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 CA5BDC43441 for ; Mon, 12 Nov 2018 09:01:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 90CAF223C6 for ; Mon, 12 Nov 2018 09:01:11 +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="Uwgjpo1E" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 90CAF223C6 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 S1729176AbeKLSxY (ORCPT ); Mon, 12 Nov 2018 13:53:24 -0500 Received: from merlin.infradead.org ([205.233.59.134]:39374 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728400AbeKLSxY (ORCPT ); Mon, 12 Nov 2018 13:53:24 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=VTRX66HXL1eoQPasuf74XvFsWBt0U52yRurmwc48kbA=; b=Uwgjpo1EfhAPUdN6QtBDiTxFd xxXUbFJAqSQFpF0eSSkWFrDvx88scxx9u+m8cN3AXAFktvHWeXrbk3FfNO77xvZoMNe+4RIyJO88X MbPkJW0Tn0Z82SA+FMXyiDTvSVk35quJGiock5C9QZfPFt3gPexh2gRU1iqjsdgD60e0aUVABkTSG NzljVMJ2iAmYjm9xaOutTXQTSHl2lL//O/cskCO0WmZQmxfmT18H33L9e09l7ygzHt1ieSTyxyG2z 1BV3Fj0T2UQcA4j4lB6S/flzqIcIp6s8QEnSpaI0cUQYMhXjkLU0rnURXkgjOhEmrU1f1PW1sq7v8 P8QQslfsA==; Received: from [64.114.255.114] (helo=worktop) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gM85R-0006rF-2K; Mon, 12 Nov 2018 09:00:49 +0000 Received: by worktop (Postfix, from userid 1000) id E7CD46E03B0; Mon, 12 Nov 2018 10:00:47 +0100 (CET) Date: Mon, 12 Nov 2018 10:00:47 +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: <20181112090047.GN3056@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> <20181112020710.GJ3056@worktop> <20181112022455.GD4170@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112022455.GD4170@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 06:24:55PM -0800, Paul E. McKenney wrote: > > > 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 :/ > > In theory, sure. In practice, people have switched any number of > things from RCU-sched to RCU and back without problems. Still, better safe than sorry. It was a rather big change in behaviour, so it wouldn't have been strange to call that out. > > 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. > > Again, there are quite a few places that have managed that transition > without issue. Why do you expect this change to have problems that have > not been seen elsewhere? I'm not, I'm just taking issue with the Changelog. > > 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. > > I can easily make that change. Please, sufficient doesn't imply necessary etc.. A changelog should always clarify why we do the patch.