From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 D194D36C0C7; Fri, 6 Feb 2026 17:13:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770398030; cv=none; b=IreBVSST62V7XmGjrdvJgUZqp+/W3WpH1DipWnUY+8HBoBwIbe27bJr4jDzJtHT9QRd3C6Zu0ucLh5je/gPYTiT7NWhxS9x78zAL4NT1m7XtuKmATwSJ8oin03zAFloWp0EQR85oW8JoKwz7+NXcxiMggrk3ZWjwYzt6GZnl1W0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770398030; c=relaxed/simple; bh=GLzNiltKbkC1Pow+qhGIJkVK+++Chfm8SxgjlZAIaN4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fBr0DNLnLE7QXwz9nSy3aiw2WPOY7U4KqBIQKexikmOY0YVhQqhdb7jtD++mkNLMGL9eXAPqi7ZXH0vTmpDeRwk1FqfVwQ5/jc5JTpYrwbYbgBqcHPTOWKj72HImcQWx6xukw6AoYiBX0cdIDORrtZFO2e4XjHrgsVS0BkyZJVU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=pp+u/PbV; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="pp+u/PbV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DDF1BC19423; Fri, 6 Feb 2026 17:13:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770398030; bh=GLzNiltKbkC1Pow+qhGIJkVK+++Chfm8SxgjlZAIaN4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pp+u/PbVSWQA84fO+VdvbGB/pOEKDgdosJkXeD0tCqvEV90UhoYRHKLTzUHj8E8nX YQV+xGkSD/dw/IsCbKVBx9lXoSmiNHZFCKt8Jlvg0oJw1eeTZOzbmvt/M1ohftVvKI EV60JNljbvlJRoMM7KMY7gW2wRhqcGyGShr+9dOweHsc4g9SriZVo/+C6oaeKoPA92 mntzdpAZkd3B4mM1cznnn5AXOd3XUvtWNQ1xe96fO02TABaj7jjhoK9RvOVOgkpYKj uM9xgJPcp/Uc1QH+gg7OeHpbKjR+IaYYRVJQ7Nuw8wd84UeS2TdMwCPgpehYxDndzY 3BCnL1Uco332g== Message-ID: Date: Fri, 6 Feb 2026 18:13:44 +0100 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: Stalls when starting a VSOCK listening socket: soft lockups, RCU stalls, timeout Content-Language: en-GB, fr-BE To: Stefano Garzarella Cc: Stefan Hajnoczi , kvm@vger.kernel.org, virtualization@lists.linux.dev, Netdev , rcu@vger.kernel.org, MPTCP Linux , Linux Kernel , Peter Zijlstra , Thomas Gleixner , Shinichiro Kawasaki , "Paul E. McKenney" References: From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; keydata= xsFNBFXj+ekBEADxVr99p2guPcqHFeI/JcFxls6KibzyZD5TQTyfuYlzEp7C7A9swoK5iCvf YBNdx5Xl74NLSgx6y/1NiMQGuKeu+2BmtnkiGxBNanfXcnl4L4Lzz+iXBvvbtCbynnnqDDqU c7SPFMpMesgpcu1xFt0F6bcxE+0ojRtSCZ5HDElKlHJNYtD1uwY4UYVGWUGCF/+cY1YLmtfb WdNb/SFo+Mp0HItfBC12qtDIXYvbfNUGVnA5jXeWMEyYhSNktLnpDL2gBUCsdbkov5VjiOX7 CRTkX0UgNWRjyFZwThaZADEvAOo12M5uSBk7h07yJ97gqvBtcx45IsJwfUJE4hy8qZqsA62A nTRflBvp647IXAiCcwWsEgE5AXKwA3aL6dcpVR17JXJ6nwHHnslVi8WesiqzUI9sbO/hXeXw TDSB+YhErbNOxvHqCzZEnGAAFf6ges26fRVyuU119AzO40sjdLV0l6LE7GshddyazWZf0iac nEhX9NKxGnuhMu5SXmo2poIQttJuYAvTVUNwQVEx/0yY5xmiuyqvXa+XT7NKJkOZSiAPlNt6 VffjgOP62S7M9wDShUghN3F7CPOrrRsOHWO/l6I/qJdUMW+MHSFYPfYiFXoLUZyPvNVCYSgs 3oQaFhHapq1f345XBtfG3fOYp1K2wTXd4ThFraTLl8PHxCn4ywARAQABzSRNYXR0aGlldSBC YWVydHMgPG1hdHR0YmVAa2VybmVsLm9yZz7CwZEEEwEIADsCGwMFCwkIBwIGFQoJCAsCBBYC AwECHgECF4AWIQToy4X3aHcFem4n93r2t4JPQmmgcwUCZUDpDAIZAQAKCRD2t4JPQmmgcz33 EACjROM3nj9FGclR5AlyPUbAq/txEX7E0EFQCDtdLPrjBcLAoaYJIQUV8IDCcPjZMJy2ADp7 /zSwYba2rE2C9vRgjXZJNt21mySvKnnkPbNQGkNRl3TZAinO1Ddq3fp2c/GmYaW1NWFSfOmw