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Date:   Fri, 26 May 2023 14:52:03 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Tejun Heo <tj@kernel.org>
Cc:     jiangshanlai@gmail.com, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, Wang Yugui <wangyugui@e16-tech.com>,
        Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 08/13] btrfs: Use alloc_ordered_workqueue() to create
 ordered workqueues
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On Thu, May 25, 2023 at 01:33:08PM -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.
> 
> BTRFS
> =====
> 
> * fs_info->scrub_workers initialized in scrub_workers_get() was setting
>   @max_active to 1 when @is_dev_replace is set and it seems that the
>   workqueue actually needs to be ordered if @is_dev_replace. Update the code
>   so that alloc_ordered_workqueue() is used if @is_dev_replace.
> 
> * fs_info->discard_ctl.discard_workers initialized in
>   btrfs_init_workqueues() was directly using alloc_workqueue() w/
>   @max_active==1. Converted to alloc_ordered_workqueue().
> 
> * fs_info->fixup_workers and fs_info->qgroup_rescan_workers initialized in
>   btrfs_queue_work() use the btrfs's workqueue wrapper, btrfs_workqueue,
>   which are allocated with btrfs_alloc_workqueue().
> 
>   btrfs_workqueue implements automatic @max_active adjustment which is
>   disabled when the specified max limix is below a certain threshold, so
>   calling btrfs_alloc_workqueue() with @limit_active==1 yields an ordered
>   workqueue whose @max_active won't be changed as the auto-tuning is
>   disabled.
> 
>   This is rather brittle in that nothing clearly indicates that the two
>   workqueues should be ordered or btrfs_alloc_workqueue() must disable
>   auto-tuning when @limit_active==1.
> 
>   This patch factors out the common btrfs_workqueue init code into
>   btrfs_init_workqueue() and add explicit btrfs_alloc_ordered_workqueue().
>   The two workqueues are converted to use the new ordered allocation
>   interface.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Wang Yugui <wangyugui@e16-tech.com>
> Cc: Chris Mason <clm@fb.com>
> Cc: Josef Bacik <josef@toxicpanda.com>
> Cc: David Sterba <dsterba@suse.com>
> Cc: linux-btrfs@vger.kernel.org
> ---
> Hello,
> 
> David, I think this is a bit too invasive to carry through workqueue tree.
> If this looks okay, can you plase apply route it through the btrfs tree?

Yesd and I actually prefer to take such patches via btrfs tree unless
there's a strong dependency on other patches from another subsystem.
Thanks.