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Date:   Fri, 21 Apr 2023 03:15:56 -0500
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Subject: Re: [PATCH 02/22] greybus: Use alloc_ordered_workqueue() to create
 ordered workqueues
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To:     Tejun Heo <tj@kernel.org>, jiangshanlai@gmail.com
Cc:     linux-kernel@vger.kernel.org, kernel-team@meta.com,
        Johan Hovold <johan@kernel.org>, Alex Elder <elder@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        greybus-dev@lists.linaro.org
References: <20230421025046.4008499-1-tj@kernel.org>
 <20230421025046.4008499-3-tj@kernel.org>
From:   Alex Elder <elder@ieee.org>
In-Reply-To: <20230421025046.4008499-3-tj@kernel.org>
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On 4/20/23 9:50 PM, 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.

Both of the workqueues affected here should be ordered.

Acked-by: Alex Elder <elder@linaro.org>

> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Johan Hovold <johan@kernel.org>
> Cc: Alex Elder <elder@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: greybus-dev@lists.linaro.org
> ---
>   drivers/greybus/connection.c | 4 ++--
>   drivers/greybus/svc.c        | 2 +-
>   2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/greybus/connection.c b/drivers/greybus/connection.c
> index e3799a53a193..9c88861986c8 100644
> --- a/drivers/greybus/connection.c
> +++ b/drivers/greybus/connection.c
> @@ -187,8 +187,8 @@ _gb_connection_create(struct gb_host_device *hd, int hd_cport_id,
>   	spin_lock_init(&connection->lock);
>   	INIT_LIST_HEAD(&connection->operations);
>   
> -	connection->wq = alloc_workqueue("%s:%d", WQ_UNBOUND, 1,
> -					 dev_name(&hd->dev), hd_cport_id);
> +	connection->wq = alloc_ordered_workqueue("%s:%d", 0, dev_name(&hd->dev),
> +						 hd_cport_id);
>   	if (!connection->wq) {
>   		ret = -ENOMEM;
>   		goto err_free_connection;
> diff --git a/drivers/greybus/svc.c b/drivers/greybus/svc.c
> index 16cced80867a..0d7e749174a4 100644
> --- a/drivers/greybus/svc.c
> +++ b/drivers/greybus/svc.c
> @@ -1318,7 +1318,7 @@ struct gb_svc *gb_svc_create(struct gb_host_device *hd)
>   	if (!svc)
>   		return NULL;
>   
> -	svc->wq = alloc_workqueue("%s:svc", WQ_UNBOUND, 1, dev_name(&hd->dev));
> +	svc->wq = alloc_ordered_workqueue("%s:svc", 0, dev_name(&hd->dev));
>   	if (!svc->wq) {
>   		kfree(svc);
>   		return NULL;