MvB5CJaN0UK5l0/drnaA6Hxsu62V5UnpvxWgexqDuo0wfpEeP1PEqMNzyiVPvJ8bJxgM8qoC cpXLp1Rq/jq7pbUycY8GeYw2j+FVZJHlhL0w0Zm9CFHThHxRAm1tsIPc+oTorx7haXP+nN0J iqBXVAxLK2KxrHtMygim50xk2QpUotWYfZpRRv8dMygEPIB3f1Vi5JMwP4M47NZNdpqVkHrm jvcNuLfDgf/vqUvuXs2eA2/BkIHcOuAAbsvreX1WX1rTHmx5ud3OhsWQQRVL2rt+0p1DpROI 3Ob8F78W5rKr4HYvjX2Inpy3WahAm7FzUY184OyfPO/2zadKCqg8n01mWA9PXxs84bFEV2mP VzC5j6K8U3RNA6cb9bpE5bzXut6T2gxj6j+7TsgMQFhbyH/tZgpDjWvAiPZHb3sV29t8XaOF BwzqiI2AEkiWMySiHwCCMsIH9WUH7r7vpwROko89Tk+InpEbiphPjd7qAkyJ+tNIEWd1+MlX ZPtOaFLVHhLQ3PLFLkrU3+Yi3tXqpvLE3gO3LM7BTQRV4/npARAA5+u/Sx1n9anIqcgHpA7l 5SUCP1e/qF7n5DK8LiM10gYglgY0XHOBi0S7vHppH8hrtpizx+7t5DBdPJgVtR6SilyK0/mp 9nWHDhc9rwU3KmHYgFFsnX58eEmZxz2qsIY8juFor5r7kpcM5dRR9aB+HjlOOJJgyDxcJTwM 1ey4L/79P72wuXRhMibN14SX6TZzf+/XIOrM6TsULVJEIv1+NdczQbs6pBTpEK/G2apME7vf mjTsZU26Ezn+LDMX16lHTmIJi7Hlh7eifCGGM+g/AlDV6aWKFS+sBbwy+YoS0Zc3Yz8zrdbi Kzn3kbKd+99//mysSVsHaekQYyVvO0KD2KPKBs1S/ImrBb6XecqxGy/y/3HWHdngGEY2v2IP Qox7mAPznyKyXEfG+0rrVseZSEssKmY01IsgwwbmN9ZcqUKYNhjv67WMX7tNwiVbSrGLZoqf Xlgw4aAdnIMQyTW8nE6hH/Iwqay4S2str4HZtWwyWLitk7N+e+vxuK5qto4AxtB7VdimvKUs x6kQO5F3YWcC3vCXCgPwyV8133+fIR2L81R1L1q3swaEuh95vWj6iskxeNWSTyFAVKYYVskG V+OTtB71P1XCnb6AJCW9cKpC25+zxQqD2Zy0dK3u2RuKErajKBa/YWzuSaKAOkneFxG3LJIv Hl7iqPF+JDCjB5sAEQEAAcLBXwQYAQIACQUCVeP56QIbDAAKCRD2t4JPQmmgc5VnD/9YgbCr HR1FbMbm7td54UrYvZV/i7m3dIQNXK2e+Cbv5PXf19ce3XluaE+wA8D+vnIW5mbAAiojt3Mb 6p0WJS3QzbObzHNgAp3zy/L4lXwc6WW5vnpWAzqXFHP8D9PTpqvBALbXqL06smP47JqbyQxj Xf7D2rrPeIqbYmVY9da1KzMOVf3gReazYa89zZSdVkMojfWsbq05zwYU+SCWS3NiyF6QghbW voxbFwX1i/0xRwJiX9NNbRj1huVKQuS4W7rbWA87TrVQPXUAdkyd7FRYICNW+0gddysIwPoa KrLfx3Ba6Rpx0JznbrVOtXlihjl4KV8mtOPjYDY9u+8x412xXnlGl6AC4HLu2F3ECkamY4G6 UxejX+E6vW6Xe4n7H+rEX5UFgPRdYkS1TA/X3nMen9bouxNsvIJv7C6adZmMHqu/2azX7S7I vrxxySzOw9GxjoVTuzWMKWpDGP8n71IFeOot8JuPZtJ8omz+DZel+WCNZMVdVNLPOd5frqOv mpz0VhFAlNTjU1Vy0CnuxX3AM51J8dpdNyG0S8rADh6C8AKCDOfUstpq28/6oTaQv7QZdge0 JY6dglzGKnCi/zsmp2+1w559frz4+IC7j/igvJGX4KDDKUs0mlld8J2u2sBXv7CGxdzQoHaz lzVbFe7fduHbABmYz9cefQpO7wDE/Q== Organization: NGI0 Core In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Stefano, Thank you for your reply! On 06/02/2026 17:38, Stefano Garzarella wrote: > On Fri, Feb 06, 2026 at 12:54:13PM +0100, Matthieu Baerts wrote: >> Hi Stefan, Stefano, + VM, RCU, sched people, > > Hi Matt, > >> >> First, I'm sorry to cc a few MLs, but I'm still trying to locate the >> origin of the issue I'm seeing. >> >> Our CI for the MPTCP subsystem is now regularly hitting various stalls >> before even starting the MPTCP test suite. These issues are visible on >> top of the latest net and net-next trees, which have been sync with >> Linus' tree yesterday. All these issues have been seen on a "public CI" >> using GitHub-hosted runners with KVM support, where the tested kernel is >> launched in a nested (I suppose) VM. I can see the issue with or without > > Just to be sure I'm on the same page, the issue is in the most nested > guest, right? (the last VM started) That's