both with and without 1d6e569b7d0 > >> reverted. > > Thank you very much for your bisect! > > As the author, I am sorry for that regression! No worries. That's why we test :-) > > - does it help to increase the tun queue size? > > I agree, this would be great to know. > > However, even then we must act. I am considering IFF_BACKPRESSURE > as a feature flag, defaulting to off. It would just enable/disable > the stopping logic in tun_net_xmit() and the waking logic > in __tun_wake_queue(). If disabled, it would result in the same logic > as before. > > I could provide such a patch as [net] material. I'm going to make myself a strong cup of tea and dig into it a bit more here and will let you know if I find anything worth reporting. If you need me to try re-testing with specific settings or test a patch I'm happy to do so. Cheers, Brett -- Brett Sheffield (he/him) Librecast - Decentralising the Internet with Multicast https://librecast.net/ https://blog.brettsheffield.com/