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 C89F2C4167B for ; Tue, 5 Dec 2023 22:16:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346490AbjLEWQV (ORCPT ); Tue, 5 Dec 2023 17:16:21 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50560 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346478AbjLEWQT (ORCPT ); Tue, 5 Dec 2023 17:16:19 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 12D65137 for ; Tue, 5 Dec 2023 14:16:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701814584; 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=h2Vem31mI/E1rxmbZRCBYAPkrSR+YjFerajC0+h+2EI=; b=dVat5q1A35wXT8Nf76O/hEt5AccD2IDRmLUFLYZ1qCJdVHhmdINS3DrF6alqiduTNoJCi1 AvV/iayvbS+ZVgHTDJlo9YDguBo/+15AXixy4M3mo+gDmODm9w/LC7HPjsRAyleTuglDnU 2oXkYH/hw1EULws8XcuNJ1U+ShZXaKU= 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-433-3LYAxG9aN0qe_CfFGknouw-1; Tue, 05 Dec 2023 17:16:16 -0500 X-MC-Unique: 3LYAxG9aN0qe_CfFGknouw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (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 F03511C0BB41; Tue, 5 Dec 2023 22:16:15 +0000 (UTC) Received: from [10.22.8.88] (unknown [10.22.8.88]) by smtp.corp.redhat.com (Postfix) with ESMTP id DCC8C112131D; Tue, 5 Dec 2023 22:16:14 +0000 (UTC) Message-ID: <7284ef19-ba26-46cd-9630-cad18c2e3ce7@redhat.com> Date: Tue, 5 Dec 2023 17:16:14 -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 10.11.54.3 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/1/23 12:06, Tejun Heo wrote: > Hello, > > On Wed, Nov 29, 2023 at 11:01:04AM -0500, Waiman Long wrote: > ... >>>> 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 > Setting and clearing a bit is as atomic as it gets, right? Yes, I think so. > >> 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 just can't think of a case where this would be broken. The data being read > and written is atomic. There's no way to break a bit operation into multiple > pieces. It is possible to write a really bone-headed bitmask operations > (like, if you shift the bits into place or sth) to make the bits go through > unintended changes but that'd just be a flat-out broken implementation. Even > for a bitmask where write accesses are synchronized through a spinlock, we > should still be able to use test_bit() without holding the lock. This seems > like a pretty basic assumption. > > Adding unnecessary synchronization confuses the readers. If we don't need > it, we shouldn't have it. OK, I will send a simplified v2 patch. Cheers, Longman