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 X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9C9F4C433E0 for ; Thu, 14 May 2020 15:50:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 675EB2065F for ; Thu, 14 May 2020 15:50:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589471434; bh=5tSjNChQ6lIWRMBCkikt2nPuu1u6XyTfuXs2AiZn6Ws=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=LKjYEclXOdkoRpF+xeucOyosuwoWTQdDIvJSnep6hVrjM7Sw52vNwg+YnnUMygMcS HxCcXcc1bvLiuXHj+x9kKxaFsGFb9z5ohFQmTLZn0NFQ44QXpbmJXIe+6nQpkhjVnV aJDgfA9GPCFH3a7Zcqy6CoJ8YAe5/8+lO9sS/T1s= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727838AbgENPud (ORCPT ); Thu, 14 May 2020 11:50:33 -0400 Received: from mail.kernel.org ([198.145.29.99]:45406 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726088AbgENPud (ORCPT ); Thu, 14 May 2020 11:50:33 -0400 Received: from paulmck-ThinkPad-P72.home (unknown [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id A3C24206A5; Thu, 14 May 2020 15:50:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589471432; bh=5tSjNChQ6lIWRMBCkikt2nPuu1u6XyTfuXs2AiZn6Ws=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=PJPRRTfSUaf9HnO7ggpV1YaRRzlO36Yg5u5DvUb3QyKw34quNnibbBMuEqvwvaR5s rPOIao0VbaPs5gIhJRITJEvZwsQzlNZvZfJfMdbjVPNX9Z9pGpN/vT7B+660iGyMjB rNj1sUjoWWp1KF65oOlaWbiLYS18Y4C2f478GyY4= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 86A6C35229C5; Thu, 14 May 2020 08:50:32 -0700 (PDT) Date: Thu, 14 May 2020 08:50:32 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: LKML , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Josh Triplett Subject: Re: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB Message-ID: <20200514155032.GN2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200513164714.22557-1-frederic@kernel.org> <20200513164714.22557-8-frederic@kernel.org> <20200513182527.GU2869@paulmck-ThinkPad-P72> <20200513230827.GC18303@lenoir> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200513230827.GC18303@lenoir> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 14, 2020 at 01:08:28AM +0200, Frederic Weisbecker wrote: > On Wed, May 13, 2020 at 11:25:27AM -0700, Paul E. McKenney wrote: > > On Wed, May 13, 2020 at 06:47:11PM +0200, Frederic Weisbecker wrote: > > > So far nohz_full CPUs had to be nocb. This requirement may change > > > temporarily as we are working on preparing RCU to be able to toggle the > > > nocb state of a CPU. Once that is done and nohz_full can be toggled as > > > well dynamically, we'll restore that initial requirement. > > > > Would it simplify anything to make the CPU exit nohz_full first and > > then exit rcu_nocb and vice versa in the other direction? That way the > > assumption about nohz_full CPUs always being rcu_nocb could remain while > > still allowing runtime changes to both states. > > That's the future plan but for now nohz_full can't even be exited yet. > RCU is unlucky enough to be chosen as the starting point of this whole work :-) But testing could still start with CPUs marked rcu_nocb but not marked nohz_full, right? I must confess that I am a bit concerned about the increase in state space. > > Of course, given that setup, it would not be possible to cause a CPU to > > exit rcu_nocb state if it was still in nohz_full state. > > Right. > > > My fear is that allowing a CPU to be in nohz_full state without also > > being in rcu_nocb state will cause needless confusion and bug reports. > > Well, it should only be visible to those who work on it since there > won't be a proper interface before we achieve the whole. Fair point, but I am also concerned about the welfare of the people working on it. ;-) Thanx, Paul