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=-8.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,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 52DE0C070C3 for ; Mon, 15 Oct 2018 02:48:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0A5B120652 for ; Mon, 15 Oct 2018 02:48:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="Pmoy8HpV" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0A5B120652 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 S1726524AbeJOKbO (ORCPT ); Mon, 15 Oct 2018 06:31:14 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:39400 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726319AbeJOKbO (ORCPT ); Mon, 15 Oct 2018 06:31:14 -0400 Received: by mail-pg1-f194.google.com with SMTP id r9-v6so8429042pgv.6 for ; Sun, 14 Oct 2018 19:48:01 -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=CfP9bckGZAz9VDsWFkEm+7rjWCsijRJ0HBC+jzrBiDI=; b=Pmoy8HpVGV3aorg0pZ8Xg9o7wELAyl7Ts5KdYmWq8olmgzEIdQSh6R2RSjUWssWwQ9 rZ+Fi3aSsDODiD1/XrLkdGNLknZKyhHXYBgJcCCALMi+u4Uyx0sqKCJ4BsLiJobYM+PW nLUiC+x6AZxrql4e/eXgp2z2shTxmgPCHUBSc= 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=CfP9bckGZAz9VDsWFkEm+7rjWCsijRJ0HBC+jzrBiDI=; b=Lrj5+ngXar0+NesetiO+9kVm6gLi7jg73oeLVKGu+2GpLGq25z3W/4Zyqa0s88KECm kHFmJ4slHG96Js+Uyz+Z4Sosl4S7+Oz29TTVaD+TjVCxHsWZ0fssTBjZAkso7sk++G86 ldgMG6R2351XD8Uv5b76TMu3YcUE4jU8Up4qdXt59rzdAJ4y2jmrrBclv9qFcWYkRxjJ E20wL7hxcbjZAVihRFD8MaXfnBQe1VqzVztI4sYlHdzUnk9FJgbE3PbUS0i5ttAJZlxM wyal7QBnhVSRR1+wYxS054WH070mO4qPLlbH/wJwEtHDUj2pv1PGz1etilvS4ai+E5+A Fdwg== X-Gm-Message-State: ABuFfojuQkk10c05FyT8UHQplIJStJC/eX5IhhFrVZMaODK4d2LX6Eoa 9VrucI3zQ/ee/eietyQMPexx4A== X-Google-Smtp-Source: ACcGV62qI5HWbNu6YiaJ63JAr/FAQXuq2ZRMVTqQoMfB58rzSr/QopzvJh1vl6d6yOoOsbL5C0644w== X-Received: by 2002:a62:384c:: with SMTP id f73-v6mr15756296pfa.242.1539571680364; Sun, 14 Oct 2018 19:48:00 -0700 (PDT) Received: from localhost ([2620:0:1000:1601:3aef:314f:b9ea:889f]) by smtp.gmail.com with ESMTPSA id 6-v6sm10562838pgl.6.2018.10.14.19.47.59 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 14 Oct 2018 19:47:59 -0700 (PDT) Date: Sun, 14 Oct 2018 19:47:58 -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: <20181015024758.GA227989@joelaf.mtv.corp.google.com> References: <20181014212955.95267-1-joel@joelfernandes.org> <20181014231731.GN2674@linux.ibm.com> <20181015020827.GA217384@joelaf.mtv.corp.google.com> <20181015021349.GB217384@joelaf.mtv.corp.google.com> <20181015023328.GP2674@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181015023328.GP2674@linux.ibm.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 07:33:28PM -0700, Paul E. McKenney wrote: > On Sun, Oct 14, 2018 at 07:13:49PM -0700, Joel Fernandes wrote: > > 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 ;-) > > In normal testing, 2-6 milliseconds is indeed excessive. Could you please > point me at Oleg's experiment? Also, what CONFIG_PREEMPT setting were > you using? (My guess is CONFIG_PREEMPT=y.) The CONFIG_PREEMPT config I am using is CONFIG_PREEMPT=y. > > 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. > > I used rcuperf, which might not be doing the same thing as Oleg's > experiment. The experiment is mentioned at: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg912055.html If you apply the below diff, it applies cleanly on rcu/dev. And then run: taskset 2 perl -e 'syscall 157, 666, 5000' & taskset 1 perl -e 'syscall 157, 777' diff --git a/kernel/sys.c b/kernel/sys.c index cf5c67533ff1..b654b7566ca3 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2261,6 +2261,9 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which, return -EINVAL; } +#include + + SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, unsigned long, arg4, unsigned long, arg5) { @@ -2274,6 +2277,19 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, error = 0; switch (option) { + case 666: + preempt_disable(); + pr_crit("SPIN start\n"); + while (arg2--) + mdelay(1); + pr_crit("SPIN done!\n"); + preempt_enable(); + break; + case 777: + pr_crit("SYNC start\n"); + synchronize_rcu(); + pr_crit("SYNC done!\n"); + break; case PR_SET_PDEATHSIG: if (!valid_signal(arg2)) { error = -EINVAL;