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=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 6383AC48BE9 for ; Mon, 24 Jun 2019 14:24:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 43E8D208E4 for ; Mon, 24 Jun 2019 14:24:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728415AbfFXOYZ (ORCPT ); Mon, 24 Jun 2019 10:24:25 -0400 Received: from outbound-smtp15.blacknight.com ([46.22.139.232]:50426 "EHLO outbound-smtp15.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726263AbfFXOYZ (ORCPT ); Mon, 24 Jun 2019 10:24:25 -0400 Received: from mail.blacknight.com (unknown [81.17.254.17]) by outbound-smtp15.blacknight.com (Postfix) with ESMTPS id 5FDCF1C26F2 for ; Mon, 24 Jun 2019 15:24:22 +0100 (IST) Received: (qmail 26894 invoked from network); 24 Jun 2019 14:24:22 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.21.36]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 24 Jun 2019 14:24:22 -0000 Date: Mon, 24 Jun 2019 15:24:20 +0100 From: Mel Gorman To: Matt Fleming Cc: Peter Zijlstra , "Lendacky, Thomas" , "linux-kernel@vger.kernel.org" , "Suthikulpanit, Suravee" , Borislav Petkov Subject: Re: [PATCH] sched/topology: Improve load balancing on AMD EPYC Message-ID: <20190624142420.GC2978@techsingularity.net> References: <20190605155922.17153-1-matt@codeblueprint.co.uk> <20190605180035.GA3402@hirez.programming.kicks-ass.net> <20190610212620.GA4772@codeblueprint.co.uk> <18994abb-a2a8-47f4-9a35-515165c75942@amd.com> <20190618104319.GB4772@codeblueprint.co.uk> <20190618123318.GG3419@hirez.programming.kicks-ass.net> <20190619213437.GA6909@codeblueprint.co.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20190619213437.GA6909@codeblueprint.co.uk> 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 Wed, Jun 19, 2019 at 10:34:37PM +0100, Matt Fleming wrote: > On Tue, 18 Jun, at 02:33:18PM, Peter Zijlstra wrote: > > On Tue, Jun 18, 2019 at 11:43:19AM +0100, Matt Fleming wrote: > > > This works for me under all my tests. Thoughts? > > > > > > --->8--- > > > > > > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c > > > index 80a405c2048a..4db4e9e7654b 100644 > > > --- a/arch/x86/kernel/cpu/amd.c > > > +++ b/arch/x86/kernel/cpu/amd.c > > > @@ -8,6 +8,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > #include > > > #include > > > @@ -824,6 +825,8 @@ static void init_amd_zn(struct cpuinfo_x86 *c) > > > { > > > set_cpu_cap(c, X86_FEATURE_ZEN); > > > > > > > I'm thinking this deserves a comment. Traditionally the SLIT table held > > relative memory latency. So where the identity is 10, 16 would indicate > > 1.6 times local latency and 32 would be 3.2 times local. > > > > Now, even very early on BIOS monkeys went about their business and put > > in random values in an attempt to 'tune' the system based on how > > $random-os behaved, which is all sorts of fu^Wwrong. > > > > Now, I suppose my question is; is that 32 Zen puts in an actual relative > > memory latency metric, or a random value we somehow have to deal with. > > And can we pretty please describe the whole sordid story behind this > > 'tunable' somewhere? > > This is one for the AMD folks. I don't know if the memory latency > really is 3.2 times or not, only that that's the value in all the Zen > machines I have access to. Even this 2-socket one: > > node distances: > node 0 1 > 0: 10 32 > 1: 32 10 > > Tom, Suravee? Do not consider this an authorative response but based on what I know of the physical topology, it is not unreasonable to use 32 in the SLIT table. There is a small latency when accessing another die on the same socket (details are generation specific). It's not quite a local access but it's not as much as a traditional remote access either (hence 16 being the base unit for another die to hint that it's not quite local but not quite remote either). 32 is based on accessing a die on a remote socket based on the expected performance and latency of the interconnect. To the best of my knowledge, the magic numbers are reflective of the real topology and not just a gamification of the numbers for a random OS. If anything, the fact that there is a load balancing issue on Linux would indicate that they were not picking random numbers for Linux at least :P -- Mel Gorman SUSE Labs