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 10BA3C433EF for ; Mon, 13 Dec 2021 19:00:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242062AbhLMTAJ (ORCPT ); Mon, 13 Dec 2021 14:00:09 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47984 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232416AbhLMTAI (ORCPT ); Mon, 13 Dec 2021 14:00:08 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 22B8DC061574; Mon, 13 Dec 2021 11:00:08 -0800 (PST) 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 ams.source.kernel.org (Postfix) with ESMTPS id DF811B81255; Mon, 13 Dec 2021 19:00:06 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A90AAC34600; Mon, 13 Dec 2021 19:00:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1639422005; bh=7r2b3zIykqzLviEJZiXs7snpV80iotoqxiNSSC/IIdQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=bhWWvKxJqH1GrV3KQJ4HIG943VnstdXGJUBUbcpftai8/XZtnux5qInMFxr9IKiPv OP1Y2pbz0wSdlKpfH4I/Hr5aoiHUWO5Rix8Ri0TeEJSJKMCtmqnqFO5/z2PmA1R+u+ 0APpNLLUuKxHfjCiUWPIyuaWMbQDZm+fyveadObECvWL6u4gC2sgjACdy7Q9wmfkC7 Inyb9witJxd4CLY79U4GeGPJls+Iqj1LJmUczy9+lBkHr6Hqj06sC0He1JYARESX26 BpSBo3mNQVQqOax1kcg6Q2/lKEMtk3AAqaXzz5G9ArditI9ef8Fu5EjasL9Cfz/35D 2WSZ95tgmJouQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 680BC5C0B9E; Mon, 13 Dec 2021 11:00:05 -0800 (PST) Date: Mon, 13 Dec 2021 11:00:05 -0800 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: David Woodhouse , Neeraj Upadhyay , josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, joel@joelfernandes.org, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, urezki@gmail.com, boqun.feng@gmail.com Subject: Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation Message-ID: <20211213190005.GP641268@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20211211170139.27711-1-quic_neeraju@quicinc.com> <6c184b9ffc5c641736d53bb7598f814d6b4c3fe0.camel@infradead.org> <601ecb12-ae2e-9608-7127-c2cddc8038a6@quicinc.com> <20211213112246.GA782195@lothringen> <984a63d4c11d04e2ee8a83fc9c61006413bf209e.camel@infradead.org> <20211213131407.GD782195@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211213131407.GD782195@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 13, 2021 at 02:14:07PM +0100, Frederic Weisbecker wrote: > On Mon, Dec 13, 2021 at 11:28:45AM +0000, David Woodhouse wrote: > > On Mon, 2021-12-13 at 12:22 +0100, Frederic Weisbecker wrote: > > > I was about to ack the patch but, should we really add code that isn't going to > > > be necessary before a long while? > > > > Yeah, I'm torn on that. In this case it's harmless enough and it makes > > the code reentrant in its own right instead of relying on the fact that > > the cpuhp code won't invoke it multiple times in parallel. So I think > > that's reasonable defensive programming. > > The thing is that RCU code is already quite complicated. Are we even at least > sure that we'll ever make CPU hotplug allow concurrent CPU onlining/offlining? > > This will require much more thoughts and a new hotplug concurrency > infrastructure that we'll need to base RCU on. IMHO it's a bit early to handle > that on hotplug individual callbacks. > > But anyway, let's see what Paul thinks about it... We need to at least think through parallelizing the various RCU CPU-hotplug notifiers. For one thing, it might turn out to be necessary to parallelize those notifiers, perhaps on some non-x86 architecture or on some non-Amazon .config. For another thing, doing so might suggest some simplifications, as has happened in my ongoing rcu_barrier() work. My thought is to pull in the patches and at the very least leave a tag recording them for later possible use. Thanx, Paul