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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS 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 1B1E4C282CE for ; Tue, 9 Apr 2019 20:41:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E14822084B for ; Tue, 9 Apr 2019 20:41:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726558AbfDIUlP (ORCPT ); Tue, 9 Apr 2019 16:41:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34806 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726337AbfDIUlP (ORCPT ); Tue, 9 Apr 2019 16:41:15 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A448B31649AA; Tue, 9 Apr 2019 20:41:14 +0000 (UTC) Received: from intel-knightslanding-lb-02.khw1.lab.eng.bos.redhat.com (intel-knightslanding-lb-02.khw1.lab.eng.bos.redhat.com [10.16.200.87]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9517719022; Tue, 9 Apr 2019 20:41:11 +0000 (UTC) From: Joel Savitz To: linux-kernel@vger.kernel.org Cc: Joel Savitz , Phil Auld , Waiman Long , Tejun Heo , Li Zefan , cgroups@vger.kernel.org Subject: [PATCH v2] cpuset: restore sanity to cpuset_cpus_allowed_fallback() Date: Tue, 9 Apr 2019 16:40:03 -0400 Message-Id: <20190409204003.6428-1-jsavitz@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Tue, 09 Apr 2019 20:41:14 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If a process is limited by taskset (i.e. cpuset) to only be allowed to run on cpu N, and then cpu N is offlined via hotplug, the process will be assigned the current value of its cpuset cgroup's effective_cpus field in a call to do_set_cpus_allowed() in cpuset_cpus_allowed_fallback(). This argument's value does not makes sense for this case, because task_cs(tsk)->effective_cpus is modified by cpuset_hotplug_workfn() to reflect the new value of cpu_active_mask after cpu N is removed from the mask. While this may make sense for the cgroup affinity mask, it does not make sense on a per-task basis, as a task that was previously limited to only be run on cpu N will be limited to every cpu _except_ for cpu N after it is offlined/onlined via hotplug. Pre-patch behavior: $ grep Cpus /proc/$$/status Cpus_allowed: ff Cpus_allowed_list: 0-7 $ taskset -p 4 $$ pid 19202's current affinity mask: f pid 19202's new affinity mask: 4 $ grep Cpus /proc/self/status Cpus_allowed: 04 Cpus_allowed_list: 2 # echo off > /sys/devices/system/cpu/cpu2/online $ grep Cpus /proc/$$/status Cpus_allowed: 0b Cpus_allowed_list: 0-1,3 # echo on > /sys/devices/system/cpu/cpu2/online $ grep Cpus /proc/$$/status Cpus_allowed: 0b Cpus_allowed_list: 0-1,3 On a patched system, the final grep produces the following output instead: $ grep Cpus /proc/$$/status Cpus_allowed: ff Cpus_allowed_list: 0-7 This patch changes the above behavior by instead resetting the mask to task_cs(tsk)->cpus_allowed by default, and cpu_possible mask in legacy mode. This fallback mechanism is only triggered if _every_ other valid avenue has been traveled, and it is the last resort before calling BUG(). Signed-off-by: Joel Savitz --- kernel/cgroup/cpuset.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 4834c4214e9c..6c9deb2cc687 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -3255,10 +3255,23 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) spin_unlock_irqrestore(&callback_lock, flags); } +/** + * cpuset_cpus_allowed_fallback - final fallback before complete catastrophe. + * @tsk: pointer to task_struct with which the scheduler is struggling + * + * Description: In the case that the scheduler cannot find an allowed cpu in + * tsk->cpus_allowed, we fall back to task_cs(tsk)->cpus_allowed. In legacy + * mode however, this value is the same as task_cs(tsk)->effective_cpus, + * which will not contain a sane cpumask during cases such as cpu hotplugging. + * This is the absolute last resort for the scheduler and it is only used if + * _every_ other avenue has been traveled. + **/ + void cpuset_cpus_allowed_fallback(struct task_struct *tsk) { rcu_read_lock(); - do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus); + do_set_cpus_allowed(tsk, is_in_v2_mode() ? + task_cs(tsk)->cpus_allowed : cpu_possible_mask); rcu_read_unlock(); /* -- 2.18.1