From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F14FEC4332F for ; Thu, 17 Nov 2022 17:42:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234299AbiKQRm2 (ORCPT ); Thu, 17 Nov 2022 12:42:28 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32916 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232097AbiKQRm1 (ORCPT ); Thu, 17 Nov 2022 12:42:27 -0500 Received: from mail-oa1-x33.google.com (mail-oa1-x33.google.com [IPv6:2001:4860:4864:20::33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 731BD2792C for ; Thu, 17 Nov 2022 09:42:26 -0800 (PST) Received: by mail-oa1-x33.google.com with SMTP id 586e51a60fabf-13ae8117023so2970961fac.9 for ; Thu, 17 Nov 2022 09:42:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=RfORcEnRNI40YvWBc5y1Em70ZVblOHNV3oKHjYQR7dc=; b=j+DFMuYmsxvGLfPcmv0ANKN1bbBdxegl3OjLWAop7KTeXYyQulIUZRoaf+DJBfe6k/ 5/yZWx9ZkhFavd8PgTjvwuWIgvpbm4IM64k9HwhLS27NrFLILA/L+OUl7qYbmmxX2PNy NjNPCV6fr5oArPBq/g+1IU4x7MZq0eMGMUl8Y= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=RfORcEnRNI40YvWBc5y1Em70ZVblOHNV3oKHjYQR7dc=; b=qwe4rx/xpeISUs5bi3k5D758MFKVFWIHN3uBjAzaEMDm1P8HRKu6Jf7+uXWflGdzXK Fj6wvOsLVu3pD+DSQyDFP17WkN+MY+XTfiXVZicYQkcwVxS0rH//HFEOiulRIYnSmTYF q6qF79s64zqZCB7Qp8gZb/XSEd0k70zDQOLTKS16QZfMaIdXkzWt1XbPr0CevSGkzWF8 qm3Zs3s5R+ILwTKVterhdHOA10IsnKSrCHg6xi0Wfsiu5/YNUiooMbBaNdtPYqDxzdlV Fg9fNmG3n/D90UFH5/ukYRYmf00TJ8rO9miJE4lVbJRV82AM2TiOoX0AT7VXgSeXolZf Ndzw== X-Gm-Message-State: ANoB5pkYZTB1reZ1gC7RnnPlqz7xDk6QSU3OLy2iHIS7S5lnERATHHv8 oG6yo8FOQbO6afNSNSVlXhptDerJt3mdOOOcNtnZpA== X-Google-Smtp-Source: AA0mqf5TwZQoOYT1jHxYgRDVMEmTtUN232TguQIURbIWHB70y5QL2vtLGeTJlyytNW9PS+z7/JOeWKuHxfcM1lgMOPw= X-Received: by 2002:a05:6870:591:b0:13b:bbbb:1623 with SMTP id m17-20020a056870059100b0013bbbbb1623mr1886559oap.115.1668706945539; Thu, 17 Nov 2022 09:42:25 -0800 (PST) MIME-Version: 1.0 References: <20221117031551.1142289-1-joel@joelfernandes.org> <20221117031551.1142289-3-joel@joelfernandes.org> In-Reply-To: From: Joel Fernandes Date: Thu, 17 Nov 2022 17:42:14 +0000 Message-ID: Subject: Re: [PATCH rcu/dev 3/3] net: Use call_rcu_flush() for dst_destroy_rcu To: Eric Dumazet Cc: linux-kernel@vger.kernel.org, Cong Wang , David Ahern , "David S. Miller" , Hideaki YOSHIFUJI , Jakub Kicinski , Jamal Hadi Salim , Jiri Pirko , netdev@vger.kernel.org, Paolo Abeni , rcu@vger.kernel.org, rostedt@goodmis.org, paulmck@kernel.org, fweisbec@gmail.com, jiejiang@google.com, Thomas Glexiner Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, Nov 17, 2022 at 5:40 PM Eric Dumazet wrote: > > On Thu, Nov 17, 2022 at 9:38 AM Joel Fernandes wrote: > > > > On Thu, Nov 17, 2022 at 5:17 PM Eric Dumazet wrote: > > > > > > On Thu, Nov 17, 2022 at 7:58 AM Joel Fernandes wrote: > > > > > > > > Hello Eric, > > > > > > > > On Wed, Nov 16, 2022 at 07:44:41PM -0800, Eric Dumazet wrote: > > > > > On Wed, Nov 16, 2022 at 7:16 PM Joel Fernandes (Google) > > > > > wrote: > > > > > > > > > > > > In a networking test on ChromeOS, we find that using the new CONFIG_RCU_LAZY > > > > > > causes a networking test to fail in the teardown phase. > > > > > > > > > > > > The failure happens during: ip netns del > > > > > > > > > > And ? What happens then next ? > > > > > > > > The test is doing the 'ip netns del ' and then polling for the > > > > disappearance of a network interface name for upto 5 seconds. I believe it is > > > > using netlink to get a table of interfaces. That polling is timing out. > > > > > > > > Here is some more details from the test's owner (copy pasting from another > > > > bug report): > > > > In the cleanup, we remove the netns, and thus will cause the veth pair being > > > > removed