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 66285ECAAD3 for ; Thu, 1 Sep 2022 11:29:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234513AbiIAL35 (ORCPT ); Thu, 1 Sep 2022 07:29:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57030 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232877AbiIAL3y (ORCPT ); Thu, 1 Sep 2022 07:29:54 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8C535B48; Thu, 1 Sep 2022 04:29:52 -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 ams.source.kernel.org (Postfix) with ESMTPS id F07EEB824F5; Thu, 1 Sep 2022 11:29:50 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3B578C433D7; Thu, 1 Sep 2022 11:29:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1662031789; bh=DjYCxepA2d0Zh1cPmPtLxRdp/vMrxB28w9asViiX7Qs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=junecTn4Ih0IUcrqTCrMjncSBOcDJhRNTLuuo4o1gEmRQ7OElTwICJY2m3CDL71yF H8C4QHZr9nEy4FtqZPNMpSJyIl5KpJDUU3XzSCiNEZVQpTsB511StY4tAMOfrAOrLi 05TGVfMYcWDI5zfo/qsQK8yzbVQj8/rQMgb5MqJV53/Q11nyF4BRQ0RI5TQWjZMiwC l0BiVDAU2Q04Chpx1oWCDh5asLnM3OytkYPuqnda/Nmh9jpaWrzUWeD9iKghfRkgqj wBIxdSdaZr+8mUH+p/zwj9zmfMI7aQRgKfiHnkmzqveJXeU9UDVKsWrSwBvBAOUMD2 QDGduz4pKB6iw== Date: Thu, 1 Sep 2022 13:29:47 +0200 From: Frederic Weisbecker To: Uladzislau Rezki Cc: "Paul E. McKenney" , Joel Fernandes , Dietmar Eggemann , LKML , Rushikesh S Kadam , Neeraj upadhyay , Steven Rostedt , rcu , Vineeth Pillai Subject: Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes Message-ID: <20220901112947.GA105556@lothringen> References: <20220829134045.GA54589@lothringen> <1f7dd31b-f4d0-5c1c-ce28-c27f75c17f05@joelfernandes.org> <20220829194622.GA58291@lothringen> <20220829204202.GQ6159@paulmck-ThinkPad-P17-Gen-1> <20220830105324.GA71266@lothringen> <20220830114343.GS6159@paulmck-ThinkPad-P17-Gen-1> <20220830160316.GC71266@lothringen> <20220830162244.GA73392@lothringen> 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 Tue, Aug 30, 2022 at 06:44:51PM +0200, Uladzislau Rezki wrote: > Hello, Frederic. > > > > > Although who knows, may be some periodic file operation while idle are specific > > to Android. I'll try to trace lazy callbacks while idle and the number of grace > > periods associated. > > > > > Everything related to lazy call-backs is about not waking "nocb" > kthreads in order to offload one or i should say few callbacks > because it is more or less useless. Currently if incoming callback > is the only one, it will kick a GP whereas a GP will kick nocb_kthread > to offload. Not sure this is only about not waking "nocb" kthreads. The grace period kthread is also awaken in !NOCB and has quite some work to do. And there, having a server expands the issue because you may have a lot of CPUs's extended quiescent states to check. Also in !NOCB, pending callbacks retain the timer tick of a CPU (see rcu_needs_cpu()), and cpuidle relies on the tick to be stopped before allowing the CPU into low power mode. So a lazy callback may delay a CPU from entering into low power mode for a few milliseconds. And I can observe those retained ticks on my idle box. Thanks.