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

[Jan Kara]

On Wed 23-05-18 10:56:32, 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.

OK, but that means that you have to have 256 block devices, don't you? As
we have a bdi per device and we call synchronize_rcu_expedited() only when
unregistering bdi (and the corresponding request queue must be gone at that
point as well as that's otherwise holding a reference). Am I understanding
the situation correctly?

>   [<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.

As Rik wrote, some paralelism is good to reduce number of forced grace
periods so raising this to 16 is good. I was thinking whether we could not
batch rcu grace periods in some explicit way but that would be difficult to
do.

But thinking a bit more about this, if we made bdi RCU freed, we could just
avoid the synchronize_rcu_expedited() in bdi_remove_from_list() altogether.
The uses of bdi list are pretty limited and everybody ends up testing
WB_registered bit before doing anything anyway... What do you think?
 
Other than that you can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

to this patch when updated to increase concurrency as that's a good short
term solution (for stable kernels) anyway.

								Honza

> 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@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx



> ---
>  mm/backing-dev.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 7441bd9..8fe3ebd 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,
>  						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)
> -- 
> 2.9.5
> 
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR