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 34E74C4167B for ; Wed, 29 Nov 2023 16:01:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343595AbjK2QBJ (ORCPT ); Wed, 29 Nov 2023 11:01:09 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37074 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232910AbjK2QBH (ORCPT ); Wed, 29 Nov 2023 11:01:07 -0500 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 BA279D5C for ; Wed, 29 Nov 2023 08:01:13 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701273672; 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=8tNd18nd+bxJ1BZqHDEg4dU9YjfOiYLR5XBMfKbMPiU=; b=GPvIzOjCAuZ/tXt2/skzU+WTeQ+mdCBM8sWxAWfUXUPdrNy8tGLo+5NQUOwBGICfMSzmRK clB8K+5tPtYEDz3U1AzTX0/pYYdbZ+JGD0QlkPaWLto20q3wDYstyPB7zSWmo7Da0FVYyE 7sTuLaH3TFaCVKW/S1J7zJQABuNScYU= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-344-3O6M-KuyMKW4NpqGVr1ctA-1; Wed, 29 Nov 2023 11:01:06 -0500 X-MC-Unique: 3O6M-KuyMKW4NpqGVr1ctA-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id CCDBA3C0C119; Wed, 29 Nov 2023 16:01:05 +0000 (UTC) Received: from [10.22.34.102] (unknown [10.22.34.102]) by smtp.corp.redhat.com (Postfix) with ESMTP id 145FAC1596F; Wed, 29 Nov 2023 16:01:05 +0000 (UTC) Message-ID: Date: Wed, 29 Nov 2023 11:01:04 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH-cgroup 2/2] cgroup/cpuset: Include isolated cpuset CPUs in cpu_is_isolated() check Content-Language: en-US To: Tejun Heo Cc: Zefan Li , Johannes Weiner , Andrew Morton , Michal Hocko , Frederic Weisbecker , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Mrunal Patel , Ryan Phillips , Brent Rowsell , Peter Hunt References: <20231127041956.266026-1-longman@redhat.com> <20231127041956.266026-3-longman@redhat.com> <8de482b5-1942-4312-8de4-6f54565ab517@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.8 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/28/23 17:12, Tejun Heo wrote: > Hello, > > On Tue, Nov 28, 2023 at 01:32:53PM -0500, Waiman Long wrote: >> On 11/28/23 11:56, Tejun Heo wrote: >>> Hello, >>> >>> On Sun, Nov 26, 2023 at 11:19:56PM -0500, Waiman Long wrote: >>>> +bool cpuset_cpu_is_isolated(int cpu) >>>> +{ >>>> + unsigned int seq; >>>> + bool ret; >>>> + >>>> + do { >>>> + seq = read_seqcount_begin(&isolcpus_seq); >>>> + ret = cpumask_test_cpu(cpu, isolated_cpus); >>>> + } while (read_seqcount_retry(&isolcpus_seq, seq)); >>>> + return ret; >>>> +} >>>> +EXPORT_SYMBOL_GPL(cpuset_cpu_is_isolated); >>> We're testing a bit in a bitmask. I don't think we need to worry about value >>> integrity from RMW updates being broken up. ie. We can just test the bit >>> without seqlock and drop the first patch? >> My concern is that if we have an isolated partition with a set of isolated >> CPUs (say 2-4), I don't want any addition, deletion of changes made to >> another isolated partition affects the test of the pre-existing one. Testing >> result of the partition being change is fair game. >> >> Depending on how the cpumask operators are implemented, we may not have a >> guarantee that testing CPU 2, for instance, will always return true. That is > Can you please elaborate this part a bit? I'm having a difficult time > imagining the sequence of operations where this would matter but that could > easily be me not being familiar with the details. I may be a bit paranoid about incorrect result due to racing as I had been burned before. Just testing a bit in the bitmask may probably be OK. I don't think it will be a problem for x86, but I am less certain about other more exotic architectures like arm64 or PPC which I am less familiar about. I add a seqcount for synchronization just for the peace of mind. I can take the seqcount out if you don't it is necessary. I have also been thinking about an alternative helper that returns the whole isolated cpumask since in both cases where cpu_is_isolated() is used, we will have to iterate all the possible CPUs anyway, it will be more efficient to have the whole cpumask available. In that case, we may want to have a seqcount to avoid returning an intermediate result. Anyway, this is just a thought for now, I am not planning to do that at the moment. Cheers, Longman