Commit f1834646 authored May 23, 2018 by Tejun Heo Committed by Jens Axboe May 23, 2018

bdi: Move cgroup bdi_writeback to a dedicated low concurrency workqueue

From 0aa2e9b921d6db71150633ff290199554f0842a8 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 23 May 2018 10:29:00 -0700

cgwb_release() punts the actual release to cgwb_release_workfn() on
system_wq.  Depending on the number of cgroups or block devices, there
can be a lot of cgwb_release_workfn() in flight at the same time.

We're periodically seeing close to 256 kworkers getting stuck with the
following stack trace and overtime the entire system gets stuck.

  [<ffffffff810ee40c>] _synchronize_rcu_expedited.constprop.72+0x2fc/0x330
  [<ffffffff810ee634>] synchronize_rcu_expedited+0x24/0x30
  [<ffffffff811ccf23>] bdi_unregister+0x53/0x290
  [<ffffffff811cd1e9>] release_bdi+0x89/0xc0
  [<ffffffff811cd645>] wb_exit+0x85/0xa0
  [<ffffffff811cdc84>] cgwb_release_workfn+0x54/0xb0
  [<ffffffff810a68d0>] process_one_work+0x150/0x410
  [<ffffffff810a71fd>] worker_thread+0x6d/0x520
  [<ffffffff810ad3dc>] kthread+0x12c/0x160
  [<ffffffff81969019>] ret_from_fork+0x29/0x40
  [<ffffffffffffffff>] 0xffffffffffffffff

The events leading to the lockup are...

1. A lot of cgwb_release_workfn() is queued at the same time and all
   system_wq kworkers are assigned to execute them.

2. They all end up calling synchronize_rcu_expedited().  One of them
   wins and tries to perform the expedited synchronization.

3. However, that invovles queueing rcu_exp_work to system_wq and
   waiting for it.  Because #1 is holding all available kworkers on
   system_wq, rcu_exp_work can't be executed.  cgwb_release_workfn()
   is waiting for synchronize_rcu_expedited() which in turn is waiting
   for cgwb_release_workfn() to free up some of the kworkers.

We shouldn't be scheduling hundreds of cgwb_release_workfn() at the
same time.  There's nothing to be gained from that.  This patch
updates cgwb release path to use a dedicated percpu workqueue with
@max_active of 1.

While this resolves the problem at hand, it might be a good idea to
isolate rcu_exp_work to its own workqueue too as it can be used from
various paths and is prone to this sort of indirect A-A deadlocks.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 6df133a1

mm/backing-dev.c

+17 −1

Original line number	Diff line number	Diff line
		@@ -412,6 +412,7 @@ static void wb_exit(struct bdi_writeback *wb)
		* protected.
		*/
		static DEFINE_SPINLOCK(cgwb_lock);
		static struct workqueue_struct *cgwb_release_wq;

		/**
		* wb_congested_get_create - get or create a wb_congested
		@@ -522,7 +523,7 @@ static void cgwb_release(struct percpu_ref *refcnt)
		{
		struct bdi_writeback *wb = container_of(refcnt, struct bdi_writeback,
		refcnt);
		schedule_work(&wb->release_work);
		queue_work(cgwb_release_wq, &wb->release_work);
		}

		static void cgwb_kill(struct bdi_writeback *wb)
		@@ -784,6 +785,21 @@ static void cgwb_bdi_register(struct backing_dev_info *bdi)
		spin_unlock_irq(&cgwb_lock);
		}

		static int __init cgwb_init(void)
		{
		/*
		* There can be many concurrent release work items overwhelming
		* system_wq. Put them in a separate wq and limit concurrency.
		* There's no point in executing many of these in parallel.
		*/
		cgwb_release_wq = alloc_workqueue("cgwb_release", 0, 1);
		if (!cgwb_release_wq)
		return -ENOMEM;

		return 0;
		}
		subsys_initcall(cgwb_init);

		#else /* CONFIG_CGROUP_WRITEBACK */

		static int cgwb_bdi_init(struct backing_dev_info *bdi)

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