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Date: Mon, 5 Feb 2024 19:10:25 -0500
From: Kent Overstreet <kent.overstreet@linux.dev>
To: Tejun Heo <tj@kernel.org>
Cc: Brian Foster <bfoster@redhat.com>, linux-bcachefs@vger.kernel.org, 
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] bcachefs: Use alloc_ordered_workqueue() to create
 ordered workqueues
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On Mon, Feb 05, 2024 at 01:58:29PM -1000, Tejun Heo wrote:
> BACKGROUND
> ==========
> 
> When multiple work items are queued to a workqueue, their execution order
> doesn't match the queueing order. They may get executed in any order and
> simultaneously. When fully serialized execution - one by one in the queueing
> order - is needed, an ordered workqueue should be used which can be created
> with alloc_ordered_workqueue().
> 
> However, alloc_ordered_workqueue() was a later addition. Before it, an
> ordered workqueue could be obtained by creating an UNBOUND workqueue with
> @max_active==1. This originally was an implementation side-effect which was
> broken by 4c16bd327c74 ("workqueue: restore WQ_UNBOUND/max_active==1 to be
> ordered"). Because there were users that depended on the ordered execution,
> 5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered")
> made workqueue allocation path to implicitly promote UNBOUND workqueues w/
> @max_active==1 to ordered workqueues.
> 
> While this has worked okay, overloading the UNBOUND allocation interface
> this way creates other issues. It's difficult to tell whether a given
> workqueue actually needs to be ordered and users that legitimately want a
> min concurrency level wq unexpectedly gets an ordered one instead. With
> planned UNBOUND workqueue updates to improve execution locality and more
> prevalence of chiplet designs which can benefit from such improvements, this
> isn't a state we wanna be in forever.
> 
> This patch series audits all callsites that create an UNBOUND workqueue w/
> @max_active==1 and converts them to alloc_ordered_workqueue() as necessary.
> 
> WHAT TO LOOK FOR
> ================
> 
> The conversions are from
> 
>   alloc_workqueue(WQ_UNBOUND | flags, 1, args..)
> 
> to 
> 
>   alloc_ordered_workqueue(flags, args...)
> 
> which don't cause any functional changes. If you know that fully ordered
> execution is not ncessary, please let me know. I'll drop the conversion and
> instead add a comment noting the fact to reduce confusion while conversion
> is in progress.
> 
> If you aren't fully sure, it's completely fine to let the conversion
> through. The behavior will stay exactly the same and we can always
> reconsider later.
> 
> As there are follow-up workqueue core changes, I'd really appreciate if the
> patch can be routed through the workqueue tree w/ your acks. Thanks.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Kent Overstreet <kent.overstreet@linux.dev>
> Cc: Brian Foster <bfoster@redhat.com>
> ---
>  fs/bcachefs/btree_update_interior.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/fs/bcachefs/btree_update_interior.c
> +++ b/fs/bcachefs/btree_update_interior.c
> @@ -2484,7 +2484,7 @@ void bch2_fs_btree_interior_update_init_
>  int bch2_fs_btree_interior_update_init(struct bch_fs *c)
>  {
>  	c->btree_interior_update_worker =
> -		alloc_workqueue("btree_update", WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> +		alloc_ordered_workqueue("btree_update", WQ_MEM_RECLAIM);
>  	if (!c->btree_interior_update_worker)
>  		return -BCH_ERR_ENOMEM_btree_interior_update_worker_init;
>  

This one doesn't actually need to be ordered - if anything, we might
want to bump up max_active.

Tejun, can you drop this patch? If you're trying to get rid of all
WQ_UNBOUND with max_active=1 workqueues for your own auditing, either
you or I could change that to 8.