From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751581AbdILSJY (ORCPT <rfc822;w@1wt.eu>);
        Tue, 12 Sep 2017 14:09:24 -0400
Received: from terminus.zytor.com ([65.50.211.136]:41025 "EHLO
        terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751217AbdILSJR (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 12 Sep 2017 14:09:17 -0400
Date: Tue, 12 Sep 2017 11:05:09 -0700
From: tip-bot for Peter Zijlstra <tipbot@zytor.com>
Message-ID: <tip-edd8e41d2e3cbd6ebe13ead30eb1adc6f48cbb33@git.kernel.org>
Cc: mingo@kernel.org, torvalds@linux-foundation.org, hpa@zytor.com,
        linux-kernel@vger.kernel.org, peterz@infradead.org, tglx@linutronix.de
Reply-To: torvalds@linux-foundation.org, mingo@kernel.org, hpa@zytor.com,
        linux-kernel@vger.kernel.org, tglx@linutronix.de, peterz@infradead.org
In-Reply-To: <20170907150614.044460912@infradead.org>
References: <20170907150614.044460912@infradead.org>
To: linux-tip-commits@vger.kernel.org
Subject: [tip:sched/urgent] sched/fair: Plug hole between hotplug and
 active_load_balance()
Git-Commit-ID: edd8e41d2e3cbd6ebe13ead30eb1adc6f48cbb33
X-Mailer: tip-git-log-daemon
Robot-ID: <tip-bot.git.kernel.org>
Robot-Unsubscribe: Contact <mailto:hpa@kernel.org> to get blacklisted from
 these emails
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset=UTF-8
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

Commit-ID:  edd8e41d2e3cbd6ebe13ead30eb1adc6f48cbb33
Gitweb:     http://git.kernel.org/tip/edd8e41d2e3cbd6ebe13ead30eb1adc6f48cbb33
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 7 Sep 2017 17:03:51 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 12 Sep 2017 17:41:04 +0200

sched/fair: Plug hole between hotplug and active_load_balance()

The load balancer applies cpu_active_mask to whatever sched_domains it
finds, however in the case of active_balance there is a hole between
setting rq->{active_balance,push_cpu} and running the stop_machine
work doing the actual migration.

The @push_cpu can go offline in this window, which would result in us
moving a task onto a dead cpu, which is a fairly bad thing.

Double check the active mask before the stop work does the migration.

  CPU0					CPU1

  <SoftIRQ>
					stop_machine(takedown_cpu)
    load_balance()			cpu_stopper_thread()
      ...				  work = multi_cpu_stop
      stop_one_cpu_nowait(		    /* wait for CPU0 */
	.func = active_load_balance_cpu_stop
      );
  </SoftIRQ>

  cpu_stopper_thread()
    work = multi_cpu_stop
      /* sync with CPU1 */
					    take_cpu_down()
					<idle>
					  play_dead();

    work = active_load_balance_cpu_stop
      set_task_cpu(p, CPU1); /* oops!! */

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170907150614.044460912@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3bcea40..efeebed 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8560,6 +8560,13 @@ static int active_load_balance_cpu_stop(void *data)
 	struct rq_flags rf;
 
 	rq_lock_irq(busiest_rq, &rf);
+	/*
+	 * Between queueing the stop-work and running it is a hole in which
+	 * CPUs can become inactive. We should not move tasks from or to
+	 * inactive CPUs.
+	 */
+	if (!cpu_active(busiest_cpu) || !cpu_active(target_cpu))
+		goto out_unlock;
 
 	/* make sure the requested cpu hasn't gone down in the meantime */
 	if (unlikely(busiest_cpu != smp_processor_id() ||