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=-3.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,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 199E7C433ED for ; Fri, 2 Apr 2021 16:45:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E1B6961055 for ; Fri, 2 Apr 2021 16:45:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235204AbhDBQpB (ORCPT ); Fri, 2 Apr 2021 12:45:01 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:38230 "EHLO mx0b-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229722AbhDBQo6 (ORCPT ); Fri, 2 Apr 2021 12:44:58 -0400 Received: from pps.filterd (m0127361.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.43/8.16.0.43) with SMTP id 132GXp6k149512; Fri, 2 Apr 2021 12:44:35 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=from : to : cc : subject : in-reply-to : references : date : message-id : mime-version : content-type : content-transfer-encoding; s=pp1; bh=gkdE+H6UoslkeqtMNb7IPN0mwGr/JpXzRpWAcK7ID8g=; b=cmxmgJYD6zF37Ml4Xm7Xvxgd52NeOf6pcYAvdLxICgKmSQTV7ZVPNB0LkY5kelvuZ7Vm tgczNHZzYFVjm11/QyHlLm4qQHZrbxf1/KJ31b9KXLnUL/IAG8IUiL5bFBUtnou1NVuQ SglAwVLdBvCGk0yf2IHjEYZV//OvzQtUvqlSwLNvHU4IOsp7VfHLQNGfIhGp8VM1B7bZ 8YQDMyjPMcSHyjLlcWHGoVo4uz81/BSVc6hWJXsKINu/j/VnBA2Xpk8d06Usr4AdVvnG 4WHiMHTxhO5MLMCPPZy8LM+FtRtRlSj9qayJHQi7mx2adBf0uCdVh7rcuFVgg0CSICAi ng== Received: from ppma04wdc.us.ibm.com (1a.90.2fa9.ip4.static.sl-reverse.com [169.47.144.26]) by mx0a-001b2d01.pphosted.com with ESMTP id 37ntw1sgyq-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Fri, 02 Apr 2021 12:44:34 -0400 Received: from pps.filterd (ppma04wdc.us.ibm.com [127.0.0.1]) by ppma04wdc.us.ibm.com (8.16.0.43/8.16.0.43) with SMTP id 132Gh6IT007334; Fri, 2 Apr 2021 16:44:34 GMT Received: from b01cxnp23033.gho.pok.ibm.com (b01cxnp23033.gho.pok.ibm.com [9.57.198.28]) by ppma04wdc.us.ibm.com with ESMTP id 37n2984607-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Fri, 02 Apr 2021 16:44:34 +0000 Received: from b01ledav001.gho.pok.ibm.com (b01ledav001.gho.pok.ibm.com [9.57.199.106]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 132GiXVh31392050 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 2 Apr 2021 16:44:33 GMT Received: from b01ledav001.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D32152805C; Fri, 2 Apr 2021 16:44:33 +0000 (GMT) Received: from b01ledav001.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9156528059; Fri, 2 Apr 2021 16:44:33 +0000 (GMT) Received: from localhost (unknown [9.163.15.116]) by b01ledav001.gho.pok.ibm.com (Postfix) with ESMTP; Fri, 2 Apr 2021 16:44:33 +0000 (GMT) From: Nathan Lynch To: Laurent Dufour Cc: cheloha@linux.ibm.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org, Srikar Dronamraju Subject: Re: [PATCH v2] pseries: prevent free CPU ids to be reused on another node In-Reply-To: References: <20210325093512.57856-1-ldufour@linux.ibm.com> <87a6qgbyk6.fsf@linux.ibm.com> Date: Fri, 02 Apr 2021 11:44:32 -0500 Message-ID: <877dlkbpqn.fsf@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-TM-AS-GCONF: 00 X-Proofpoint-GUID: HJD8wR6yREK7Ai04wfNu96h7_W125uRM X-Proofpoint-ORIG-GUID: HJD8wR6yREK7Ai04wfNu96h7_W125uRM X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.369,18.0.761 definitions=2021-04-02_09:2021-04-01,2021-04-02 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 priorityscore=1501 mlxscore=0 phishscore=0 spamscore=0 clxscore=1015 adultscore=0 suspectscore=0 malwarescore=0 lowpriorityscore=0 impostorscore=0 mlxlogscore=999 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2103310000 definitions=main-2104020116 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Laurent Dufour writes: > Le 02/04/2021 =C3=A0 15:34, Nathan Lynch a =C3=A9crit=C2=A0: >> Laurent Dufour writes: >>> When a CPU is hot added, the CPU ids are taken from the available mask = from >>> the lower possible set. If that set of values was previously used for C= PU >>> attached to a different node, this seems to application like if these C= PUs >>> have migrated from a node to another one which is not expected in real >>> life. >>=20 >> This seems like a problem that could affect other architectures or >> platforms? I guess as long as arch code is responsible for placing new >> CPUs in cpu_present_mask, that code will have the responsibility of >> ensuring CPU IDs' NUMA assignments remain stable. > > Actually, x86 is already handling this issue in the arch code specific > code, see 8f54969dc8d6 ("x86/acpi: Introduce persistent storage for > cpuid <-> apicid mapping"). I didn't check for other architectures but > as CPU id allocation is in the arch part, I believe this is up to each > arch to deal with this issue. > > Making the CPU id allocation common to all arch is outside the scope > of this patch. Well... we'd better avoid a situation where architectures impose different policies in this area. (I guess we're already in this situation, which is the problem.) A more maintainable way to achieve that would be to put the higher-level policy in arch-independent code, as much as possible. I don't insist, though. >> I don't know, should we not fail the request instead of doing the >> ABI-breaking thing the code in this change is trying to prevent? I >> don't think a warning in the kernel log is going to help any >> application that would be affected by this. > > That's a really good question. One should argue that the most > important is to satisfy the CPU add operation, assuming that only few > are interested in the CPU numbering, while others would prefer the CPU > adding to fail (which may prevent adding CPUs on another nodes if the > whole operation is aborted as soon as a CPU add is failing). > > I was conservative here, but if failing the operation is the best > option, then this will make that code simpler, removing the backward > jump. > > Who is deciding? I favor failing the request. Apart from the implications for user space, it's not clear to me that allowing the cpu-node relationship to change once initialized is benign in terms of internal kernel assumptions (e.g. sched domains, workqueues?). And as you say, it would make for more straightforward code.