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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 5334BC2D0DB for ; Fri, 24 Jan 2020 15:13:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 21ED220709 for ; Fri, 24 Jan 2020 15:13:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="W3qX1D7e" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731184AbgAXPNu (ORCPT ); Fri, 24 Jan 2020 10:13:50 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:48224 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726293AbgAXPNt (ORCPT ); Fri, 24 Jan 2020 10:13:49 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=35gCEfpxVAtjI0TfMCZ1JhtcqIkWEo2HG4/U2DcwoqA=; b=W3qX1D7eKIlfLC6pgoA9hEyYm AJ3QDv5UwmVIhwcakfZzw+VrHsNExGoljlLqgbrtiokyNRGEkGZQrFFnygTlaj9ZlBZnvXUB3+g2M I/dgy/eQnq38v47EWpW+gKk0tJ1jSmN2DOio8ltCdwjJZ1lTAg53pO/bmLRl5BXQbjbh/l8d5G8YE L+0zhq/GvAYo/asdkBTOU31ZGwmb1op68byQ2U+R5cU7a4gCAsDcKyNeT0AicZPLeR785qtJNDGD0 XgDS+sl3pZusZgrxRZdf9ezFTYwk/LWT8vsFw1Z73vn2QsbTTZbeWbk1c5FeekrarX9l+3cLDg6FY ihbNnrDNg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1iv0e0-0007Qp-QC; Fri, 24 Jan 2020 15:13:12 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 63A913012DC; Fri, 24 Jan 2020 16:11:29 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id E889620AFECE6; Fri, 24 Jan 2020 16:13:09 +0100 (CET) Date: Fri, 24 Jan 2020 16:13:09 +0100 From: Peter Zijlstra To: Waiman Long Cc: Alex Kogan , linux@armlinux.org.uk, Ingo Molnar , Will Deacon , Arnd Bergmann , linux-arch@vger.kernel.org, linux-arm-kernel , linux-kernel@vger.kernel.org, Thomas Gleixner , Borislav Petkov , hpa@zytor.com, x86@kernel.org, Hanjun Guo , Jan Glauber , Steven Sistare , Daniel Jordan , dave.dice@oracle.com Subject: Re: [PATCH v8 4/5] locking/qspinlock: Introduce starvation avoidance into CNA Message-ID: <20200124151309.GE14879@hirez.programming.kicks-ass.net> References: <20191230194042.67789-1-alex.kogan@oracle.com> <20191230194042.67789-5-alex.kogan@oracle.com> <20200121132949.GL14914@hirez.programming.kicks-ass.net> <3862F8A1-FF9B-40AD-A88E-2C0BA7AF6F58@oracle.com> <20200124075235.GX14914@hirez.programming.kicks-ass.net> <2c6741c5-d89d-4b2c-cebe-a7c7f6eed884@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2c6741c5-d89d-4b2c-cebe-a7c7f6eed884@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 24, 2020 at 09:42:42AM -0500, Waiman Long wrote: > On 1/24/20 2:52 AM, Peter Zijlstra wrote: > > On Thu, Jan 23, 2020 at 04:33:54PM -0500, Alex Kogan wrote: > >> Let me put this question to you. What do you think the number should be? > > I think it would be very good to keep the inter-node latency below 1ms. > It is hard to guarantee that given that lock hold times can vary quite a > lot depending on the workload. What we can control is just how many > later lock waiters can jump ahead before a given waiter. We're not into this for easy. And exactly because it depends on a lot we need a lot of data. Worst case lock acquisition times directly translate into worst case IRQ-off latencies, and even the most die hard throughput oriented workloads don't like to experience multiple ticks worth of irq-off latencies. > > But to realize that we need data on the lock hold times. Specifically > > for the heavily contended locks that make CNA worth it in the first > > place. > > > > I don't see that data, so I don't see how we can argue about this let > > alone call something reasonable. > > > In essence, CNA lock is for improving throughput on NUMA machines at the > expense of increasing worst case latency. If low latency is important, Latency is _always_ important. Otherwise we'd never have put so much time and effort into fair locks to begin with. Unbounded latency sucks unconditionally. > it should be disabled. If CONFIG_PREEMPT_RT is on, > CONFIG_NUMA_AWARE_SPINLOCKS should be off. You're spouting nonsense. You cannot claim any random number is reasonable without argument.