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=-5.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 5BA26C7112A for ; Mon, 15 Oct 2018 02:13:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1D4AB2086A for ; Mon, 15 Oct 2018 02:13:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="bl7y2h3K" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1D4AB2086A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joelfernandes.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726553AbeJOJ46 (ORCPT ); Mon, 15 Oct 2018 05:56:58 -0400 Received: from mail-pl1-f194.google.com ([209.85.214.194]:34162 "EHLO mail-pl1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726400AbeJOJ45 (ORCPT ); Mon, 15 Oct 2018 05:56:57 -0400 Received: by mail-pl1-f194.google.com with SMTP id f18-v6so8504462plr.1 for ; Sun, 14 Oct 2018 19:13:51 -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=DAG6do8Qbuvxl3VxASK5DL9Y/7NWoA5FaquGDm42Tx0=; b=bl7y2h3KoQJLFBCan8Shl5mib2K10vQrbMsGoEFQPCAw6gRYbQrXblrpTKW0Hlm3sz jRBi9oaIvnADEdevUdePVxYy6M7ns/etK0XTxWDmdvorsNEdxS+ZQxIANv2aZ+vhCWeu +mFvlmAEAs6KMaH/kBpSx4norievrrm+ZgZpU= 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=DAG6do8Qbuvxl3VxASK5DL9Y/7NWoA5FaquGDm42Tx0=; b=kSUmwbh7acH0KeD5WIPd4r/4Gk9faBL/U2cmkLgkht9dqXqw99lcpx0NcstCwcKY/N rqe/pt9w/HtZpBD3sAgNHP3qIkQ56kouLDt85lz4YIaXMnN4MB0Dam0KFuYjvVt6bPGY f87bThKqtVMrhsXU6R0PaMUtanCIagfMsT9rgL8jzwylLZ6dQM6/sECOiYa1tkcgNge4 UOO0aMxELcIO4DTqVxs/luXll/DtgtGPKkco1n8O8DukIQedJFGvF+tAMBJ36bS66ZEI WqGkj4WdMfBozI8fCSilksOR6Hc2fc1yg6ncmrHCcTxTEC2LqhS3gjxc4yxpV4TqaV63 rU2g== X-Gm-Message-State: ABuFfoiC/xBMsO6W80QOEFRmo3AanEc2BE7tsvIhtJRytzRnvotvgaxj HiNwpi7KjlfzvqnkZnqLnAl6Uw== X-Google-Smtp-Source: ACcGV62Vc33AnYOkOhiJUPeSo9TLudulRpEn5c4v/8a12isUmHzPeSRWIxW4fTt8MuGAu2xwlGkKtA== X-Received: by 2002:a17:902:9896:: with SMTP id s22-v6mr14891091plp.113.1539569631208; Sun, 14 Oct 2018 19:13:51 -0700 (PDT) Received: from localhost ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id q2-v6sm12402174pfc.17.2018.10.14.19.13.49 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 14 Oct 2018 19:13:50 -0700 (PDT) Date: Sun, 14 Oct 2018 19:13:49 -0700 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers , Steven Rostedt Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption Message-ID: <20181015021349.GB217384@joelaf.mtv.corp.google.com> References: <20181014212955.95267-1-joel@joelfernandes.org> <20181014231731.GN2674@linux.ibm.com> <20181015020827.GA217384@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181015020827.GA217384@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 14, 2018 at 07:08:27PM -0700, Joel Fernandes wrote: > On Sun, Oct 14, 2018 at 04:17:31PM -0700, Paul E. McKenney wrote: > > On Sun, Oct 14, 2018 at 02:29:55PM -0700, Joel Fernandes (Google) wrote: > > > The Requirements.html document says "Disabling Preemption Does Not Block > > > Grace Periods". However this is no longer true with the RCU > > > consolidation. Lets remove the obsolete (non-)requirement entirely. > > > > > > Signed-off-by: Joel Fernandes (Google) > > > > Good catch, queued, thank you! > > Thanks! By the way after I sent the patch, I also tried Oleg's experiment to > confirm that this is indeed obsolete. :) > > One thing interesting came up when I tried synchronize_rcu_expedited() > instead of synchronize_rcu() in Oleg's experiment, I still saw a multiple > millisecond delay between when the rcu read section completely and the > synchronize_rcu_expedited returns: > > For example, with synchronize_rcu_expedited, the 'SPIN done' and the 'SYNC > done' are about 3 millisecond apart: > [ 77.599142] SPIN start > [ 77.601595] SYNC start > [ 82.604950] SPIN done! > [ 82.607836] SYNC done! > I saw anywhere from 2-6 milliseconds. > > The reason I bring this up is according to Requirements.html: In some cases, > the multi-millisecond synchronize_rcu() latencies are unacceptable. In these > cases, synchronize_rcu_expedited() may be used instead,.. so either I messed > something up in the experiment, or I need to update this part of the document ;-) So I realized I'm running in Qemu so it could also be a scheduling delay of the vcpu thread. So apologies about the noise if the experiment works fine for you. thanks, - Joel