From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793369721; b=PAnKrYNxgnwuAS9wYYPQJ4mKv7zMJPzAr2er3gbeNvW6eMiC8Ij2LD6CcGLuto2CoGDy iTw0HZzjfxBa8Jkd4KEXrD26IZ+TO4jc3pq0Hs1bOyC1keevMHKEonvn8Fs02EM5iSpGv ZdjoS0ebVH62AgIbi9TEaSGHIbx84j2Tgujip5oQcuSv63M9MbcNxb036S44xL8JT+x/A ME8EqxHSJGNaL82YWU3jgz0/ZRr10XbXd9NsTEoonMWqC5yC7bo58EemXfmqNH/2+hCkg VuaxeWXjHCnRTnpEwup2ezUUWT7nOWW+IA2m1g/1d3ZZiScEXnXm9pMs/ZOoVzzIM7g== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793369721; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition: Content-Transfer-Encoding:In-Reply-To; bh=6o/wru8YGsBJ8nISiaIwKRCzFTUDQcQKEKhFrnSO15Q=; b=s3jsZQJkZnzM4L0YG9HfsimFlS3oprlRJuUjPwbtELA+8ersyLIvWK/FniUx4KbbtuYX 2Xih/Itcp0fhnLs5Tc3dVkCiy9ZPrOLVPTOmtjY7Qciev5lchmD41Q+HGfVHqMikwRqy1 8TWtRrBskLJpXVbRGn2CZeuVLY87Um5BasTrxNE3XQyhT3KQip9mSapi2+Xk9gTQxvLj9 KireQ9pGOOw1PU85jRhxV2pKkSk6FB+rpWDJ/21zZGxlEm968nflKtxRdZG08uBdfq/KP YNuLZTEWlaTML2PQqs15osZnDULPUgT+VF4sV+QhItWf6SEVx6fuuAa6WPk2RlJDUFg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=2ThaJ8qB; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9BB8D606A320; Fri, 30 Oct 2026 14:15:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=2ThaJ8qB Received: by smtp.kernel.org (Postfix) with ESMTPSA id 841A61F000C1; Fri, 30 Oct 2026 14:15:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1793369720; bh=6o/wru8YGsBJ8nISiaIwKRCzFTUDQcQKEKhFrnSO15Q=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=2ThaJ8qBhraU6rDhRkC/D6TOscfYLnqxHXJG6QdzkJxpd7V+dj3zuwzSBw9XblvFY aC57fqDmqaCcYWSVA4Xxom0B/xj/62qrlM4aBxN6EKJMAmhr1M0QiIqgg5hy7Euhx3 dhTKsaLu7JtgjBUcQCdAihs79tw4eSXLgrA3ipWw= Date: Fri, 30 Oct 2026 15:15:17 +0100 From: Greg KH To: Jason Xing Cc: Paolo Abeni , Gyutae Bae , stable@vger.kernel.org, Thomas Gleixner , Eric Dumazet , Jakub Kicinski , Sebastian Andrzej Siewior , Peter Zijlstra , "Paul E. McKenney" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Minsu Jeon , Jonghyeon Kim , Siwan Kim , Gyutae Bae Subject: Re: Backport request: Revert "softirq: Let ksoftirqd do its job" (d15121be7485) Message-ID: <2026103018-gothic-sheath-5c34@gregkh> References: <20261029054200.97234-1-gyutae.opensource@navercorp.com> <9024faed-854d-411d-96c1-7cefd4d68dc6@redhat.com> X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Oct 30, 2026 at 02:16:57PM +0800, Jason Xing wrote: > Hi Paolo, > > On Thu, Oct 29, 2026 at 4:33 PM Paolo Abeni wrote: > > > > On 10/29/26 06:42, Gyutae Bae wrote: > > > Please backport the following upstream commit to 6.1.y and 5.15.y: > > > > > > d15121be7485655129101f3960ae6add40204463 > > > Revert "softirq: Let ksoftirqd do its job" > > > > > > On a production 5.15 PREEMPT_NONE node, long-running overlayfs > > > getdents64() calls prevented ksoftirqd from running. RPS IPIs > > > continued to arrive, but NET_RX processing on that CPU stalled for > > > up to about 3 seconds. Over 15 days, the node logged about 1,000 > > > bonding ARP monitor failovers, with no link-state changes recorded > > > during that period. The failovers stopped after the workload was > > > removed. > > > > > > On a 5.15.27 PREEMPT_NONE test node, we used a livepatch to make > > > ksoftirqd_running() always return false. This removes the same > > > softirq deferral checks as the revert on this build. Under the same > > > load, we measured three 10-minute windows: before patching, with the > > > livepatch active, and after removing it. The number of HTTP requests > > > taking more than 200 ms was 17, 0 and 11, respectively. TCPTimeouts > > > increased by 25, 1 and 21. > > > > > > We verified with patch --dry-run that the revert applies cleanly > > > to v6.1.190 and v5.15.223. Both already contain the related RCU > > > fix 1dd1eff161bd. 6.6.y and later already contain the revert. > > > > > > The original commit was intended to prevent user-task starvation > > > under a softirq flood. The revert's changelog points to threaded > > > NAPI as an alternative, but it is disabled by default. > > > > > > The revert was merged in v6.5 without a Cc: stable tag. > > > Syzbot has also reported bisections to this revert [1]. > > > Paolo, Thomas, Eric: I would appreciate your thoughts on whether > > > this revert is suitable for 6.1.y and 5.15.y. > > > > > > [1] https://lore.kernel.org/all/000000000000a249880603717c15@google.com/ > > IMHO such change should not land to stable lightly. It's a behavioural > > change with subtle side effects. The threaded napi feature is disabled > > Technically speaking, yes, it's actually not a bug but an optimization. > > > by default for the same reason. Also the revert has some soft functional > > dependencies in napi threaded cleanup and improvements landed into v6.4, > > i.e. 6bb218b5a6e3f3c5e2e76faf8fc976f3f6f72496, likely more. > > Maybe I have a slightly different thought on this: since I really > received/noticed there are not a few reports showing the 'issue' in > the older kernels. It's not easy for some people or company to upgrade > the stable kernel. I'm wondering if it's still possible to land it > with more careful consideration in the patch if Gyutae volunteers to > do so? Please feel free to reject the proposal :) Given that 5.15.y is about to go end-of-life in a month, that's a non-starter. For 6.1.y, you better be planning how to move off of that _now_ given that you only have 1 more year before that is gone. Anyone hitting this issue should be moving now as that gives you an incentive to do so :) thanks, greg k-h