linux-kernel - Re: [PATCH 12/13] media: coda: Use alloc_ordered

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Message-ID: <63bf19dc9ce534703597e830799aa0866a6156b8.camel@pengutronix.de>
Date:   Tue, 09 May 2023 12:46:33 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Tejun Heo <tj@...nel.org>, jiangshanlai@...il.com
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH 12/13] media: coda: Use alloc_ordered_workqueue() to
 create ordered workqueues

On Mo, 2023-05-08 at 15:50 -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@...nel.org>

Reviewed-by: Philipp Zabel <p.zabel@...gutronix.de>

regards
Philipp