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 B2D8EC43441 for ; Mon, 12 Nov 2018 02:07:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7606C208A3 for ; Mon, 12 Nov 2018 02:07:23 +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="Ul+rtrIt" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7606C208A3 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 S1730202AbeKLL6O (ORCPT ); Mon, 12 Nov 2018 06:58:14 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:37140 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729952AbeKLL6O (ORCPT ); Mon, 12 Nov 2018 06:58:14 -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=n4x0JQIDgYsZMqYxMLcRzz/KLTwieUnDwyzO35zu4m0=; b=Ul+rtrIt1YZj50KE0KoJp9ckU e3G1zLDpgEJJKbU66cTerzgx7s2I0+QjKrRxSGuS8b4atgVIDJexID56WTactXVcFUCewsktcykOn WsF5olZPjUoXKc0BfpmhHtGnlAnRSMqncjZLMmSWUZVELEpplADB40fUamzvEatc9rkb2723WTc6j z26j2GsyOkgvykyG9l+BDpzZEV/IFn26aoF/DI6vqCkZxKGvE90d0CWA4/AovSbK4iRPp1EEP2H7e Zna6MKvsjjO4cHlp/baNJRtZHdy7U1dw5G1O25CzjYjBvcq99Lv/KTvtW7FS4u4lV3YxkBu2yg37m OovhtVgfg==; Received: from [64.114.255.114] (helo=worktop) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gM1d8-0000jF-VY; Mon, 12 Nov 2018 02:07:11 +0000 Received: by worktop (Postfix, from userid 1000) id 542EA6E0795; Mon, 12 Nov 2018 03:07:10 +0100 (CET) Date: Mon, 12 Nov 2018 03:07:10 +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: <20181112020710.GJ3056@worktop> References: <20181111194104.GA4787@linux.ibm.com> <20181111194410.6368-23-paulmck@linux.ibm.com> <20181112001233.GC3056@worktop> <20181112004528.GA4170@linux.ibm.com> <20181112005329.GG3056@worktop> <20181112014736.GB4170@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112014736.GB4170@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 05:47:36PM -0800, Paul E. McKenney wrote: > On Mon, Nov 12, 2018 at 01:53:29AM +0100, Peter Zijlstra wrote: > > 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? > > The Changelog says this: > > 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. > > The "synchronize_rcu() waits for preempt-disable regions of code as > well as RCU read-side critical sections" seems pretty unambiguous to me. > Exactly what more are you wanting said there? The quoted bit only states that synchronize_rcu() is sufficient; it does not say it is equivalent and the patch is a nop. It also doesn't say that the purpose is to get rid of the synchronize_sched() function. > There were quite a few commits involved in making this happen. Perhaps > the most pertinent are these: > > 3e3100989869 ("rcu: Defer reporting RCU-preempt quiescent states when disabled") > 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds") The latter; it does not mention that this will possible make synchronize_sched() quite a bit more expensive on PREEMPT=y builds :/ > Normal grace periods are almost always quite long compared to typical > read-side critical sections, preempt-disable regions of code, and so on. > So in the common case this should be OK. Or are you instead worried > about synchronize_sched_expedited()? No, I still feel expedited should not exist at all ;-) But for PREEMPT=y synchronize_sched() can be quite a bit shorter than synchronize_rcu(), since we don't have to wait for preempted read side stuff. > > > > 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. > > ??? Here is the current definition of synchronize_sched() in mainline: > > static inline void synchronize_sched(void) > { > synchronize_rcu(); > } Again, the patch didn't say that. If the Changelog would've read something like: "Since synchronize_sched() is now equivalent to synchronize_rcu(), replace the synchronize_sched() usage such that we can eventually remove the interface." It would've been clear that the patch is a nop and what the purpose was.