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=-16.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 4E142C4320A for ; Mon, 30 Aug 2021 18:46:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 331FB60F5B for ; Mon, 30 Aug 2021 18:46:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232710AbhH3SrU (ORCPT ); Mon, 30 Aug 2021 14:47:20 -0400 Received: from mail.kernel.org ([198.145.29.99]:54754 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232695AbhH3SrE (ORCPT ); Mon, 30 Aug 2021 14:47:04 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 71A9960E98; Mon, 30 Aug 2021 18:46:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1630349170; bh=J23JT3g5G0Rq/agAJPJs93+AxpcQHeDG5rGcBdcVQmw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=AESHqYHp5trjKc05XVKr0DmGCOB4OApA5VeZrTR7vguCf3WEoazddObwXaYtaXoaP P3df2vVNJdsdWCKI4NUR0E+HcCxM134uwomPCnYGrTM5cu77AAh057E/ax5p+W4i3W 1q5NbXta4J1mi3ngNvH7LcZwkMQI4A5ZXwk1T+0uK0Kr3dlzWyXKR23RizWn00kfxj MxTFvuRAZAOWe1b2Ix4Yd0e6egDQAMASER45suA2o9yKdCZ0Atls4b0n1kgIZu57el IqaDtYMRYpHuBQ2XubFzClv5ll0eaUYmpbp4myR6n0PIL8+puu/XH1CNPLswpAkSjr fndMhomjaiF0A== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 3CFAF5C0566; Mon, 30 Aug 2021 11:46:10 -0700 (PDT) Date: Mon, 30 Aug 2021 11:46:10 -0700 From: "Paul E. McKenney" To: Andrii Nakryiko Cc: Waiman Long , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org, open list , Alexei Starovoitov , Andrii Nakryiko Subject: Re: [PATCH] rcu: Avoid unneeded function call in rcu_read_unlock() Message-ID: <20210830184610.GX4156@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210827022122.15816-1-longman@redhat.com> <20210827183455.GP4156@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 30, 2021 at 11:36:51AM -0700, Andrii Nakryiko wrote: > On Fri, Aug 27, 2021 at 11:34 AM Paul E. McKenney wrote: > > > > On Thu, Aug 26, 2021 at 10:21:22PM -0400, Waiman Long wrote: > > > Since commit aa40c138cc8f3 ("rcu: Report QS for outermost > > > PREEMPT=n rcu_read_unlock() for strict GPs"). A real function call > > > rcu_read_unlock_strict() is added to the inlined rcu_read_unlock(). > > > The rcu_read_unlock_strict() call is only needed if the performance > > > sagging CONFIG_RCU_STRICT_GRACE_PERIOD option is set. This config > > > option isn't set for most production kernels while the function call > > > overhead remains. > > > > > > To provide a slight performance improvement, the > > > CONFIG_RCU_STRICT_GRACE_PERIOD config check is moved from > > > rcu_read_unlock_strict() to __rcu_read_unlock() so that the function > > > call can be compiled out in most cases. > > > > > > Besides, the GPL exported rcu_read_unlock_strict() also impact the > > > the compilation of non-GPL kernel modules as rcu_read_unlock() is a > > > frequently used kernel API. > > > > > > Signed-off-by: Waiman Long > > > > Nice, and good eyes!!! > > > > I have queued this for v5.16, that is, not the upcoming merge window > > but the one after that. > > > > I did my usual wordsmithing, so please check the following in case I > > messed something up. I intentionally omitted the EXPORT_SYMBOL_GPL() > > discussion because: > > > > 1. Kernels built with CONFIG_PREEMPT=y have the same issue > > with the __rcu_read_lock() and __rcu_read_unlock() functions. > > > > 2. Many other RCU functions are EXPORT_SYMBOL_GPL() and have > > been for almost two decades. > > > > But if someone does use RCU readers within CONFIG_PREEMPT=n kernels from > > a binary module, I will happily refer them to you for any RCU issues > > that they encounter. ;-) > > > > I am also CCing the BPF guys in case my interpretation of the code in > > the BPF verifier is incorrect. > > > > LGTM from the BPF side, nothing really changed about when > rcu_read_unlock_strict is an actual function vs no-op macro. It's also > important to minimize the number of function calls in the context of > recent LBR on-demand work done by Song, so this is a great > improvement! Thank you for looking this over! May I add your Acked-by or similar? Thanx, Paul > > ------------------------------------------------------------------------ > > > > commit 4a9f53b997b809c0256838e31c604aeeded2345a > > Author: Waiman Long > > Date: Thu Aug 26 22:21:22 2021 -0400 > > > > rcu: Avoid unneeded function call in rcu_read_unlock() > > > > Since commit aa40c138cc8f3 ("rcu: Report QS for outermost PREEMPT=n > > rcu_read_unlock() for strict GPs") the function rcu_read_unlock_strict() > > is invoked by the inlined rcu_read_unlock() function. However, > > rcu_read_unlock_strict() is an empty function in production kernels, > > which are built with CONFIG_RCU_STRICT_GRACE_PERIOD=n. > > > > There is a mention of rcu_read_unlock_strict() in the BPF verifier, > > but this is in a deny-list, meaning that BPF does not care whether > > rcu_read_unlock_strict() is ever called. > > > > This commit therefore provides a slight performance improvement > > by hoisting the check of CONFIG_RCU_STRICT_GRACE_PERIOD from > > rcu_read_unlock_strict() into rcu_read_unlock(), thus avoiding the > > pointless call to an empty function. > > > > Cc: Alexei Starovoitov > > Cc: Andrii Nakryiko > > Signed-off-by: Waiman Long > > Signed-off-by: Paul E. McKenney > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 434d12fe2d4f..5e0beb5c5659 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -71,7 +71,8 @@ static inline void __rcu_read_lock(void) > > static inline void __rcu_read_unlock(void) > > { > > preempt_enable(); > > - rcu_read_unlock_strict(); > > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) > > + rcu_read_unlock_strict(); > > } > > > > static inline int rcu_preempt_depth(void) > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 7a4876a3a882..0b55c647ab80 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -814,8 +814,7 @@ void rcu_read_unlock_strict(void) > > { > > struct rcu_data *rdp; > > > > - if (!IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) || > > - irqs_disabled() || preempt_count() || !rcu_state.gp_kthread) > > + if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread) > > return; > > rdp = this_cpu_ptr(&rcu_data); > > rcu_report_qs_rdp(rdp);