Re: [PATCH] bdi: Move cgroup bdi_writeback to a dedicated low concurrency workqueue

[Jens Axboe]

On 5/23/18 11:56 AM, Tejun Heo wrote:
> From 0aa2e9b921d6db71150633ff290199554f0842a8 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@xxxxxxxxxx>
> 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.

Applied, thanks.

-- 
Jens Axboe