From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, RCVD_IN_DNSWL_HI,SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.2 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id C85D67D910 for ; Thu, 29 Aug 2019 19:00:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728408AbfH2TAt (ORCPT ); Thu, 29 Aug 2019 15:00:49 -0400 Received: from mail-pf1-f193.google.com ([209.85.210.193]:43937 "EHLO mail-pf1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727959AbfH2TAt (ORCPT ); Thu, 29 Aug 2019 15:00:49 -0400 Received: by mail-pf1-f193.google.com with SMTP id v12so2678504pfn.10 for ; Thu, 29 Aug 2019 12:00:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=YFO4ToJTqzSQELe3odwwkEfeFs84ACj71p8BdwBVqGc=; b=d0VFIDowYpvAl/u3SmvC0to24Kdmxtpz2/UnnHd8VZyq1Od8buoXAZgDmyshy3xKpv E4JQSe8CmUGbPMBhy30yzfA9+zY3kE2Oveu4m/7CuPoW/IOsvPx2E/fQ8csrW2zCDVFZ YCaTL8mvBoul3+p5hEYzMhXdMc/Th4xD/Oc+c= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=YFO4ToJTqzSQELe3odwwkEfeFs84ACj71p8BdwBVqGc=; b=B8z6pYIp8e7OyDBzqXhTeDG+NEmCghFOC1DHyxmOKejmd9ch+eh9crSrz9AUXIbvQf 535bTE6OEsVvkxmAaPbnhIAD/KlonXxWLFGx1RNUOML8iIHYCKxMGfDpp+UzqX4AIrVH jYKGZ2ZLFnY4XjhUuDktYx+XkROgsVDY9supu4O2z31IGrESydBpxutLLbNtLjp/XhhO sM8vLzuJswYTUoofTmf53CjkZPYpR10K3cnjn15qa2SMqsMp40tbWkEaGqPjrCX66Whm m5pVTaotyiscQhRxZZ6a6kjs7I3jeDZOgI5SnniSxzPi56UmuTGcNRjMbetxpbDP0s1N /j3w== X-Gm-Message-State: APjAAAXf3M6jd3TBnn9mmQ3UrFA4Dcb756B6vfACzMtmsrZMnPI+/0pS 3fm4NoO39nNcirW7Jm1BEmCnhw== X-Google-Smtp-Source: APXvYqw8LkU0kSGTofdVPMh2aqBnJCfeAMn73MuHa4izANAy35sEF6IaMEn0o0lntDwre+4hF5klZA== X-Received: by 2002:a17:90a:d345:: with SMTP id i5mr11528135pjx.16.1567105248246; Thu, 29 Aug 2019 12:00:48 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id 5sm2846842pgh.93.2019.08.29.12.00.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Aug 2019 12:00:47 -0700 (PDT) Date: Thu, 29 Aug 2019 15:00:46 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: Andy Lutomirski , 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 Subject: Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi_nesting counter Message-ID: <20190829190046.GB115245@google.com> References: <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> <20190829165407.GT4125@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190829165407.GT4125@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org Hi Paul, On Thu, Aug 29, 2019 at 09:54:07AM -0700, Paul E. McKenney wrote: > On Thu, Aug 29, 2019 at 09:21:46AM -0700, Andy Lutomirski wrote: > > 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. > > Yeah, PTI was quite annoying. Still is, from what I can see. :-/ > > > So I think we should just drop this :( > > OK, thank you! I will put a tag into -rcu marking its removal in case > it should prove useful whenever for whatever. > > Joel, would you like to remove this, or would you rather that I did? > It is in code you are working with right now, so if I do it, I need to > wait until yours is finalized. Which wouldn't be a problem. I can remove it in my series, made a note to do so. thanks, - Joel