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 5A2CDC433F5 for ; Sat, 9 Apr 2022 15:56:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242564AbiDIP6W (ORCPT ); Sat, 9 Apr 2022 11:58:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44902 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234296AbiDIP6V (ORCPT ); Sat, 9 Apr 2022 11:58:21 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id ED4C0B8213 for ; Sat, 9 Apr 2022 08:56:13 -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 86F3060B69 for ; Sat, 9 Apr 2022 15:56:13 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id DF7B8C385A4; Sat, 9 Apr 2022 15:56:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1649519772; bh=iEsZf4yCox/8RyXdErEUAGOd+SMekJ2heiJThUv/iec=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=J6tLAqJf75T0x3NLhkSTXBPTy5UqyrqO7ag0uuW0AXQMs8Vcrcb6y2I8DlY3Egr51 MfUioqWHjCZosSeV7z/rqhOHyIVqUG+Ol4WvqW0OT3BTGeEzzPP+XT2U3l/vDZ/5ks dKTIxIW3fpbZEemvHE90D9TYVfEr8cxRyqlXM5r/ybFgJeITyuyDKFZ1nGpnx2vDWY KMkFYw4NzIPX28InYDh2Hcn3hwOSOxUfl7efqmJjXb8vhr7ydnsvEbL1Uk0e0/GdmD 28ej/nN8fRJP1H+ZyOJAiH9ipS8p6YmcqNYGdOphYgVAVG5GEpc5I0XQk9+W9i4L4U UA37kelcXdfIw== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 812025C09EF; Sat, 9 Apr 2022 08:56:12 -0700 (PDT) Date: Sat, 9 Apr 2022 08:56:12 -0700 From: "Paul E. McKenney" To: Hillf Danton Cc: Kalesh Singh , Joel Fernandes , LKML Subject: Re: [PATCH v2] EXP rcu: Move expedited grace period (GP) work to RT kthread_worker Message-ID: <20220409155612.GO4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220408045734.1158817-1-kaleshsingh@google.com> <20220408143444.GC4285@paulmck-ThinkPad-P17-Gen-1> <20220408153447.GE4285@paulmck-ThinkPad-P17-Gen-1> <20220408173908.GJ4285@paulmck-ThinkPad-P17-Gen-1> <20220409071740.6024-1-hdanton@sina.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220409071740.6024-1-hdanton@sina.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Apr 09, 2022 at 03:17:40PM +0800, Hillf Danton wrote: > On Fri, 8 Apr 2022 10:53:53 -0700 Kalesh Singh wrote > > Thanks for the discussion everyone. > > > > We didn't fully switch to kthread workers to avoid changing the > > behavior for users that dont need this low latency exp GPs. Another > > (and perhaps more important) reason is because kthread_worker offers > > reduced concurrency than workqueues which Pual reported can pose > > issues on systems with a large number of CPUs. > > A second ... what issues were reported wrt concurrency, given the output > of grep -nr workqueue block mm drivers. > > Feel free to post a URL link to the issues. The issues can be easily seen by inspecting kthread_queue_work() and the functions that it invokes. In contrast, normal workqueues uses per-CPU mechanisms to avoid contention, as can equally easily be seen by inspecting queue_work_on() and the functions that it invokes. Please do feel free to take a look. If taking a look does not convince you, please construct some in-kernel benchmarks to test the scalability of these two mechanisms. Please note that some care will be required to make sure that you are doing a valid apples-to-apples comparison. Thanx, Paul