From patchwork Fri Aug 14 20:54:24 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: 276411 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=-12.8 required=3.0 tests=BAYES_00, DKIM_ADSP_CUSTOM_MED, DKIM_INVALID, DKIM_SIGNED, FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 0B397C433DF for ; Sat, 15 Aug 2020 16:10:34 +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 BFF1623B18 for ; Sat, 15 Aug 2020 16:10:33 +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="fX6amXTx" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BFF1623B18 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+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:55218 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1k6ylN-0002qD-1I for qemu-devel@archiver.kernel.org; Sat, 15 Aug 2020 12:10:33 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56636) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1k6yk4-0001Wv-Dl; Sat, 15 Aug 2020 12:09:12 -0400 Received: from mail-qk1-x72a.google.com ([2607:f8b0:4864:20::72a]:35294) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1k6yk2-0002UQ-CG; Sat, 15 Aug 2020 12:09:12 -0400 Received: by mail-qk1-x72a.google.com with SMTP id p25so11202908qkp.2; Sat, 15 Aug 2020 09:09:09 -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=foegIg+R6xGelzocg1qpmzjKagMlOEuwe5qYOlr3pzs=; b=fX6amXTxJw3yKN5HikX+764lKobkINSWRwiCU3kdeD7rI1EQNbhUOBbOWS3jMONLPR QZzUdlcdMQ+VMOTrxF7hl3m3k94Pj4WEGUp8Zyz4sduy511U/c6ZK92Z3dLNBN+Ha0Bs uVwWqRqpOccRKsR+EBKFw3D4vp9cnMf6f2qB/XoFYUpG646nqWd3EN5kUPuwPk0XwMtk D+EESfyx4Jr5qj5YV0xV2lenR0xFogbb8I6vfo3XwYnWHkWYG8EnGwPOhYPlTbnzcFIw 9epvkT5ciH+owLvLYa/ImJvZZ1JrxGRI03DV9CeyZGdeU+7K7h0FgeOPO8elzR9Vytaf es8w== 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=foegIg+R6xGelzocg1qpmzjKagMlOEuwe5qYOlr3pzs=; b=MosR95BZSgMr8nkuDwGCYRMA3GDvwDoPh9EcU45xlXm3ho4zRSaSRfhJcWY5qzYojv lFJYLtRvXG4Dv+HsoK9NywhlexSH2sOE5PCFJJXV+dRhTM80KcQ6OECLBNY439b2PSDi clLh6wlXJhrG8/4uP3o3WtrNK24uUMwxdqvLITc5/yWSXAMkH39+Jh6kNUlvfFfbJiO5 o5y/DkTZt7lpF81GaHuMdS/niKYbe5BLy7aaheL6U2XUlyqZoyanzkwdeR4MJ2Rm6Yaq 2kzP+wWRtZBOb2WcAfpDOB8TqQwJ8jkxyZfi7F6lujR6dcumV9erceogTuPzlKDyigme s/eg== X-Gm-Message-State: AOAM531vOiUi53HeBvhNFKoFvARMXrrLmKSDwui9YYgZl6WVd2+y6qJQ LmAeAGTJLoQ9Tc/METkOyLdkBkaIPMYJqA== X-Google-Smtp-Source: ABdhPJydDn0sx120VMqKlvwEZlpXbFioI1mkTrLBAnIUyix//GwiPDJ9Sb4CKLjnbZlRA+bRkBynVw== X-Received: by 2002:ac8:dce:: with SMTP id t14mr3637537qti.314.1597438494467; Fri, 14 Aug 2020 13:54:54 -0700 (PDT) Received: from rekt.ibmuc.com ([2804:431:c7c6:303f:d1dc:35d8:e9f6:c8b]) by smtp.gmail.com with ESMTPSA id p33sm12301018qtp.49.2020.08.14.13.54.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 14 Aug 2020 13:54:54 -0700 (PDT) From: Daniel Henrique Barboza To: qemu-devel@nongnu.org Subject: [PATCH 10/10] specs/ppc-spapr-numa: update with new NUMA support Date: Fri, 14 Aug 2020 17:54:24 -0300 Message-Id: <20200814205424.543857-11-danielhb413@gmail.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200814205424.543857-1-danielhb413@gmail.com> References: <20200814205424.543857-1-danielhb413@gmail.com> MIME-Version: 1.0 Received-SPF: pass client-ip=2607:f8b0:4864:20::72a; envelope-from=danielhb413@gmail.com; helo=mail-qk1-x72a.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, david@gibson.dropbear.id.au Errors-To: qemu-devel-bounces+qemu-devel=archiver.