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=-7.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 D670FC3A5A3 for ; Thu, 29 Aug 2019 17:14:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A415122CF5 for ; Thu, 29 Aug 2019 17:14:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="ES+v6LcC" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727364AbfH2RO6 (ORCPT ); Thu, 29 Aug 2019 13:14:58 -0400 Received: from mail-pg1-f193.google.com ([209.85.215.193]:38876 "EHLO mail-pg1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727403AbfH2RO5 (ORCPT ); Thu, 29 Aug 2019 13:14:57 -0400 Received: by mail-pg1-f193.google.com with SMTP id e11so1923079pga.5 for ; Thu, 29 Aug 2019 10:14:56 -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=mPDM53/6fx4rnRkVTsQ5BY1RiGRNlZ4KUune0Ro7yNA=; b=ES+v6LcCwtNJsRIRE18/qNv6QT20nJphH77qku4h5nuifb2Wj1/18f5TeoE4dY3IPA rhhxr+Wqb3oUpX46hfOnzYY3HDNWdMUd+HJV5GYySJ/TEv4Yg5h3x3lj4DcKeSKvoCWS TN7fwkfTE6IjogQx7daWws5OBakVs4pcljWHU= 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=mPDM53/6fx4rnRkVTsQ5BY1RiGRNlZ4KUune0Ro7yNA=; b=rzoslvU6SlEs+DAydbWVF+IxXk1aVSOTIjDfJ9txciZKW5u881X6XE2PCrGiRssIzU V9JimYJezG1WFzTaqgzxrpsn+GDusPNIYDSPiGBM5b9fx0YhrH3mSF35xuJ0eD8wvPI3 jF6WCdF72+Lc1NQTT6YVS5flrdx53F9E4CARaVXZIMlZ7uIFBMzyRGh66Ufw459Yc1bR rDQYRH1mWUDIhoVMHR4KyZOL+Z1npRFaYIDQlUozu6U7LxGJCWBbcuPekqjL0qUoXQPL ZF/wDs3Dio5MLu8suKd2TrMye8YE6LL+XyZEbut/2r3xVz6qwx8d2kpH4AviFggYdl8w H43w== X-Gm-Message-State: APjAAAVQHNhSywAj8rLME1c+rC3SxvYWyM1cpdzOwLvBqN8tOZkatKPA iQyMwZFcflgNdUs06gxBsG+0MA== X-Google-Smtp-Source: APXvYqzLu5wk3DxAGdv97u2fDeuvOrok07/5HmEdMMy5kzt5jkkZmQC2p7EkfjQqRXSQajATsBsnrg== X-Received: by 2002:a17:90a:e286:: with SMTP id d6mr11225418pjz.61.1567098896195; Thu, 29 Aug 2019 10:14:56 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id x17sm3516199pff.62.2019.08.29.10.14.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Aug 2019 10:14:55 -0700 (PDT) Date: Thu, 29 Aug 2019 13:14:54 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Frederic Weisbecker , Jonathan Corbet , Josh Triplett , kernel-team@android.com, Lai Jiangshan , linux-doc@vger.kernel.org, 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: <20190829171454.GA115245@google.com> References: <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> <20190829151325.GF63638@google.com> <20190829161301.GQ4125@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190829161301.GQ4125@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Aug 29, 2019 at 09:13:01AM -0700, Paul E. McKenney wrote: > On Thu, Aug 29, 2019 at 11:13:25AM -0400, Joel Fernandes wrote: > > On Thu, Aug 29, 2019 at 10:43:55AM -0400, Joel Fernandes wrote: > > > On Wed, Aug 28, 2019 at 08:43:36PM -0700, Paul E. McKenney wrote: > > > [snip] > > > > > > > > This change is not fixing a bug, so there is no need for an emergency fix, > > > > > > > > and thus no point in additional churn. I understand that it is a bit > > > > > > > > annoying to code and test something and have your friendly maintainer say > > > > > > > > "sorry, wrong rocks", and the reason that I understand this is that I do > > > > > > > > that to myself rather often. > > > > > > > > > > > > > > The motivation for me for this change is to avoid future bugs such as with > > > > > > > the following patch where "== 2" did not take the force write of > > > > > > > DYNTICK_IRQ_NONIDLE into account: > > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/commit/?h=dev&id=13c4b07593977d9288e5d0c21c89d9ba27e2ea1f > > > > > > > > > > > > Yes, the current code does need some simplification. > > > > > > > > > > > > > I still don't see it as pointless churn, it is also a maintenance cost in its > > > > > > > current form and the simplification is worth it IMHO both from a readability, > > > > > > > and maintenance stand point. > > > > > > > > > > > > > > I still don't see what's technically wrong with the patch. I could perhaps > > > > > > > add the above "== 2" point in the patch? > > > > > > > > > > > > I don't know of a crash or splat your patch would cause, if that is > > > > > > your question. But that is also true of the current code, so the point > > > > > > is simplification, not bug fixing. And from what I can see, there is an > > > > > > opportunity to simplify quite a bit further. And with something like > > > > > > RCU, further simplification is worth -serious- consideration. > > > > > > > > > > > > > We could also discuss f2f at LPC to see if we can agree about it? > > > > > > > > > > > > That might make a lot of sense. > > > > > > > > > > Sure. I am up for a further redesign / simplification. I will think more > > > > > about your suggestions and can also further discuss at LPC. > > > > > > > > One question that might (or might not) help: Given the compound counter, > > > > where the low-order hex digit indicates whether the corresponding CPU > > > > is running in a non-idle kernel task and the rest of the hex digits > > > > indicate the NMI-style nesting counter shifted up by four bits, what > > > > could rcu_is_cpu_rrupt_from_idle() be reduced to? > > > > > > > > > And this patch is on LKML archives and is not going anywhere so there's no > > > > > rush I guess ;-) > > > > > > > > True enough! ;-) > > > > > > 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") > > > > > > Paul, also what what happens in the following scenario: > > > > CPU0 CPU1 > > > > A syscall causes rcu_eqs_exit() > > rcu_read_lock(); > > ---> FQS loop waiting on > > dyntick_snap > > usermode-upcall entry -->causes rcu_eqs_enter(); > > > > usermode-upcall exit -->causes rcu_eqs_exit(); > > > > ---> FQS loop sees > > dyntick snap > > increment and > > declares CPU0 is > > in a QS state > > before the > > rcu_read_unlock! > > > > rcu_read_unlock(); > > --- > > > > Does the context tracking not call rcu_user_enter() in this case, or did I > > really miss something? > > Holding rcu_read_lock() across usermode execution (in this case, > the usermode upcall) is a bad idea. Why is CPU 0 doing that? Oh, ok. I was just hypothesizing that since usermode upcalls from something as heavy as interrupts, it could also mean we had the same from some path that held an rcu_read_lock() as well. It was just a theoretical concern, if it is not an issue, no problem. The other question I had was, in which cases would dyntick_nesting in current RCU code be > 1 (after removing the lower bit and any crowbarring) ? In the scenarios I worked out on paper, I can only see this as 1 or 0. But the wording of it is 'dynticks_nesting'. May be I am missing a nesting scenario? We can exit RCU-idleness into process context only once (either exiting idle mode or user mode). Both cases would imply a value of 1. thanks! - Joel