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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED 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 1D6D7C28CF8 for ; Mon, 15 Oct 2018 06:05:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C5E0E2086A for ; Mon, 15 Oct 2018 06:05:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C5E0E2086A Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com 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 S1726566AbeJONtO (ORCPT ); Mon, 15 Oct 2018 09:49:14 -0400 Received: from mx2.suse.de ([195.135.220.15]:38090 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726381AbeJONtO (ORCPT ); Mon, 15 Oct 2018 09:49:14 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 15914AD6B; Mon, 15 Oct 2018 06:05:25 +0000 (UTC) Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption To: Joel Fernandes , "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers , Steven Rostedt 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> <20181015024758.GA227989@joelaf.mtv.corp.google.com> From: Nikolay Borisov Openpgp: preference=signencrypt Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= xsFNBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABzSJOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuZGU+wsF4BBMBAgAiBQJYijkSAhsDBgsJCAcDAgYVCAIJ CgsEFgIDAQIeAQIXgAAKCRBxvoJG5T8oV/B6D/9a8EcRPdHg8uLEPywuJR8URwXzkofT5bZE IfGF0Z+Lt2ADe+nLOXrwKsamhweUFAvwEUxxnndovRLPOpWerTOAl47lxad08080jXnGfYFS Dc+ew7C3SFI4tFFHln8Y22Q9075saZ2yQS1ywJy+TFPADIprAZXnPbbbNbGtJLoq0LTiESnD w/SUC6sfikYwGRS94Dc9qO4nWyEvBK3Ql8NkoY0Sjky3B0vL572Gq0ytILDDGYuZVo4alUs8 LeXS5ukoZIw1QYXVstDJQnYjFxYgoQ5uGVi4t7FsFM/6ykYDzbIPNOx49Rbh9W4uKsLVhTzG BDTzdvX4ARl9La2kCQIjjWRg+XGuBM5rxT/NaTS78PXjhqWNYlGc5OhO0l8e5DIS2tXwYMDY LuHYNkkpMFksBslldvNttSNei7xr5VwjVqW4vASk2Aak5AleXZS+xIq2FADPS/XSgIaepyTV tkfnyreep1pk09cjfXY4A7qpEFwazCRZg9LLvYVc2M2eFQHDMtXsH59nOMstXx2OtNMcx5p8 0a5FHXE/HoXz3p9bD0uIUq6p04VYOHsMasHqHPbsMAq9V2OCytJQPWwe46bBjYZCOwG0+x58 fBFreP/NiJNeTQPOa6FoxLOLXMuVtpbcXIqKQDoEte9aMpoj9L24f60G4q+pL/54ql2VRscK d87BTQRYigc+ARAAyJSq9EFk28++SLfg791xOh28tLI6Yr8wwEOvM3wKeTfTZd+caVb9gBBy wxYhIopKlK1zq2YP7ZjTP1aPJGoWvcQZ8fVFdK/1nW+Z8/NTjaOx1mfrrtTGtFxVBdSCgqBB jHTnlDYV1R5plJqK+ggEP1a0mr/rpQ9dFGvgf/5jkVpRnH6BY0aYFPprRL8ZCcdv2DeeicOO YMobD5g7g/poQzHLLeT0+y1qiLIFefNABLN06Lf0GBZC5l8hCM3Rpb4ObyQ4B9PmL/KTn2FV Xq/c0scGMdXD2QeWLePC+yLMhf1fZby1vVJ59pXGq+o7XXfYA7xX0JsTUNxVPx/MgK8aLjYW hX+TRA4bCr4uYt/S3ThDRywSX6Hr1lyp4FJBwgyb8iv42it8KvoeOsHqVbuCIGRCXqGGiaeX Wa0M/oxN1vJjMSIEVzBAPi16tztL/wQtFHJtZAdCnuzFAz8ue6GzvsyBj97pzkBVacwp3/Mw qbiu7sDz7yB0d7J2tFBJYNpVt/Lce6nQhrvon0VqiWeMHxgtQ4k92Eja9u80JDaKnHDdjdwq FUikZirB28UiLPQV6PvCckgIiukmz/5ctAfKpyYRGfez+JbAGl6iCvHYt/wAZ7Oqe/3Cirs5 KhaXBcMmJR1qo8QH8eYZ+qhFE3bSPH446+5oEw8A9v5oonKV7zMAEQEAAcLBXwQYAQIACQUC WIoHPgIbDAAKCRBxvoJG5T8oV1pyD/4zdXdOL0lhkSIjJWGqz7Idvo0wjVHSSQCbOwZDWNTN JBTP0BUxHpPu/Z8gRNNP9/k6i63T4eL1xjy4umTwJaej1X15H8Hsh+zakADyWHadbjcUXCkg OJK4NsfqhMuaIYIHbToi9K5pAKnV953xTrK6oYVyd/Rmkmb+wgsbYQJ0Ur1Ficwhp6qU1CaJ mJwFjaWaVgUERoxcejL4ruds66LM9Z1Qqgoer62ZneID6ovmzpCWbi2sfbz98+kW46aA/w8r 7sulgs1KXWhBSv5aWqKU8C4twKjlV2XsztUUsyrjHFj91j31pnHRklBgXHTD/pSRsN0UvM26 lPs0g3ryVlG5wiZ9+JbI3sKMfbdfdOeLxtL25ujs443rw1s/PVghphoeadVAKMPINeRCgoJH zZV/2Z/myWPRWWl/79amy/9MfxffZqO9rfugRBORY0ywPHLDdo9Kmzoxoxp9w3uTrTLZaT9M KIuxEcV8wcVjr+Wr9zRl06waOCkgrQbTPp631hToxo+4rA1jiQF2M80HAet65ytBVR2pFGZF zGYYLqiG+mpUZ+FPjxk9kpkRYz61mTLSY7tuFljExfJWMGfgSg1OxfLV631jV1TcdUnx+h3l Sqs2vMhAVt14zT8mpIuu2VNxcontxgVr1kzYA/tQg32fVRbGr449j1gw57BV9i0vww== Message-ID: <5151da01-343b-bb37-353e-b6652ae530f5@suse.com> Date: Mon, 15 Oct 2018 09:05:22 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181015024758.GA227989@joelaf.mtv.corp.google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15.10.2018 05:47, Joel Fernandes wrote: > 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"); But you are using the console printing infrastructure which is rather heavyweight. Try replacing pr_* calls with trace_printk so that you write to the lock-free ring buffer, this will reduce the noise from the heavy console printing infrastructure. > + break; > case PR_SET_PDEATHSIG: > if (!valid_signal(arg2)) { > error = -EINVAL; >