From patchwork Tue Sep 29 13:38:17 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Daniel Henrique Barboza X-Patchwork-Id: 11805929 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 300B9112E for ; Tue, 29 Sep 2020 13:46:41 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A0886207C4 for ; Tue, 29 Sep 2020 13:46:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="dPDTaKwZ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A0886207C4 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=gmail.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Received: from localhost ([::1]:58436 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kNFxm-0006tU-I8 for patchwork-qemu-devel@patchwork.kernel.org; Tue, 29 Sep 2020 09:46:38 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40764) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kNFqK-0007ZN-Lo; Tue, 29 Sep 2020 09:38:57 -0400 Received: from mail-qk1-x744.google.com ([2607:f8b0:4864:20::744]:34614) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1kNFqI-0001Db-Dy; Tue, 29 Sep 2020 09:38:56 -0400 Received: by mail-qk1-x744.google.com with SMTP id c62so4303193qke.1; Tue, 29 Sep 2020 06:38:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=jijcRPvylkUY2TozcNpYKw9C8qqxbdoidV8N19tjEm4=; b=dPDTaKwZtQTJfQYvuPYpUl+Irb8UZ3NOao1kHBopyQx/+Q9bJFc7IIwSTPfBShfbqa wgrxPnFHZ80dr610BbiS4i3IurQymTfur3Oojmmr4Myy7VvATNlLQdEa+BhDvkf/Pd3T bOB/80p4v5KXBqjTQxlGvktuWAIXU6fB5+Kel1ZV69OQYTLjrga7zGOcnyL+fwv78pnX EOjdQ//5f515kZcGpCpQRUaxKjAzxpjFHkYFrrFTGLCV0L/3quX33WT43X8pd+dRDqVA qjhBVNqNRWYcN+7z84+jXceGbU5t3ESEWpIUUxCm0B2bvky3QAaeixOq663WbTJE3GcS kDkw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=jijcRPvylkUY2TozcNpYKw9C8qqxbdoidV8N19tjEm4=; b=kf4dgA9d02iT3G+lbhyhWH1d3xyAJeoly+Yv6ti62DslkDe66IHEjt+mzhJLmYRj3b l3/k6ZH2F8NseAxOWOlmJBi7cpIUlaHADBsmMcT1EL47TEj/slwtts/A+fRNOgLSTqXH wAVdltFHO0nNuvRqPC8Xi6FW7aKIxiG5NzFYA8pQhOfZirt7CI6+9AMjq/6ck3xMM3cC q21lo7umd/sEryAiREUDG2QLF6+Hnjmi7Xxg5jtW7rUueUb/4c6KQ2zD3KhTYw1xuSVf 9MI2uwxRIVjB4TddTdFFfgC1qNyLOjMiZzULiv+qe+qwqnkVIpZOlnwuR9iG458JKRSs aW2g== X-Gm-Message-State: AOAM533uA7Ld/goZxZ7Q/WJ1IwWx/sHRKYLr1N0r+/hGAGL4asgohzud aFCbdk6lUovEZvtADfOTT7KKwFTHQ9c= X-Google-Smtp-Source: ABdhPJyvaH1zhujglLKoZrXU3wefcyWi8IBxw+5RJ6ecGEWvlZkrfzElu2+O7jgT2s9mjax2WPeYGA== X-Received: by 2002:a05:620a:231:: with SMTP id u17mr4553983qkm.166.1601386732262; Tue, 29 Sep 2020 06:38:52 -0700 (PDT) Received: from rekt.ibmuc.com ([2804:431:c7c7:c625:6c0e:4720:8228:5f68]) by smtp.gmail.com with ESMTPSA id j88sm5239938qte.96.2020.09.29.06.38.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 29 Sep 2020 06:38:51 -0700 (PDT) From: Daniel Henrique Barboza To: qemu-devel@nongnu.org Subject: [PATCH v3 5/5] specs/ppc-spapr-numa: update with new NUMA support Date: Tue, 29 Sep 2020 10:38:17 -0300 Message-Id: <20200929133817.560278-6-danielhb413@gmail.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200929133817.560278-1-danielhb413@gmail.com> References: <20200929133817.560278-1-danielhb413@gmail.com> MIME-Version: 1.0 Received-SPF: pass client-ip=2607:f8b0:4864:20::744; envelope-from=danielhb413@gmail.com; helo=mail-qk1-x744.google.com X-detected-operating-system: by eggs.gnu.org: No matching host in p0f cache. That's all we know. X-Spam_score_int: -17 X-Spam_score: -1.8 X-Spam_bar: - X-Spam_report: (-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Daniel Henrique Barboza , qemu-ppc@nongnu.org, groug@kaod.org, david@gibson.dropbear.id.au Errors-To: qemu-devel-bounces+patchwork-qemu-devel=patchwork.kernel.org@nongnu.org Sender: "Qemu-devel" This update provides more in depth information about the choices and drawbacks of the new NUMA support for the spapr machine. Signed-off-by: Daniel Henrique Barboza --- docs/specs/ppc-spapr-numa.rst | 206 +++++++++++++++++++++++++++++++++- 1 file changed, 205 insertions(+), 