From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752726AbdARKPF (ORCPT ); Wed, 18 Jan 2017 05:15:05 -0500 Received: from mx2.suse.de ([195.135.220.15]:42626 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752683AbdARKNz (ORCPT ); Wed, 18 Jan 2017 05:13:55 -0500 Date: Wed, 18 Jan 2017 10:55:23 +0100 From: Petr Mladek To: Florian Fainelli Cc: Russell King - ARM Linux , linux-arm-kernel@lists.infradead.org, Chris Metcalf , open list , Aaron Tomlin , Andrew Morton , "Paul E. McKenney" , Thomas Gleixner , Kees Cook , Sergey Senozhatsky Subject: Re: [PATCH] ARM: smp: Remove CPU: shutdown notice Message-ID: <20170118095523.GK20462@pathway.suse.cz> References: <20170117230714.6799-1-f.fainelli@gmail.com> <20170117232314.GI27312@n2100.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 2017-01-17 15:39:45, Florian Fainelli wrote: > On 01/17/2017 03:23 PM, Russell King - ARM Linux wrote: > > On Tue, Jan 17, 2017 at 03:07:12PM -0800, Florian Fainelli wrote: > >> This message is not particularly informative, and is not paired with an > >> identical message when a CPU is brought online. Finally, it slows the > >> CPU hotplug path down, thus allowing less CPU hotplug operations per > >> second. Just remove it. > > > > CPU hotplug isn't a fast operation anyway - it's also fairly disruptive > > in that it uses stop_machine() to halt activity everywhere while taking > > the CPU offline. > > We have a test that consists in shutting down all CPUs as frequently as > we can and do this for about 2 million iterations which takes roughly > 24h, and this printk slows thing down by a reasonable amount. Here are > some numbers on 500 hotplug operations: > > w/ printk: > real 0m9.997s > user 0m0.725s > sys 0m3.030s > # > > w/o printk: > real 0m8.547s > user 0m0.436s > sys 0m1.838s I am curious that a single printk() might make such a big difference. One reason might be that the messages are pushed to a "slow" console. Another reason might be that there are many other messages printed on the system and there is a contention on logbuf_lock or other console related locks. There might be also the opposite problem. The messages are also read by userspace tools that store them into /var/log/messages or systemd logs. If these are the only messages printed to the log and if there is no other activity on the system. Then the waken loggers might make a difference, especially if all CPUs are getting disabled and only one is available at some point. Well, I am not sure what other operations are needed to do the CPU hotplug operation. I cannot judge how the message is useful and if the speed up is worth removing it. Best Regards, Petr