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 7F4F21917F4 for ; Sun, 15 Jun 2025 21:29:51 +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=1750022991; cv=none; b=tRXtYGSYCDUjwqKmb0Q+X0xCa08No4Y/s7iKlh+BuklJn6n7RyE3lNilfJmQBOOxeMc0aPYS6ninZFs7qh/KD3rk1l6HxKsvpzyO1CCc0YWp1yhPcezFz+05H5fCV25H0FyGaQcD2JHdiVrUjvx8yzMxzOFuVyC8biOlNwZM8q0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750022991; c=relaxed/simple; bh=NBk2SggHtqUnItBtE5c4/+AJ67vh/JJ50i+ZV+Iv+A0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=dvMu3FhRPkjn24Ac5B2uOWE44xTeT230xFTJZozis9e3AzFB9dtZ6ZiFLWe/m+QdEWjB0SLVNv51g8uuwltLidp/ZZVaw7lEm7DdlXd8HLST63trhW75MdJD3AmI/MXOjPuMH6qWH/Pj/ga7TT+1Jy+MuhZxuhCOTMqy5088ZWo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Nelwp51+; 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="Nelwp51+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C385AC4CEE3; Sun, 15 Jun 2025 21:29:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1750022990; bh=NBk2SggHtqUnItBtE5c4/+AJ67vh/JJ50i+ZV+Iv+A0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Nelwp51+YOhN8keTCUnj+H+Oc0jNX2W4VF9vZ5yaSsGgtP4gVzifIqc0VqTzrG0es PyvYQRpOpC4KboSyUgSYzUnTL4/10lbWLkcXJLFVYUiee1GRRDZzX+sl3xWkzUIc83 tC6Vs2BbnbTFLmRckmfjaSpyRdneHCF2HS9Z4Sa+YtN8R2ROiGCoP5xvUVuS6WkFWo Ykt6/tkiwc6Lit0oMoUbgGt9QVzVWV4XGgr2FYGyAqsqmY0Dz3ruPHy5bcbo+xUUWv zZC4szR6GMlWHfG52Cadb+BteXZX4nhw2hgLC9YTXcmzDPZTRtGFYrM17cFN3Fvhb0 4s9wi+LQvKJLQ== Message-ID: <23fd5569-f383-441e-9be4-48a8430954d9@kernel.org> Date: Sun, 15 Jun 2025 23:29:47 +0200 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH mptcp-next v3 0/3] send() fails with EAGAIN in blocking IO mode #487 Content-Language: en-GB, fr-BE To: Mat Martineau , Geliang Tang Cc: mptcp@lists.linux.dev, Geliang Tang References: From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Mat, Geliang, On 14/06/2025 01:11, Mat Martineau wrote: > On Thu, 29 May 2025, Geliang Tang wrote: > >> From: Geliang Tang >> >> Good news! I finally solved the unstable issue of MPTCP BPF sched >> selftests >> I reported a year ago, #487 "send() fails with EAGAIN in blocking IO >> mode". >> >> The fix is simple, it can be solved by explicitly setting SO_SNDBUF >> sockopt, but be sure not to set SO_RCVBUF at the same time >> (see sk->sk_userlocks & SOCK_RCVBUF_LOCK in mptcp_rcv_space_adjust()). >> >> With this fix, BPF sched selftests are now very stable, I run loop >> testing >> using mptcp-upstream-virtme-docker (run_loop run_bpftest_all), and can >> run >> it normally for hundreds of times without error: >> > > Hi Geliang - > > I can see how changing SO_SNDBUF on the sending socket side would shift > timing behavior in a way that affect the test outcome, but it doesn't > address the root issue with bug #487: > > It is either OK to get an EAGAIN from a blocking send(), or it's not OK. > > > If it's not ok to ever return EAGAIN from a blocking send, the existing > test code is a reproducer for a bug, and changing the test is hiding > that bug. > > If EAGAIN is ok, then we should change the code in send_recv_server() to > allow it. It is now a bit hidden in the middle of #487, but if I'm not mistaken, it is OK to get EAGAIN with a blocking send() **if** SO_SNDTIMEO is used, and in case of timeout. See: https://github.com/multipath-tcp/mptcp_net-next/issues/487#issuecomment-2485577676 So I think the question should be: is it normal to block for longer than the timeout period (which is a "long" period, no?)? If yes, then limiting the send buffer might be a solution, but as Mat said, it looks better to understand the root cause than hiding a bug :) Cheers, Matt -- Sponsored by the NGI0 Core fund.