1 deletion(-) diff --git a/docs/specs/ppc-spapr-numa.rst b/docs/specs/ppc-spapr-numa.rst index e762038022..6dd13bf97b 100644 --- a/docs/specs/ppc-spapr-numa.rst +++ b/docs/specs/ppc-spapr-numa.rst @@ -158,9 +158,213 @@ kernel tree). This results in the following distances: * resources four NUMA levels apart: 160 -Consequences for QEMU NUMA tuning +pseries NUMA mechanics +====================== + +Starting in QEMU 5.2, the pseries machine considers user input when setting NUMA +topology of the guest. The following changes were made: + +* ibm,associativity-reference-points was changed to {0x4, 0x3, 0x2, 0x1}, allowing + for 4 distinct NUMA distance values based on the NUMA levels + +* ibm,max-associativity-domains was changed to support multiple associativity + domains in all NUMA levels. This is needed to ensure user flexibility + +* ibm,associativity for all resources now varies with user input + +These changes are only effective for pseries-5.2 and newer machines that are +created with more than one NUMA node (disconsidering NUMA nodes created by +the machine itself, e.g. NVLink 2 GPUs). The now legacy support has been +around for such a long time, with users seeing NUMA distances 10 and 40 +(and 80 if using NVLink2 GPUs), and there is no need to disrupt the +existing experience of those guests. + +To bring the user experience x86 users have when tuning up NUMA, we had +to operate under the current pseries Linux kernel logic described in +`How the pseries Linux guest calculates NUMA distances`_. The result +is that we needed to translate NUMA distance user input to pseries +Linux kernel input. + +Translating user distance to kernel distance +-------------------------------------------- + +User input for NUMA distance can vary from 10 to 254. We need to translate +that to the values that the Linux kernel operates on (10, 20, 40, 80, 160). +This is how it is being done: + +* user distance 11 to 30 will be interpreted as 20 +* user distance 31 to 60 will be interpreted as 40 +* user distance 61 to 120 will be interpreted as 80 +* user distance 121 and beyond will be interpreted as 160 +* user distance 10 stays 10 + +The reasoning behind this aproximation is to avoid any round up to the local +distance (10), keeping it exclusive to the 4th NUMA level (which is still +exclusive to the node_id). All other ranges were chosen under the developer +discretion of what would be (somewhat) sensible considering the user input. +Any other strategy can be used here, but in the end the reality is that we'll +have to accept that a large array of values will be translated to the same +NUMA topology in the guest, e.g. this user input: + +:: + + 0 1 2 + 0 10 31 120 + 1 31 10 30 + 2 120 30 10 + +And this other user input: + +:: + + 0 1 2 + 0 10 60 61 + 1 60 10 11 + 2 61 11 10 + +Will both be translated to the same values internally: + +:: + + 0 1 2 + 0 10 40 80 + 1 40 10 20 + 2 80 20 10 + +Users are encouraged to use only the kernel values in the NUMA definition to +avoid being taken by surprise with that the guest is actually seeing in the +topology. There are enough potential surprises that are inherent to the +associativity domain assignment process, discussed below. + + +How associativity domains are assigned +-------------------------------------- + +LOPAPR allows more than one associativity array (or 'string') per allocated +resource. This would be used to represent that the resource has multiple +connections with the board, and then the operational system, when deciding +NUMA distancing, should consider the associativity information that provides +the shortest distance. + +The spapr implementation does not support multiple associativity arrays per +resource, neither does the pseries Linux kernel. We'll have to represent the +NUMA topology using one associativity per resource, which means that