path: root/mm/backing-dev.c
diff options
authorTejun Heo <tj@kernel.org>2018-05-23 10:56:32 -0700
committerJens Axboe <axboe@kernel.dk>2018-05-23 15:28:50 -0600
commitf183464684190bacbfb14623bd3e4e51b7575b4c (patch)
tree4141affe6a532a73d8072feaa0f5d3b7b07131de /mm/backing-dev.c
parentnbd: set discard granularity properly (diff)
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>
Diffstat (limited to '')
1 files changed, 17 insertions, 1 deletions
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 7441bd93b732..8fe3ebd6ac00 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -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,
- 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)
+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;
static int cgwb_bdi_init(struct backing_dev_info *bdi)