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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 EFBCAC54FCC for ; Tue, 21 Apr 2020 13:18:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D0CAF2076C for ; Tue, 21 Apr 2020 13:18:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728769AbgDUNST (ORCPT ); Tue, 21 Apr 2020 09:18:19 -0400 Received: from foss.arm.com ([217.140.110.172]:34860 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726018AbgDUNST (ORCPT ); Tue, 21 Apr 2020 09:18:19 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 1D76E31B; Tue, 21 Apr 2020 06:18:18 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 9FEAE3F68F; Tue, 21 Apr 2020 06:18:15 -0700 (PDT) References: <20200414150556.10920-1-qais.yousef@arm.com> <20200421121305.ziu3dfqwo7cw6ymu@e107158-lin.cambridge.arm.com> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Qais Yousef Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Andrew Morton , Thomas Gleixner , Yury Norov , Paul Turner , Alexey Dobriyan , Josh Don , Pavan Kondeti , linux-kernel@vger.kernel.org, Marc Zyngier , "Rafael J. Wysocki" Subject: Re: [PATCH 0/4] sched/rt: Distribute tasks in find_lowest_rq() In-reply-to: <20200421121305.ziu3dfqwo7cw6ymu@e107158-lin.cambridge.arm.com> Date: Tue, 21 Apr 2020 14:18:10 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 21/04/20 13:13, Qais Yousef wrote: > On 04/14/20 19:58, Valentin Schneider wrote: >> >> I'm a bit wary about such blanket changes. I feel like most places impacted >> by this change don't gain anything by using the random thing. In sched land >> that would be: > > The API has always been clear that cpumask_any return a random cpu within the > mask. And the fact it's a one liner with cpumask_first() directly visible, > a user made the choice to stick to cpumask_any() indicates that that's what > they wanted. > > Probably a lot of them they don't care what cpu is returned and happy with the > random value. I don't see why it has to have an effect. Some could benefit, > like my use case here. Or others truly don't care, then it's fine to return > anything, as requested. > Exactly, *some* (which AFAICT is a minority) might benefit. So why should all the others pay the price for a functionality they do not need? I don't think your change would actually cause a splat somewhere; my point is about changing existing behaviour without having a story for it. The thing said 'pick a "random" cpu', sure, but it never did that, it always picked the first. I've pointed out two examples that want to be cpumask_first(), and I'm absolutely certain there are more than these two out there. What if folks ran some performance test and were completely fine with the _first() behaviour? What tells you randomness won't degrade some cases? IMO the correct procedure is to keep everything as it is and improve the specific callsites that benefit from randomness. I get your point that using cpumask_any() should be a good enough indicator of the latter, but I don't think it can realistically be followed. To give my PoV, if in the past someone had used a cpumask_any() where a cpumask_first() could do, I would've acked it (disclaimer: super representative population of sample size = 1). Flipping the switch on everyone to then have a series of patches "oh this one didn't need it", "this one neither", "I actually need this to be the first" just feels sloppy. > I CCed Marc who's the maintainer of this file who can clarify better if this > really breaks anything. > > If any interrupt expects to be affined to a specific CPU then this must be > described in DT/driver. I think the GIC controller is free to distribute them > to any cpu otherwise if !force. Which is usually done by irq_balancer anyway > in userspace, IIUC. > > I don't see how cpumask_any_and() break anything here too. I actually think it > improves on things by better distribute the irqs on the system by default. > As you say, if someone wants smarter IRQ affinity they can do irq_balancer and whatnot. The default kernel policy for now has been to shove everything on the lowest-numbered CPU, and I see no valid reason to change that.