choices +and compromises are going to be made. + +Consider the following NUMA topology entered by user input: + +:: + + 0 1 2 3 + 0 10 40 20 40 + 1 40 10 80 40 + 2 20 80 10 20 + 3 40 40 20 10 + +Honoring just the relative distances of node 0 to every other node, one possible +value for all associativity arrays would be: + +* node 0: 0 0 0 0 +* node 1: 1 0 1 1 +* node 2: 2 2 0 2 +* node 3: 3 0 3 3 + +With the reference points {0x4, 0x3, 0x2, 0x1}, for node 0: + +* distance from 0 to 1 is 40 (no match at 0x4 and 0x3, will match + at 0x2) +* distance from 0 to 2 is 20 (no match at 0x4, will match at 0x3) +* distance from 0 to 3 is 40 (no match at 0x4 and 0x3, will match + at 0x2) + +The distances related to node 0 are accounted for. For node 1, and keeping +in mind that we don't need to revisit node 0 again, the distance from +node 1 to 2 is 80, matching at 0x4, and distance from 1 to 3 is 40, +match in 0x3: + +* node 0: 0 0 0 0 +* node 1: 1 0 1 1 +* node 2: 1 2 0 2 +* node 3: 3 0 3 3 + +In the last step we will analyze just nodes 2 and 3. The desired distance +between 2 and 3 is 20, i.e. a match in 0x3. Node 2 already has a +domain assigned in 0x3, 0. We'll preserve it to avoid dissolving the +association between node 0 and node 2, and use it as a domain for +0x3 as well: + +* node 0: 0 0 0 0 +* node 1: 1 0 1 1 +* node 2: 1 2 0 2 +* node 3: 3 0 0 3 + + +The kernel will read these arrays and will calculate the following NUMA topology for +the guest: + +:: + + 0 1 2 3 + 0 10 40 20 20 + 1 40 10 80 40 + 2 20 80 10 20 + 3 20 40 20 10 + +Note that this is not what the user wanted - the desired distance between +0 and 3 is 40, we calculated it as 20. This is what the current logic and +implementation constraints of the kernel and QEMU will provide inside the +LOPAPR specification. + + +Users are welcome to use this knowledge and experiment with the input to get +the NUMA topology they want, or as closer as they want. The important thing +is to keep expectations up to par with what we are capable of provide at this +moment: an approximation. + +Limitations of the implementation --------------------------------- +As mentioned above, the pSeries NUMA distance logic is, in fact, a way to approximate +user choice. The Linux kernel, and PAPR itself, does not provide QEMU with the ways +to fully map user input to actual NUMA distance the guest will use. These limitations +creates two notable limitations in our support: + +* Asymmetrical topologies aren't supported. We only support NUMA topologies where + the distance from node A to B is always the same as B to A. We do not support + any A-B pair where the distance back and forth is asymmetric. For example, the + following topology isn't supported and the pSeries guest will not boot with this + user input: + +:: + + 0 1 + 0 10 40 + 1 20 10 + + +* 'non-transitive' topologies will be poorly translated to the guest. This is the + kind of topology where the distance from a node A to B is X, B to C is X, but + the distance A to C is not X. E.g.: + +:: + + 0 1 2 3 + 0 10 20 20 40 + 1 20 10 80 40 + 2 20 80 10 20 + 3 40 40 20 10 + + In the example above, distance 0 to 2 is 20, 2 to 3 is 20, but 0 to 3 is 40. + The kernel will always match with the shortest associativity domain possible, + and we're attempting to retain the previous established relations between the + nodes. This means that a distance equal to 20 between nodes 0 and 2 and the + same distance 20 between nodes 2 and 3 will cause the distance between 0 and 3 + to also be 20. + + +Legacy (5.1 and older) pseries NUMA mechanics +============================================= + The way the pseries Linux guest calculates NUMA distances has a direct effect on what QEMU users can expect when doing NUMA tuning. As of QEMU 5.1, this is the default ibm,associativity-reference-points being used in the pseries