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From: Peter Zijlstra <peterz@infradead.org>
To: mingo@kernel.org, tglx@linutronix.de
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org
Subject: [PATCH 2/4] sched/fair: Plug hole between hotplug and active load_balance
Date: Thu, 07 Sep 2017 17:03:51 +0200
Message-ID: <20170907150614.044460912@infradead.org> (raw)
In-Reply-To: <20170907150349.063852465@infradead.org>


[-- Attachment #0: peterz-sched-active-balance-hole.patch --]
[-- Type: text/plain, Size: 1641 bytes --]

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>
---
 kernel/sched/fair.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8560,6 +8560,13 @@ static int active_load_balance_cpu_stop(
 	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() ||

  parent reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-09-07 15:03 [PATCH 0/4] sched: Fix some load-balancer vs hotplug holes Peter Zijlstra
2017-09-07 15:03 ` [PATCH 1/4] sched/fair: Avoid newidle balance for !active CPUs Peter Zijlstra
2017-09-12 18:04   ` [tip:sched/urgent] " tip-bot for Peter Zijlstra
2017-09-07 15:03 ` Peter Zijlstra [this message]
2017-09-12 18:05   ` [tip:sched/urgent] sched/fair: Plug hole between hotplug and active_load_balance() tip-bot for Peter Zijlstra
2017-09-07 15:03 ` [PATCH 3/4] sched: WARN when migrating to an offline CPU Peter Zijlstra
2017-09-12 18:05   ` [tip:sched/urgent] sched/core: WARN() " tip-bot for Peter Zijlstra
2017-09-28  9:14   ` [PATCH 3/4] sched: WARN " Sasha Levin
2017-09-28 10:35     ` Peter Zijlstra
2017-09-28 11:03       ` Levin, Alexander (Sasha Levin)
2017-09-28 11:42         ` Peter Zijlstra
2017-09-29 11:11           ` Peter Zijlstra
2017-10-07  2:07             ` Levin, Alexander (Sasha Levin)
2017-10-07  9:15               ` Peter Zijlstra
     [not found]                 ` <20171007174327.ky6g5viokxg5ysdm@sasha-lappy>
2017-10-09  8:04                   ` Peter Zijlstra
2017-10-10  1:18                     ` Levin, Alexander (Sasha Levin)
2017-09-07 15:03 ` [PATCH 4/4] sched/debug: Add debugfs knob for "sched_debug" Peter Zijlstra
2017-09-12 18:05   ` [tip:sched/urgent] " tip-bot for Peter Zijlstra

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