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 | 213 ++++++++++++++++++++++++++++++++++ 1 file changed, 213 insertions(+) diff --git a/docs/specs/ppc-spapr-numa.rst b/docs/specs/ppc-spapr-numa.rst index e762038022..994bfb996f 100644 --- a/docs/specs/ppc-spapr-numa.rst +++ b/docs/specs/ppc-spapr-numa.rst @@ -189,3 +189,216 @@ QEMU up to 5.1, as follows: This also means that user input in QEMU command line does not change the NUMA distancing inside the guest for the pseries machine. + +New NUMA mechanics for pseries in QEMU 5.2 +========================================== + +Starting in QEMU 5.2, the pseries machine now 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 20 20 40 + 1 20 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 B A 0 +* node 1: 0 0 A 1 +* node 2: 0 0 A 2 +* node 3: 0 B 0 3 + +With the reference points {0x4, 0x3, 0x2, 0x1}, for node 0: + +* distance from 0 to 1 is 20 (no match at 0x4, will match at 0x3) +* 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 well represented. Doing 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: + +* node 1: C 0 A 1 +* node 2: C 0 A 2 + +Over here we already have the first conflict. Even if we assign a new associativity +domain at 0x4 for 1 and 2, and we do that in the code, the kernel will define +the distance between 1 and 2 as 20, not 80, because both 1 and 2 have the "A" +associativity domain from the previous step. If we decide to discard the +associativity with "A" then the node 0 distances are compromised. + +Following up with the distance from 1 to 3 being 40 (a match in 0x2) we have another +decision to make. These are the current associativity domains of each: + +* node 1: C 0 A 1 +* node 3: 0 B 0 3 + +There is already an associativity domain at 0x2 in node 3, "B", which was assigned +by the node 0 distances. If we define a new associativity domain at this level +for 1 and 3 we will overwrite the existing associativity between 0 and 3. What +the code is doing in this case is to assign the existing domain to the +current associativity, in this case, "B" is now assigned to the 0x2 of node 1, +resulting in the following associativity arrays: + +* node 0: 0 B A 0 +* node 1: C 0 A 1 +* node 2: C B A 2 +* node 3: 0 B 0 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, +A, so we do the same as we did in the previous case and assign it to node 3 +at 0x3. This is the end result for the associativity arrays: + +* node 0: 0 B A 0 +* node 1: C 0 A 1 +* node 2: C B A 2 +* node 3: 0 B A 3 + +The kernel will read these arrays and will calculate the following NUMA topology for +the guest: + +:: + + 0 1 2 3 + 0 10 20 20 20 + 1 20 10 20 20 + 2 20 20 10 20 + 3 20 20 20 10 + +Which is not what the user wanted, but it is what the current logic and implementation +constraints of the kernel and QEMU will provide inside the LOPAPR specification. + +Changing a single value, specially a low distance value, makes for drastic changes +in the result. For example, with the same user input from above, but changing the +node distance from 0 to 1 to 40: + +:: + + 0 1 2 3 + 0 10 40 20 40 + 1 40 10 80 40 + 2 20 80 10 20 + 3 40 40 20 10 + +This is the result inside the guest, applying the same heuristics: + +:: + + $ numactl -H + available: 4 nodes (0-3) + (...) + node distances: + node 0 1 2 3 + 0: 10 40 20 20 + 1: 40 10 80 40 + 2: 20 80 10 20 + 3: 20 40 20 10 + +This result is much closer to the user input and only a single distance was changed +from the original. + +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 A and B and the same distance 20 between nodes +A and F will cause the distance between B and F to also be 20. The same will happen to +other distances, but shorter distances has precedent over it to the distance calculation. + +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.