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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 987F5C4363D for ; Wed, 7 Oct 2020 13:01:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 31063206F4 for ; Wed, 7 Oct 2020 13:01:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728387AbgJGNB3 (ORCPT ); Wed, 7 Oct 2020 09:01:29 -0400 Received: from mx2.suse.de ([195.135.220.15]:56584 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728371AbgJGNB2 (ORCPT ); Wed, 7 Oct 2020 09:01:28 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C61D5AE9A; Wed, 7 Oct 2020 13:01:27 +0000 (UTC) Date: Wed, 7 Oct 2020 14:01:25 +0100 From: Mel Gorman To: Michal Hocko Cc: Peter Zijlstra , Thomas Gleixner , Frederic Weisbecker , LKML , Ingo Molnar Subject: Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE, PREEMPT_VOLUNTARY on kernel command line Message-ID: <20201007130125.GI3165@suse.de> References: <20201007120401.11200-1-mhocko@kernel.org> <20201007121939.GE2628@hirez.programming.kicks-ass.net> <20201007122923.GJ29020@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20201007122923.GJ29020@dhcp22.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 07, 2020 at 02:29:23PM +0200, Michal Hocko wrote: > On Wed 07-10-20 14:19:39, Peter Zijlstra wrote: > > On Wed, Oct 07, 2020 at 02:04:01PM +0200, Michal Hocko wrote: > > > From: Michal Hocko > > > > > > Many people are still relying on pre built distribution kernels and so > > > distributions have to provide mutliple kernel flavors to offer different > > > preemption models. Most of them are providing PREEMPT_NONE for typical > > > server deployments and PREEMPT_VOLUNTARY for desktop users. > > > > Is there actually a benefit to NONE? We were recently talking about > > removing it. > > I believe Mel can provide much better insight. We have been historically using > PREEMPT_NONE for our enterprise customers mostly for nice throughput > numbers. Many users are really targeting throughput much more than > latencies. My understanding is that even though VOLUNTARY preemption model > doesn't add too many preemption points on top of NONE it is still > something that is observable (IIRC 2-3% on hackbench). > It's marginal from the tests I ran but that was based on 5.3. At worst, it looked like roughly a hit but a lot of loads simply didn't notice. However, it might vary between architectures that I cannot cover or workloads that I didn't consider. As the impact of PREEMPT_VOLUNTARY depends on where cond_resched and might_sleep is used, it's also something that can vary over time. The intent was that by having the command-line switch, a user could test the switch if there was a suspicion that a regression was related to PREEMPT_VOLUNTARY as opposed to telling them "tough, that's the reality now". -- Mel Gorman SUSE Labs