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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 41661ECDE5F for ; Mon, 23 Jul 2018 20:10:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D58D320874 for ; Mon, 23 Jul 2018 20:10:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D58D320874 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 S2388141AbeGWVNe (ORCPT ); Mon, 23 Jul 2018 17:13:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:56640 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728087AbeGWVNe (ORCPT ); Mon, 23 Jul 2018 17:13:34 -0400 Received: from gandalf.local.home (cpe-66-24-56-78.stny.res.rr.com [66.24.56.78]) (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 BB1C020854; Mon, 23 Jul 2018 20:10:43 +0000 (UTC) Date: Mon, 23 Jul 2018 16:10:41 -0400 From: Steven Rostedt To: Lai Jiangshan Cc: "Paul E. McKenney" , Josh Triplett , Mathieu Desnoyers , LKML , Ingo Molnar , Linus Torvalds , Peter Zijlstra , oleg@redhat.com, Eric Dumazet , davem@davemloft.net, Thomas Gleixner Subject: Re: Consolidating RCU-bh, RCU-preempt, and RCU-sched Message-ID: <20180723161041.5375b54f@gandalf.local.home> In-Reply-To: References: <20180713000249.GA16907@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.16.0 (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 Sorry for the late reply, just came back from the Caribbean :-) :-) :-) On Fri, 13 Jul 2018 11:47:18 +0800 Lai Jiangshan wrote: > On Fri, Jul 13, 2018 at 8:02 AM, Paul E. McKenney > wrote: > > Hello! > > > > I now have a semi-reasonable prototype of changes consolidating the > > RCU-bh, RCU-preempt, and RCU-sched update-side APIs in my -rcu tree. > > There are likely still bugs to be fixed and probably other issues as well, > > but a prototype does exist. What's the rational for all this churn? Linus's complaining that there are too many RCU variants? > > > > Assuming continued good rcutorture results and no objections, I am > > thinking in terms of this timeline: > > > > o Preparatory work and cleanups are slated for the v4.19 merge window. > > > > o The actual consolidation and post-consolidation cleanup is slated > > for the merge window after v4.19 (v5.0?). These cleanups include > > the replacements called out below within the RCU implementation > > itself (but excluding kernel/rcu/sync.c, see question below). > > > > o Replacement of now-obsolete update APIs is slated for the second > > merge window after v4.19 (v5.1?). The replacements are currently > > expected to be as follows: > > > > synchronize_rcu_bh() -> synchronize_rcu() > > synchronize_rcu_bh_expedited() -> synchronize_rcu_expedited() > > call_rcu_bh() -> call_rcu() > > rcu_barrier_bh() -> rcu_barrier() > > synchronize_sched() -> synchronize_rcu() > > synchronize_sched_expedited() -> synchronize_rcu_expedited() > > call_rcu_sched() -> call_rcu() > > rcu_barrier_sched() -> rcu_barrier() > > get_state_synchronize_sched() -> get_state_synchronize_rcu() > > cond_synchronize_sched() -> cond_synchronize_rcu() > > synchronize_rcu_mult() -> synchronize_rcu() > > > > I have done light testing of these replacements with good results. > > > > Any objections to this timeline? > > > > I also have some questions on the ultimate end point. I have default > > choices, which I will likely take if there is no discussion. > > > > o > > Currently, I am thinking in terms of keeping the per-flavor > > read-side functions. For example, rcu_read_lock_bh() would > > continue to disable softirq, and would also continue to tell > > lockdep about the RCU-bh read-side critical section. However, > > synchronize_rcu() will wait for all flavors of read-side critical > > sections, including those introduced by (say) preempt_disable(), > > so there will no longer be any possibility of mismatching (say) > > RCU-bh readers with RCU-sched updaters. > > > > I could imagine other ways of handling this, including: > > > > a. Eliminate rcu_read_lock_bh() in favor of > > local_bh_disable() and so on. Rely on lockdep > > instrumentation of these other functions to identify RCU > > readers, introducing such instrumentation as needed. I am > > not a fan of this approach because of the large number of > > places in the Linux kernel where interrupts, preemption, > > and softirqs are enabled or disabled "behind the scenes". > > > > b. Eliminate rcu_read_lock_bh() in favor of rcu_read_lock(), > > and required callers to also disable softirqs, preemption, > > or whatever as needed. I am not a fan of this approach > > because it seems a lot less convenient to users of RCU-bh > > and RCU-sched. > > > > At the moment, I therefore favor keeping the RCU-bh and RCU-sched > > read-side APIs. But are there better approaches? > > Hello, Paul > > Since local_bh_disable() will be guaranteed to be protected by RCU > and more general. I'm afraid it will be preferred over > rcu_read_lock_bh() which will be gradually being phased out. > > In other words, keeping the RCU-bh read-side APIs will be a slower > version of the option A. So will the same approach for the RCU-sched. > But it'll still be better than the hurrying option A, IMHO. Now when all this gets done, is synchronize_rcu() going to just wait for everything to pass? (scheduling, RCU readers, softirqs, etc) Is there any worry about lengthening the time of synchronize_rcu? -- Steve > > > > o How should kernel/rcu/sync.c be handled? Here are some > > possibilities: > > > > a. Leave the full gp_ops[] array and simply translate > > the obsolete update-side functions to their RCU > > equivalents. > > > > b. Leave the current gp_ops[] array, but only have > > the RCU_SYNC entry. The __INIT_HELD field would > > be set to a function that was OK with being in an > > RCU read-side critical section, an interrupt-disabled > > section, etc. > > > > This allows for possible addition of SRCU functionality. > > It is also a trivial change. Note that the sole user > > of sync.c uses RCU_SCHED_SYNC, and this would need to > > be changed to RCU_SYNC. > > > > But is it likely that we will ever add SRCU? > > > > c. Eliminate that gp_ops[] array, hard-coding the function > > pointers into their call sites. > > > > I don't really have a preference. Left to myself, I will be lazy > > and take option #a. Are there better approaches? > > > > o Currently, if a lock related to the scheduler's rq or pi locks is > > held across rcu_read_unlock(), that lock must be held across the > > entire read-side critical section in order to avoid deadlock. > > Now that the end of the RCU read-side critical section is > > deferred until sometime after interrupts are re-enabled, this > > requirement could be lifted. However, because the end of the RCU > > read-side critical section is detected sometime after interrupts > > are re-enabled, this means that a low-priority RCU reader might > > remain priority-boosted longer than need be, which could be a > > problem when running real-time workloads. > > > > My current thought is therefore to leave this constraint in > > place. Thoughts? > > > > Anything else that I should be worried about? ;-) > > > > Thanx, Paul > >