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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,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 04527C31E46 for ; Wed, 12 Jun 2019 16:31:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D8A0121019 for ; Wed, 12 Jun 2019 16:31:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2408866AbfFLQbF (ORCPT ); Wed, 12 Jun 2019 12:31:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34614 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2405901AbfFLQbE (ORCPT ); Wed, 12 Jun 2019 12:31:04 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 279E22E97D5; Wed, 12 Jun 2019 16:31:04 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id 23FB16061E; Wed, 12 Jun 2019 16:31:03 +0000 (UTC) Subject: Re: [RESEND PATCH v3] cpuset: restore sanity to cpuset_cpus_allowed_fallback() To: Joel Savitz , linux-kernel@vger.kernel.org Cc: Li Zefan , Phil Auld , Tejun Heo , =?UTF-8?Q?Michal_Koutn=c3=bd?= , Ingo Molnar , Peter Zijlstra , cgroups@vger.kernel.org References: <1560354648-23632-1-git-send-email-jsavitz@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: <3bca146c-14bc-4188-becb-7ebc7348f646@redhat.com> Date: Wed, 12 Jun 2019 12:31:02 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <1560354648-23632-1-git-send-email-jsavitz@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Wed, 12 Jun 2019 16:31:04 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/12/19 11:50 AM, Joel Savitz wrote: > In the case that a process is constrained by taskset(1) (i.e. > sched_setaffinity(2)) to a subset of available cpus, and all of those are > subsequently offlined, the scheduler will set tsk->cpus_allowed to > the current value of task_cs(tsk)->effective_cpus. > > This is done via a call to do_set_cpus_allowed() in the context of > cpuset_cpus_allowed_fallback() made by the scheduler when this case is > detected. This is the only call made to cpuset_cpus_allowed_fallback() > in the latest mainline kernel. > > However, this is not sane behavior. > > I will demonstrate this on a system running the latest upstream kernel > with the following initial configuration: > > # grep -i cpu /proc/$$/status > Cpus_allowed: ffffffff,fffffff > Cpus_allowed_list: 0-63 > > (Where cpus 32-63 are provided via smt.) > > If we limit our current shell process to cpu2 only and then offline it > and reonline it: > > # taskset -p 4 $$ > pid 2272's current affinity mask: ffffffffffffffff > pid 2272's new affinity mask: 4 > > # echo off > /sys/devices/system/cpu/cpu2/online > # dmesg | tail -3 > [ 2195.866089] process 2272 (bash) no longer affine to cpu2 > [ 2195.872700] IRQ 114: no longer affine to CPU2 > [ 2195.879128] smpboot: CPU 2 is now offline > > # echo on > /sys/devices/system/cpu/cpu2/online > # dmesg | tail -1 > [ 2617.043572] smpboot: Booting Node 0 Processor 2 APIC 0x4 > > > We see that our current process now has an affinity mask containing > every cpu available on the system _except_ the one we originally > constrained it to: > > # grep -i cpu /proc/$$/status > Cpus_allowed: ffffffff,fffffffb > Cpus_allowed_list: 0-1,3-63 > > This is not sane behavior, as the scheduler can now not only place the > process on previously forbidden cpus, it can't even schedule it on > the cpu it was originally constrained to! > > Other cases result in even more exotic affinity masks. Take for instance > a process with an affinity mask containing only cpus provided by smt at > the moment that smt is toggled, in a configuration such as the following: > > # taskset -p f000000000 $$ > # grep -i cpu /proc/$$/status > Cpus_allowed: 000000f0,00000000 > Cpus_allowed_list: 36-39 > > A double toggle of smt results in the following behavior: > > # echo off > /sys/devices/system/cpu/smt/control > # echo on > /sys/devices/system/cpu/smt/control > # grep -i cpus /proc/$$/status > Cpus_allowed: ffffff00,ffffffff > Cpus_allowed_list: 0-31,40-63 > > This is even less sane than the previous case, as the new affinity mask > excludes all smt-provided cpus with ids less than those that were > previously in the affinity mask, as well as those that were actually in > the mask. > > With this patch applied, both of these cases end in the following state: > > # grep -i cpu /proc/$$/status > Cpus_allowed: ffffffff,ffffffff > Cpus_allowed_list: 0-63 > > The original policy is discarded. Though not ideal, it is the simplest way > to restore sanity to this fallback case without reinventing the cpuset > wheel that rolls down the kernel just fine in cgroup v2. A user who wishes > for the previous affinity mask to be restored in this fallback case can use > that mechanism instead. > > This patch modifies scheduler behavior by instead resetting the mask to > task_cs(tsk)->cpus_allowed by default, and cpu_possible mask in legacy > mode. I tested the cases above on both modes. > > Note that the scheduler uses this fallback mechanism if and only if > _every_ other valid avenue has been traveled, and it is the last resort > before calling BUG(). > > Suggested-by: Waiman Long > Suggested-by: Phil Auld > 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 6a1942ed781c..515525ff1cfd 100644 > --- a/kernel/cgroup/cpuset.c > +++ b/kernel/cgroup/cpuset.c > @@ -3254,10 +3254,23 @@ 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(); > > /* Acked-by: Waiman Long Good job!