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 E6529C001B0 for ; Wed, 5 Jul 2023 12:41:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231807AbjGEMlN (ORCPT ); Wed, 5 Jul 2023 08:41:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49208 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230268AbjGEMlL (ORCPT ); Wed, 5 Jul 2023 08:41:11 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7ADD4116; Wed, 5 Jul 2023 05:41:10 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 0129161503; Wed, 5 Jul 2023 12:41:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D0202C433C8; Wed, 5 Jul 2023 12:41:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1688560869; bh=Q7lihklVNA978w7esJOsxnmRuuyHw/4jy8/HtRgD23U=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=KgPbK1R+lK3ukRLbUfyqgffhL5OKUT+r72u8axNjes6sIDlmKKm2yPVKP2jHv5PFM DhZMRnGTiEhynTpthgP+aOO5ltcgFqyhNP5gZ49L4A1LetkOxOxEZ7vDQooDLNGwJQ oqQp2oUBpFYw+plcqxAdQqQ3UuACagbMqHa9SeFaelxsfvhz8JuyUjB094wvyAjYke 3npJw1S50ezR6vhdIxSmhuGXYVuQFuydfdjtbY5fIdLAqw5cREcMLvoAbgERooeoxQ 3Ah16NmqPBtABEnxoC5oWHep8X0u6dPKZH6bWiGdJKBKiUVOJPnfw3n6wGwH/n2jyc MseidTfhd9Ksw== Date: Wed, 5 Jul 2023 14:41:06 +0200 From: Frederic Weisbecker To: "Paul E . McKenney" Cc: LKML , rcu , Uladzislau Rezki , Neeraj Upadhyay , Joel Fernandes , Giovanni Gherdovich Subject: Re: [PATCH 0/9] rcu: Support for lazy callbacks on !CONFIG_RCU_NOCB_CPU Message-ID: References: <20230531101736.12981-1-frederic@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20230531101736.12981-1-frederic@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le Wed, May 31, 2023 at 12:17:27PM +0200, Frederic Weisbecker a écrit : > Hi, > > Here is a first attempt at providing support for lazy callbacks on > non-offloaded CPUs. I have measured interesting results on my machine > when it is mostly idle (just an idle ssh connection kept alive) with > a gain of +10% idle time. This needs more serious testing though, > especially to make sure that no performance regression is introduced. So after refining my measurements, it actually doesn't seem to bring much. Here's what I measured (it's an average after 100 runs): * 0.01% more idle time during a "sleep 10" * introduce 3% performance regression during an "scp" on the tools directory of the kernel source. * Brings less than 1% performance improvement with a "make" on tools/perf It only brings unconditional improvement when rcu boost is enabled (callbacks offloaded to rcuc) but still below 1%. The reason for the performance penalty is unclear, I initially thought it was due to the batch execution delaying other softirqs vectors but it doesn't look that way after all. So for now I'm only going to re-iterate the cleanups in the beginning of the patchset and I'll wait for a good reason before ever proceeding with the rest. Thanks.