linux-kernel - Re: [PATCH] workqueue: flush all pending jobs in destroy

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Message-ID: <aAtXApA8ggJa6sQg@google.com>
Date: Fri, 25 Apr 2025 09:33:54 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tejun Heo <tj@...nel.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>, Philipp Stanner <phasta@...lbox.org>, 
	Danilo Krummrich <dakr@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: flush all pending jobs in destroy_workqueue()

On Thu, Apr 24, 2025 at 09:57:55AM -1000, Tejun Heo wrote:
> Hello, Alice.
> 
> On Wed, Apr 23, 2025 at 05:51:27PM +0000, Alice Ryhl wrote:
> ...
> > @@ -367,6 +367,8 @@ struct workqueue_struct {
> >  	struct lockdep_map	__lockdep_map;
> >  	struct lockdep_map	*lockdep_map;
> >  #endif
> > +	raw_spinlock_t		delayed_lock;	/* protects pending_list */
> > +	struct list_head	delayed_list;	/* list of pending delayed jobs */
> 
> I think we'll have to make this per-CPU or per-pwq. There can be a lot of
> delayed work items being queued on, e.g., system_wq. Imagine that happening
> on a multi-socket NUMA system. That cacheline is going to be bounced around
> pretty hard.

Hmm. I think we would need to add a new field to delayed_work to keep
track of which list it has been added to.

Another option could be to add a boolean that disables the list. After
all, we never call destroy_workqueue() on system_wq so we don't need the
list for that workqueue.

Thoughts?

Alice