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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 E0B1DC4CECD for ; Tue, 17 Sep 2019 16:00:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C1EC5214AF for ; Tue, 17 Sep 2019 16:00:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727677AbfIQQAk (ORCPT ); Tue, 17 Sep 2019 12:00:40 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:42604 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727203AbfIQQAj (ORCPT ); Tue, 17 Sep 2019 12:00:39 -0400 Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1iAFu2-0002Qp-Ir; Tue, 17 Sep 2019 18:00:30 +0200 Date: Tue, 17 Sep 2019 18:00:30 +0200 From: Sebastian Andrzej Siewior To: Scott Wood Cc: Thomas Gleixner , Steven Rostedt , Peter Zijlstra , Juri Lelli , Daniel Bristot de Oliveira , Clark Williams , linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Subject: Re: [PATCH RT 7/8] sched: migrate_enable: Use select_fallback_rq() Message-ID: <20190917160030.i24gvyye2bpdykfy@linutronix.de> References: <20190727055638.20443-1-swood@redhat.com> <20190727055638.20443-8-swood@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190727055638.20443-8-swood@redhat.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019-07-27 00:56:37 [-0500], Scott Wood wrote: > migrate_enable() currently open-codes a variant of select_fallback_rq(). > However, it does not have the "No more Mr. Nice Guy" fallback and thus > it will pass an invalid CPU to the migration thread if cpus_mask only > contains a CPU that is !active. > > Signed-off-by: Scott Wood > --- > This scenario will be more likely after the next patch, since > the migrate_disable_update check goes away. However, it could happen > anyway if cpus_mask was updated to a CPU other than the one we were > pinned to, and that CPU subsequently became inactive. I'm unclear about the problem / side effect this has (before and after the change). It is possible (before and after that change) that a CPU is selected which is invalid / goes offline after the "preempt_enable()" statement and before stop_one_cpu() does its job, correct? > --- > kernel/sched/core.c | 25 ++++++++++--------------- > 1 file changed, 10 insertions(+), 15 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index eb27a9bf70d7..3a2d8251a30c 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -7368,6 +7368,7 @@ void migrate_enable(void) > if (p->migrate_disable_update) { > struct rq *rq; > struct rq_flags rf; > + int cpu = task_cpu(p); > > rq = task_rq_lock(p, &rf); > update_rq_clock(rq); > @@ -7377,21 +7378,15 @@ void migrate_enable(void) > > p->migrate_disable_update = 0; > > - WARN_ON(smp_processor_id() != task_cpu(p)); > - if (!cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) { > - const struct cpumask *cpu_valid_mask = cpu_active_mask; > - struct migration_arg arg; > - unsigned int dest_cpu; > - > - if (p->flags & PF_KTHREAD) { > - /* > - * Kernel threads are allowed on online && !active CPUs > - */ > - cpu_valid_mask = cpu_online_mask; > - } > - dest_cpu = cpumask_any_and(cpu_valid_mask, &p->cpus_mask); > - arg.task = p; > - arg.dest_cpu = dest_cpu; > + WARN_ON(smp_processor_id() != cpu); > + if (!cpumask_test_cpu(cpu, &p->cpus_mask)) { > + struct migration_arg arg = { p }; > + struct rq_flags rf; > + > + rq = task_rq_lock(p, &rf); > + update_rq_clock(rq); > + arg.dest_cpu = select_fallback_rq(cpu, p); > + task_rq_unlock(rq, p, &rf); > > unpin_current_cpu(); > preempt_lazy_enable(); Sebastian