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 BEBF5ECAAA1 for ; Tue, 6 Sep 2022 18:45:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229780AbiIFSpy (ORCPT ); Tue, 6 Sep 2022 14:45:54 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54446 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229529AbiIFSpq (ORCPT ); Tue, 6 Sep 2022 14:45:46 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D018F76478; Tue, 6 Sep 2022 11:45:44 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id E3AE6615BB; Tue, 6 Sep 2022 18:45:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 15E0DC433D6; Tue, 6 Sep 2022 18:45:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1662489943; bh=5srtKgHlDqAawI0Yhw64Ha/8ev79Mn9jpJ+ljzWsc00=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=akv1mp2SKOXl6Fno8v3teqLcwKi8mSOxr9Xg06S1zU9K9MiK7O4inh1rSo8bqGC0h XOn6xDQ4+8FYz/cqPxrP86enSQXcVhICaAORgai0Pb0a1lJqOZt0PDFa2eYCq7fz2U E82wPu1uy+jTOsU0cRui+pzkUVpR6d9pnjSRF1MDSOtrixnExUEzyYLgm9ZE3hr1hy M1uHT3AOgTuiWhqcb998vG37Oniw5oTUA06Q1nFuT5lIvWjRiBwljhvdTuApSecjwx 4d1ADdgGSjv7Fv8+tRvyB496HmRl6hrut8mqi32rrb5Paj14Ob6NQa5qvHWt3T0ITd oJ90bzugrM9Jw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 187FA5C0A40; Tue, 6 Sep 2022 11:45:09 -0700 (PDT) Date: Tue, 6 Sep 2022 11:45:09 -0700 From: "Paul E. McKenney" To: Pingfan Liu Cc: LKML , rcu , Frederic Weisbecker , Neeraj Upadhyay , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Thomas Gleixner , Steven Price , Mark Rutland , Kuppuswamy Sathyanarayanan , "Jason A. Donenfeld" , boqun.feng@gmail.com Subject: Re: [RFC 06/10] rcu/hotplug: Make rcutree_dead_cpu() parallel Message-ID: <20220906184509.GF4315@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220822021520.6996-1-kernelfans@gmail.com> <20220822021520.6996-7-kernelfans@gmail.com> <20220822024528.GC6159@paulmck-ThinkPad-P17-Gen-1> <20220823030125.GJ6159@paulmck-ThinkPad-P17-Gen-1> <20220824162050.GA6159@paulmck-ThinkPad-P17-Gen-1> <20220831161522.GA2582451@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 05, 2022 at 11:53:52AM +0800, Pingfan Liu wrote: > On Thu, Sep 1, 2022 at 12:15 AM Paul E. McKenney wrote: > > > > On Wed, Aug 24, 2022 at 09:20:50AM -0700, Paul E. McKenney wrote: > > > On Wed, Aug 24, 2022 at 09:53:11PM +0800, Pingfan Liu wrote: > > > > On Tue, Aug 23, 2022 at 11:01 AM Paul E. McKenney wrote: > > > > > On Tue, Aug 23, 2022 at 09:50:56AM +0800, Pingfan Liu wrote: > > > > > > On Sun, Aug 21, 2022 at 07:45:28PM -0700, Paul E. McKenney wrote: > > > > > > > On Mon, Aug 22, 2022 at 10:15:16AM +0800, Pingfan Liu wrote: > > > > > > > > In order to support parallel, rcu_state.n_online_cpus should be > > > > > > > > atomic_dec() > > > > > > > > > > > > > > > > Signed-off-by: Pingfan Liu > > > > > > > > > > > > > > I have to ask... What testing have you subjected this patch to? > > > > > > > > > > > > > > > > > > > This patch subjects to [1]. The series aims to enable kexec-reboot in > > > > > > parallel on all cpu. As a result, the involved RCU part is expected to > > > > > > support parallel. > > > > > > > > > > I understand (and even sympathize with) the expectation. But results > > > > > sometimes diverge from expectations. There have been implicit assumptions > > > > > in RCU about only one CPU going offline at a time, and I am not sure > > > > > that all of them have been addressed. Concurrent CPU onlining has > > > > > been looked at recently here: > > > > > > > > > > https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing > > > > > > > > > > You did us atomic_dec() to make rcu_state.n_online_cpus decrementing be > > > > > atomic, which is good. Did you look through the rest of RCU's CPU-offline > > > > > code paths and related code paths? > > > > > > > > I went through those codes at a shallow level, especially at each > > > > cpuhp_step hook in the RCU system. > > > > > > And that is fine, at least as a first step. > > > > > > > But as you pointed out, there are implicit assumptions about only one > > > > CPU going offline at a time, I will chew the google doc which you > > > > share. Then I can come to a final result. > > > > > > Boqun Feng, Neeraj Upadhyay, Uladzislau Rezki, and I took a quick look, > > > and rcu_boost_kthread_setaffinity() seems to need some help. As it > > > stands, it appears that concurrent invocations of this function from the > > > CPU-offline path will cause all but the last outgoing CPU's bit to be > > > (incorrectly) set in the cpumask_var_t passed to set_cpus_allowed_ptr(). > > > > > > This should not be difficult to fix, for example, by maintaining a > > > separate per-leaf-rcu_node-structure bitmask of the concurrently outgoing > > > CPUs for that rcu_node structure. (Similar in structure to the > > > ->qsmask field.) > > > > > Sorry to reply late, since I am interrupted by some other things. > I have took a different way and posted a series ([PATCH 1/3] rcu: > remove redundant cpu affinity setting during teardown) for that on > https://lore.kernel.org/rcu/20220905033852.18988-1-kernelfans@gmail.com/T/#t And I took patch #3, thank you! #1 allows the kthread to run on the outgoing CPU, which is to be avoided, and #2 depends on #1. > Besides, for the integration of the concurrency cpu hot-removing into > the rcu torture test, I begin to do it. Very good! I am looking forward to seeing what you come up with. > > > There are probably more where that one came from. ;-) > > > > And here is one more from this week's session. > > Thanks for the update. > > > The calls to tick_dep_set() and tick_dep_clear() use atomic operations, > > but they operate on a global variable. This means that the first call > > to rcutree_offline_cpu() would enable the tick and the first call to > > rcutree_dead_cpu() would disable the tick. This might be OK, but it > > is at the very least bad practice. There needs to be a counter > > mediating these calls. > > I will see what I can do here. > > > For more detail, please see the Google document: > > > > https://docs.google.com/document/d/1jymsaCPQ1PUDcfjIKm0UIbVdrJAaGX-6cXrmcfm0PRU/edit?usp=sharing > > > > Have read it and hope that both online and offline concurrency can > come to true in near future. Indeed, I suspect that a lot of people would like to see faster kexec! Thanx, Paul