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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 4551FC43441 for ; Mon, 12 Nov 2018 20:20:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E414B22506 for ; Mon, 12 Nov 2018 20:20:43 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E414B22506 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=goodmis.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 S1729428AbeKMGPa (ORCPT ); Tue, 13 Nov 2018 01:15:30 -0500 Received: from mail.kernel.org ([198.145.29.99]:58694 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725834AbeKMGPa (ORCPT ); Tue, 13 Nov 2018 01:15:30 -0500 Received: from vmware.local.home (unknown [64.114.255.114]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 10CC7224E0; Mon, 12 Nov 2018 20:20:41 +0000 (UTC) Date: Mon, 12 Nov 2018 15:20:37 -0500 From: Steven Rostedt To: Peter Zijlstra Cc: "Paul E. McKenney" , 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, 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: <20181112152037.451d8b10@vmware.local.home> In-Reply-To: <20181112181741.GA3097@worktop.psav.com> 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> <20181112090047.GN3056@worktop> <20181112132852.GH4170@linux.ibm.com> <20181112181741.GA3097@worktop.psav.com> X-Mailer: Claws Mail 3.15.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 12 Nov 2018 19:17:41 +0100 Peter Zijlstra wrote: > On Mon, Nov 12, 2018 at 05:28:52AM -0800, Paul E. McKenney wrote: > > On Mon, Nov 12, 2018 at 10:00:47AM +0100, Peter Zijlstra wrote: > > > > Still, better safe than sorry. It was a rather big change in behaviour, > > > so it wouldn't have been strange to call that out. > > > > This guy: > > > > 45975c7d21a1 ("rcu: Define RCU-sched API in terms of RCU for Tree RCU PREEMPT builds") > > > > Has a commit log that says: > > > > Now that RCU-preempt knows about preemption disabling, its > > implementation of synchronize_rcu() works for synchronize_sched(), > > and likewise for the other RCU-sched update-side API members. > > This commit therefore confines the RCU-sched update-side code > > to CONFIG_PREEMPT=n builds, and defines RCU-sched's update-side > > API members in terms of those of RCU-preempt. > > > > That last phrase seems pretty explicit. What am I missing here? > > That does not explicitly state that because RCU-preempt > synchornize_rcu() can take _much_ longer, the new synchronize_sched() > can now take _much_ longer too. I'm curious. Are there measurements to see how much longer they can take? Saying "_much_ longer" would require that one has done the timings to see what the actual impact is. > > So when someone bisects a problem to this commit; and he reads the > Changelog, he might get the impression that was unexpected. It may well be unexpected. What is the timing differences between a normal synchronize_rcu and a synchronize_sched. Of course, 99% of users wont see any difference as 99% don't run CONFIG_PREEMPT. -- Steve