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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 5D090C3A59F for ; Thu, 29 Aug 2019 16:22:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2F71D233FF for ; Thu, 29 Aug 2019 16:22:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567095721; bh=yfzsmqtHVcxU58IiLWMkZ2wW2SOsZ+0cJNaEYROK0hg=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=HPnEciL9O/Svwra5P4t3XsYb77qxdBypaEc/F8e5Js/NZ2DC4fSJq9Zszwo1lE+h6 J7unIgHUgZTIo1NysLiPfDebeqhbD7WefI9h+b5pJbO5VELg1uIBO1dRHkXycVHaBT ophV6erik28UfzrJ210yiF6G4P+hwATH9ETpMz6E= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727417AbfH2QWA (ORCPT ); Thu, 29 Aug 2019 12:22:00 -0400 Received: from mail.kernel.org ([198.145.29.99]:38922 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727161AbfH2QWA (ORCPT ); Thu, 29 Aug 2019 12:22:00 -0400 Received: from mail-wm1-f43.google.com (mail-wm1-f43.google.com [209.85.128.43]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id AC3FC23428 for ; Thu, 29 Aug 2019 16:21:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1567095719; bh=yfzsmqtHVcxU58IiLWMkZ2wW2SOsZ+0cJNaEYROK0hg=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=WTVSQe3M3O36C2hHmO4C0PE5Jk4r6myJcarVYynfCz93uYgnG0Eg6NuyMvGs6f3jL n3laAbJx7/dxUnLKSSig2sxmXClBThloqMPAElS9lREgFtKrD6iBGFkpfuNNhEdaGu 6igLeVpsbJ8U5azWAejqfCEherJlWbm6kCaOtXmc= Received: by mail-wm1-f43.google.com with SMTP id o184so4470372wme.3 for ; Thu, 29 Aug 2019 09:21:59 -0700 (PDT) X-Gm-Message-State: APjAAAXF9WxuAE7I0ryU+VMCHoOBO/mdaerikLqDeo4ftDlfD7d5GFAg /TBg5y1vSLSHbuNzenh/KD/NSsXn/9iRd4DMygSvwQ== X-Google-Smtp-Source: APXvYqzP6/jAXk8d7DB4MU1R71kThKX52pb8K9ShVzAGw7sIo6b1HNDK1m6Zf68a2BUyONA0Hh6ESmfNcso3WPAA4u4= X-Received: by 2002:a1c:f910:: with SMTP id x16mr12055557wmh.173.1567095718086; Thu, 29 Aug 2019 09:21:58 -0700 (PDT) MIME-Version: 1.0 References: <20190828202330.GS26530@linux.ibm.com> <20190828210525.GB75931@google.com> <20190828211904.GX26530@linux.ibm.com> <20190828214241.GD75931@google.com> <20190828220108.GC26530@linux.ibm.com> <20190828221444.GA100789@google.com> <20190828231247.GE26530@linux.ibm.com> <20190829015155.GB100789@google.com> <20190829034336.GD4125@linux.ibm.com> <20190829144355.GE63638@google.com> <20190829160946.GP4125@linux.ibm.com> In-Reply-To: <20190829160946.GP4125@linux.ibm.com> From: Andy Lutomirski Date: Thu, 29 Aug 2019 09:21:46 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi_nesting counter To: paulmck@kernel.org Cc: Joel Fernandes , LKML , Frederic Weisbecker , Jonathan Corbet , Josh Triplett , Android Kernel Team , Lai Jiangshan , "open list:DOCUMENTATION" , Mathieu Desnoyers , Mauro Carvalho Chehab , rcu@vger.kernel.org, Steven Rostedt , Andrew Lutomirski Content-Type: text/plain; charset="UTF-8" Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Aug 29, 2019 at 9:10 AM Paul E. McKenney wrote: > > On Thu, Aug 29, 2019 at 10:43:55AM -0400, Joel Fernandes wrote: > > [ . . . ] > > > Paul, do we also nuke rcu_eqs_special_set()? Currently I don't see anyone > > using it. And also remove the bottom most bit of dynticks? > > > > Also what happens if a TLB flush broadcast is needed? Do we IPI nohz or idle > > CPUs are the moment? > > > > All of this was introduced in: > > b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter") > > Adding Andy Lutomirski on CC. > > Andy, is this going to be used in the near term, or should we just get > rid of it? > Let's get rid of it. I'm not actually convinced it *can* be used as designed. For those who forgot the history or weren't cc'd on all of it: I had this clever idea about how we could reduce TLB flushes. I implemented some of it (but not the part that would have used this RCU feature), and it exploded in nasty and subtle ways. This caused me to learn that speculative TLB fills were a problem that I had entirely failed to account for. Then PTI happened and thoroughly muddied the water. So I think we should just drop this :(