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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,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 50A65C43441 for ; Mon, 12 Nov 2018 00:53:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 01A012084C for ; Mon, 12 Nov 2018 00:53:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="lHHgoU4R" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 01A012084C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.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 S1729896AbeKLKo0 (ORCPT ); Mon, 12 Nov 2018 05:44:26 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:51048 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729500AbeKLKo0 (ORCPT ); Mon, 12 Nov 2018 05:44:26 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=LY+A/LXYXTbuY2MwEavME25By2/pinQHuA207V/eWcU=; b=lHHgoU4RKGcsD3NSW8hktD7pK f0C768++KOn+P4pnbblYhkxeDKtBg78yd032uICoy5/RIfmyA2qMQYUMBt2B9trGXo4a8WC7/1HXq cO+vlIM2jd5SYawDey79CxZWdaLq3xCwGAdwt1ei7/twLQdCYiPjAv33oqU3FpuT4nQRApJxlACex hITK1Dgqe10h9xBXWFu+Ww4bUc9cEi3IplwaoeovvGWCVHtNaNE/SG9KMHczOKatKjpelzWJ6yzvU Ub73Y0I1Br0+eCMwZJK4Mo0XQXLNZeiEPNqpno0yK15PzgYEpsSL8ohszumU7Xd5wzbSavay4ZBQQ I8b6+OI1g==; Received: from [64.114.255.97] (helo=worktop) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gM0Tq-00027z-7y; Mon, 12 Nov 2018 00:53:32 +0000 Received: by worktop (Postfix, from userid 1000) id 1A4D26E061A; Mon, 12 Nov 2018 01:53:29 +0100 (CET) Date: Mon, 12 Nov 2018 01:53:29 +0100 From: Peter Zijlstra To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Ingo Molnar Subject: Re: [PATCH tip/core/rcu 23/41] sched: Replace synchronize_sched() with synchronize_rcu() Message-ID: <20181112005329.GG3056@worktop> References: <20181111194104.GA4787@linux.ibm.com> <20181111194410.6368-23-paulmck@linux.ibm.com> <20181112001233.GC3056@worktop> <20181112004528.GA4170@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112004528.GA4170@linux.ibm.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 11, 2018 at 04:45:28PM -0800, Paul E. McKenney wrote: > On Mon, Nov 12, 2018 at 01:12:33AM +0100, Peter Zijlstra wrote: > > On Sun, Nov 11, 2018 at 11:43:52AM -0800, Paul E. McKenney wrote: > > > Now that synchronize_rcu() waits for preempt-disable regions of code > > > as well as RCU read-side critical sections, synchronize_sched() can be > > > replaced by synchronize_rcu(). This commit therefore makes this change. > > > > Yes, but it also waits for an actual RCU quiestent state, which makes > > synchoinize_rcu() potentially much more expensive than an actual > > synchronize_sched(). > > None of the readers have changed. > > For the updaters, if CONFIG_PREEMPT=n, synchronize_rcu() and > synchronize_sched() always were one and the same. When CONFIG_PREEMPT=y, > synchronize_rcu() and synchronize_sched() are now one and the same. The Changelog does not state this; and does the commit that makes that happen state the regression potential? > > So why are we doing this? > > Given that synchronize_rcu() and synchronize_sched() are now always one > and the same, this is a distinction without a difference. The Changelog did not state a reason for the patch. Therefore it is a bad patch.