correct. From what I see [1], each GitHub-hosted runner is a new VM, and I'm launching QEmu from there. [1] https://docs.github.com/en/actions/concepts/runners/github-hosted-runners >> debug.config. According to the logs, it might have started around >> v6.19-rc0, but I was unavailable for a few weeks, and I couldn't react >> quicker, sorry for that. Unfortunately, I cannot reproduce this locally, >> and the CI doesn't currently have the ability to execute bisections. >> >> The stalls happen before starting the MPTCP test suite. The init program >> creates a VSOCK listening socket via socat [1], and different hangs are >> then visible: RCU stalls followed by a soft lockup [2], only a soft >> lockup [3], sometimes the soft lockup comes with a delay [4] [5], or >> there is no RCU stalls or soft lockups detected after one minute, but VM >> is stalled [6]. In the last case, the VM is stopped after having >> launched GDB to get more details about what was being executed. >> >> It feels like the issue is not directly caused by the VSOCK listening >> socket, but the stalls always happen after having started the socat >> command [1] in the background. >> >> One last thing: I thought my issue was linked to another one seen on XFS >> side and reported by Shinichiro Kawasaki [7], but apparently not. >> Indeed, Paul McKenney mentioned Shinichiro's issue is probably fixed by >> Thomas Gleixner's series called "sched/mmcid: Cure mode transition woes" >> [8]. I applied these patches from Peter Zijlstra's tree from >> tip/sched/urgent [9], and my issue is still present. >> >> Any idea what could cause that, where to look at, or what could help to >> find the root cause? > > Mmm, nothing comes to mind at the vsock side :-( That's OK, thank you for having checked! I hope someone else in CC can help me finding the root cause! > I understand that bisection can't be done in the CI env, but can you > confirm in some way that 6.18 is working right with the same userspace? Yes, I can confirm that. We run the tests on both the dev ("export") and fixes ("export-net") branches, but also on stable versions: https://ci-results.mptcp.dev/flakes.html (The "critical issues" have their headers red) We don't see such issues in v6.18 and old kernels. > That could help to try to identify at least if there is anything in > AF_VSOCK we merged recently that can trigger that. Our dev branch is on top of net-next, I guess I would have seen issues directly related to AF_VSOCK earlier than after the net-next freeze in January. Here, it looks like the first issues came during Linus' merge window from the beginning of December, e.g. [2] is from the 4th of December, on top of 'net' which was at commit 8f7aa3d3c732 ("Merge tag 'net-next-6.19' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next") from Linus tree. [2] https://github.com/multipath-tcp/mptcp_net-next/actions/runs/19919313666/job/57104626001#step:7:5052 Cheers, Matt -- Sponsored by the NGI0 Core fund.