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=-2.8 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 CD441C433DF for ; Wed, 13 May 2020 23:08:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CE68D2065C for ; Wed, 13 May 2020 23:08:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589411314; bh=McI+T2hmy1wwsLIjP6KHQL0XtS4SuVPV85zl2tKT4R0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=aqrHFdpo3cNjemonJF/MpEcHqIsPkMm/4xGrBhvSluoEGY2NIrcMXZYwRFhoy3BjP viVoey5xtQ8rRCfwdsLdsacYlY6Xpi8GRsqCdFvOMzR/AX+ZCWIOZp6yBANwuXeC+F A7ObRYS5jz+FCYWm7+haVCxRCnBDPSYBbsBnwQZ0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732449AbgEMXId (ORCPT ); Wed, 13 May 2020 19:08:33 -0400 Received: from mail.kernel.org ([198.145.29.99]:40262 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732404AbgEMXIb (ORCPT ); Wed, 13 May 2020 19:08:31 -0400 Received: from localhost (lfbn-ncy-1-985-231.w90-101.abo.wanadoo.fr [90.101.63.231]) (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 D02F42054F; Wed, 13 May 2020 23:08:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589411311; bh=McI+T2hmy1wwsLIjP6KHQL0XtS4SuVPV85zl2tKT4R0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=FykKUVt2XXJz9k5bULOrNTaBEImxREfbnLQ89y6LYJmgLYhxSoCOYimRQhDZAsBUG kvfisUu5mlMWAA//YJlD6+QsxMRyoZxQbIO/9TCPJFzn/5N1ivzQPzMdnqG9JDGPVj lYDQv+UxYs33lGoTs3o2hqYpksBXGCva+7Qx11OY= Date: Thu, 14 May 2020 01:08:28 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" 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: <20200513230827.GC18303@lenoir> References: <20200513164714.22557-1-frederic@kernel.org> <20200513164714.22557-8-frederic@kernel.org> <20200513182527.GU2869@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200513182527.GU2869@paulmck-ThinkPad-P72> 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 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 :-) > 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. Thanks.