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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9DDA8ECAAD4 for ; Tue, 30 Aug 2022 11:44:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231284AbiH3Lo2 (ORCPT ); Tue, 30 Aug 2022 07:44:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55662 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230522AbiH3Ln6 (ORCPT ); Tue, 30 Aug 2022 07:43:58 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4E64E7CAA2; Tue, 30 Aug 2022 04:43:45 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id DD8C3615AF; Tue, 30 Aug 2022 11:43:44 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 35207C433D6; Tue, 30 Aug 2022 11:43:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1661859824; bh=YNxOuIHy730JOT7NeJlPe+7ba0P+Vld16heKzFiPux4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=XArcn/Q3thp7S3RHjlIGT/Hvk7YPxQJ44sZrYKJHq/y2KY1aVbnWrm5IEa1QNbVsj 1cGQTYowuQLF1ryZAqzJEqwetz83myBbMCTcyz652XS2W2yNhmp1scx041WB+0zXOs EtqAz0ylLEBgd2hxefXeQ3Lx9ZfBQJBCGr4qWOG6/RHuyNgg4Bn1l3tBv2466CmmM3 bRPFFUlmVt2gDk82Wd7Zw0Rl34R0LcRXoRauwCOuSfLeOkmqvbG8bldhPBMM7PDVto 8l5z9wg7oTHK15NroENuxKR/pIhmvyzNB8X82IdleRxgoheSjd90ewjg11ogM7kypi 8uKSNxjrZvXWA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id C23925C0531; Tue, 30 Aug 2022 04:43:43 -0700 (PDT) Date: Tue, 30 Aug 2022 04:43:43 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Joel Fernandes , Dietmar Eggemann , LKML , Rushikesh S Kadam , "Uladzislau Rezki (Sony)" , Neeraj upadhyay , Steven Rostedt , rcu , Vineeth Pillai Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes Message-ID: <20220830114343.GS6159@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220819204857.3066329-1-joel@joelfernandes.org> <20220829134045.GA54589@lothringen> <1f7dd31b-f4d0-5c1c-ce28-c27f75c17f05@joelfernandes.org> <20220829194622.GA58291@lothringen> <20220829204202.GQ6159@paulmck-ThinkPad-P17-Gen-1> <20220830105324.GA71266@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220830105324.GA71266@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 30, 2022 at 12:53:24PM +0200, Frederic Weisbecker wrote: > On Mon, Aug 29, 2022 at 01:42:02PM -0700, Paul E. McKenney wrote: > > On Mon, Aug 29, 2022 at 04:36:40PM -0400, Joel Fernandes wrote: > > > On Mon, Aug 29, 2022 at 3:46 PM Frederic Weisbecker wrote: > > > > On Mon, Aug 29, 2022 at 12:45:40PM -0400, Joel Fernandes wrote: > > > > > On 8/29/2022 9:40 AM, Frederic Weisbecker wrote: > > > > [ . . . ] > > > > > > > > 2) NOCB implies performance issues. > > > > > > > > > > Which kinds of? There is slightly worse boot times, but I'm guessing that's do > > > > > with the extra scheduling overhead of the extra threads which is usually not a > > > > > problem except that RCU is used in the critical path of boot up (on ChromeOS). > > > > > > > > I never measured it myself but executing callbacks on another CPUs, with > > > > context switches and locking can only involve significant performance issues if callbacks > > > > are frequent. So it's a tradeoff between power and performance. > > > > > > In my testing of benchmarks on real systems with 8-16 CPUs, the > > > performance hit is down in the noise. It is possible though that maybe > > > one can write a non-realistic synthetic test to force the performance > > > issues, but I've not seen it in the real world. Maybe on > > > networking-heavy servers with lots of cores, you'll see it but their > > > batteries if any would be pretty big :-). > > > > To Frederic's point, if you have enough servers, even a 1% decrease in > > power consumption is a very big deal. ;-) > > The world has enough servers, for that matters ;-) True enough! Now you need only demonstrate that call_rcu_lazy() for !rcu_nocbs servers would actually deliver that 1%. ;-) Thanx, Paul