automatically, so we use a poll to check that if the veth in the root > > > > netns still exists to know whether the cleanup is done. > > > > > > > > Here is a public link to the code that is failing (its in golang): > > > > https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/tast-tests/src/chromiumos/tast/local/network/virtualnet/env/env.go;drc=6c2841d6cc3eadd23e07912ec331943ee33d7de8;l=161 > > > > > > > > Here is a public link to the line of code in the actual test leading up to the above > > > > path (this is the test that is run: > > > > network.RoutingFallthrough.ipv4_only_primary) : > > > > https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/tast-tests/src/chromiumos/tast/local/bundles/cros/network/routing_fallthrough.go;drc=8fbf2c53960bc8917a6a01fda5405cad7c17201e;l=52 > > > > > > > > > > Using ftrace, I found the callbacks it was queuing which this series fixes. Use > > > > > > call_rcu_flush() to revert to the old behavior. With that, the test passes. > > > > > > > > > > What is this test about ? What barrier was used to make it not flaky ? > > > > > > > > I provided the links above, let me know if you have any questions. > > > > > > > > > Was it depending on some undocumented RCU behavior ? > > > > > > > > This is a new RCU feature posted here for significant power-savings on > > > > battery-powered devices: > > > > https://lore.kernel.org/rcu/20221017140726.GG5600@paulmck-ThinkPad-P17-Gen-1/T/#m7a54809b8903b41538850194d67eb34f203c752a > > > > > > > > There is also an LPC presentation about the same, I can dig the link if you > > > > are interested. > > > > > > > > > Maybe adding a sysctl to force the flush would be better for functional tests ? > > > > > > > > > > I would rather change the test(s), than adding call_rcu_flush(), > > > > > adding merge conflicts to future backports. > > > > > > > > I am not too sure about that, I think a user might expect the network > > > > interface to disappear from the networking tables quickly enough without > > > > dealing with barriers or kernel iternals. However, I added the authors of the > > > > test to this email in the hopes he can provide is point of views as well. > > > > > > > > The general approach we are taking with this sort of thing is to use > > > > call_rcu_flush() which is basically the same as call_rcu() for systems with > > > > CALL_RCU_LAZY=n. You can see some examples of that in the patch series link > > > > above. Just to note, CALL_RCU_LAZY depends on CONFIG_RCU_NOCB_CPU so its only > > > > Android and ChromeOS that are using it. I am adding Jie to share any input, > > > > he is from the networking team and knows this test well. > > > > > > > > > > > > > > I do not know what is this RCU_LAZY thing, but IMO this should be opt-in > > > > You should read the links I sent you. We did already try opt-in, > > Thomas Gleixner made a point at LPC that we should not add new APIs > > for this purpose and confuse kernel developers. > > > > > For instance, only kfree_rcu() should use it. > > > > No. Most of the call_rcu() usages are for freeing memory, so the > > consensus is we should apply this as opt out and fix issues along the > > way. We already did a lot of research/diligence on seeing which users > > need conversion. > > > > > We can not review hundreds of call_rcu() call sites and decide if > > > adding arbitrary delays cou hurt . > > > > That work has already been done as much as possible, please read the > > links I sent. > > Oh well. No. > > I will leave it to other folks dealing with this crazy thing. Yes, I agree. Your comments here have not been useful (or respectful) so I am Ok with that. - Joel