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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id F40D2C6FD20 for ; Fri, 24 Mar 2023 14:42:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231596AbjCXOm5 (ORCPT ); Fri, 24 Mar 2023 10:42:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57496 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230196AbjCXOm4 (ORCPT ); Fri, 24 Mar 2023 10:42:56 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4B76A1BF3 for ; Fri, 24 Mar 2023 07:42:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1679668933; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=To5ecTC/X7saxKtjRIArD6KqaoPoVn1lZB9f4SzUO/k=; b=bvUAiKlFPl198OVeGOxbw1NSwZIZcZGBc8b+RWOqkx/Wl9NL7y4iDFpiyXiBdxk2QbiRnu su4UXKMCuImh5I9kA5NIDKwbQv7gR0Qodb7LEMNY1M7m3VlGMt5nBVpBz0fMRIiia10Drl 66IhQjMlQwF2lXCOJ6AwEDH+FIgzkJg= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-556-XODxuWeWN7ad9yu4e0v4Dw-1; Fri, 24 Mar 2023 10:42:10 -0400 X-MC-Unique: XODxuWeWN7ad9yu4e0v4Dw-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.rdu2.redhat.com [10.11.54.7]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id AD3CC802314; Fri, 24 Mar 2023 14:42:09 +0000 (UTC) Received: from [10.22.33.184] (unknown [10.22.33.184]) by smtp.corp.redhat.com (Postfix) with ESMTP id 360CD140EBF4; Fri, 24 Mar 2023 14:42:09 +0000 (UTC) Message-ID: Date: Fri, 24 Mar 2023 10:42:09 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1 Subject: Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found in current cpuset Content-Language: en-US To: Will Deacon Cc: =?UTF-8?Q?Michal_Koutn=c3=bd?= , Tejun Heo , Zefan Li , Johannes Weiner , Shuah Khan , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Peter Zijlstra References: <20230306200849.376804-1-longman@redhat.com> <20230306200849.376804-4-longman@redhat.com> <20230314181749.5b4k6selbgdhl3up@blackpad> <58a1a878-fa0b-285d-3e43-2b5103d3c770@redhat.com> <20230317122708.ax3m2d4zijkfdzjq@blackpad> <20230324143247.GA27199@willie-the-truck> From: Waiman Long In-Reply-To: <20230324143247.GA27199@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org On 3/24/23 10:32, Will Deacon wrote: > On Fri, Mar 17, 2023 at 10:59:26AM -0400, Waiman Long wrote: >> On 3/17/23 08:27, Michal Koutný wrote: >>> On Tue, Mar 14, 2023 at 04:22:06PM -0400, Waiman Long wrote: >>>> Some arm64 systems can have asymmetric CPUs where certain tasks are only >>>> runnable on a selected subset of CPUs. >>> Ah, I'm catching up. >>> >>>> This information is not captured in the cpuset. As a result, >>>> task_cpu_possible_mask() may return a mask that have no overlap with >>>> effective_cpus causing new_cpus to become empty. >>> I can see that historically, there was an approach of terminating >>> unaccomodable tasks: >>> 94f9c00f6460 ("arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores") >>> the removal of killing had been made possible with >>> df950811f4a8 ("arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system"). >>> >>> That gives two other alternatives to affinity modification: >>> 2) kill such tasks (not unlike OOM upon memory.max reduction), >>> 3) reject cpuset reduction (violates cgroup v2 delegation). >>> >>> What do you think about 2)? >> Yes, killing it is one possible solution. >> >> (3) doesn't work if the affinity change is due to hot cpu removal. So that >> leaves this patch or (2) as the only alternative. I would like to hear what >> Will and Tejun thinks about it. > The main constraint from the Android side (the lucky ecosystem where these > SoCs tend to show up) is that existing userspace (including 32-bit binaries) > continues to function without modification. So approaches such as killing > tasks or rejecting system calls tend not to work as well, since you > inevitably get divergent behaviour leading to functional breakage rather > than e.g. performance anomalies. > > Having said that, the behaviour we currently have in mainline seems to > be alright, so please don't go out of your way to accomodate these SoCs. > I'm mainly just concerned about introducing any regressions, which is why > I ran my tests on this series I agree that killing it may be too draconian. I am withholding this patch for now. Thanks, Longman