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 EC909C43441 for ; Mon, 12 Nov 2018 18:18:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B011D22419 for ; Mon, 12 Nov 2018 18:18:05 +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="I5MGIvwA" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B011D22419 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 S1730283AbeKMEM0 (ORCPT ); Mon, 12 Nov 2018 23:12:26 -0500 Received: from merlin.infradead.org ([205.233.59.134]:47802 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727247AbeKMEMZ (ORCPT ); Mon, 12 Nov 2018 23:12:25 -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=9IRJWI2EKydd/p22jhXbWXXqMesqLJXGcHCf+VedH7g=; b=I5MGIvwA7BKDZ4QM21bms6J6n dGltIaB2vXqseAe5ncfz5sTDzEL5rhPRRRIdIohVbPpJyOb6iEBFr5tNqxVt8EPiL/vKK6iNrBIqU aGGkLqXKlAiKn+kjiLzRC88/1eBjvKSKZ9vX3xfnBCJ8nYwXWz65c2XJn4IBhw6s5WP7drYImJY6w LukyqLDgQ0hSFiL5tvNYFRjWYvbTiynbY+U9ggNsPk+UURq3KlwKaXHCO+6lcCeK1pY3VQ5O7g8UL fy8TpOXd2Xyki+DhH1Taa+3NWmpil65JHrSBZ4ZcHmwv07gQ2E9Ue6NKNn4ZdnhJpUah395YjMSEp 65L6gCi3Q==; Received: from [64.114.255.114] (helo=worktop) by merlin.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gMGmN-0003JO-1y; Mon, 12 Nov 2018 18:17:43 +0000 Received: by worktop (Postfix, from userid 1000) id 1F0256E086C; Mon, 12 Nov 2018 19:17:41 +0100 (CET) Date: Mon, 12 Nov 2018 19:17:41 +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: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181112132852.GH4170@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 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. So when someone bisects a problem to this commit; and he reads the Changelog, he might get the impression that was unexpected. > Not that it matters, given that I know of no way to change a mainlined > commit log. I suppose I could ask Jon if he would be willing to take > a 2018 RCU API LWN article, if that would help. Yes, it is water under the bridge; but Changelogs should be explicit about behavioural changes. And while the merged RCU has the semantic behaviour required, the timing behaviour did change significantly. > > > > 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. > > ??? Did you mean to say "necessary doesn't imply sufficient"? If so, > what else do you feel is missing? No, I meant to say that your original Changelog only states that sync_rcu now covers rcu-sched behaviour. Which means that the change is sufficient. It completely and utterly fails to explain _why_ you're doing the change. Ie. you do not address why it is necessary. A Changelog should always explain why the change is needed. In this case because you want to get rid of the sync_sched() api.