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Date:   Fri, 21 Apr 2023 10:02:39 -0400
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Subject: Re: [PATCH 03/22] IB/hfi1: Use alloc_ordered_workqueue() to create
 ordered workqueues
Content-Language: en-US
To:     Tejun Heo <tj@kernel.org>, jiangshanlai@gmail.com
Cc:     linux-kernel@vger.kernel.org, kernel-team@meta.com,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Leon Romanovsky <leon@kernel.org>, linux-rdma@vger.kernel.org
References: <20230421025046.4008499-1-tj@kernel.org>
 <20230421025046.4008499-4-tj@kernel.org>
From:   Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
In-Reply-To: <20230421025046.4008499-4-tj@kernel.org>
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On 4/20/23 10:50 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.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> Cc: Leon Romanovsky <leon@kernel.org>
> Cc: linux-rdma@vger.kernel.org
> ---
>  drivers/infiniband/hw/hfi1/init.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/infiniband/hw/hfi1/init.c b/drivers/infiniband/hw/hfi1/init.c
> index 62b6c5020039..e03d867cda13 100644
> --- a/drivers/infiniband/hw/hfi1/init.c
> +++ b/drivers/infiniband/hw/hfi1/init.c
> @@ -755,14 +755,13 @@ static int create_workqueues(struct hfi1_devdata *dd)
>  		}
>  		if (!ppd->link_wq) {
>  			/*
> -			 * Make the link workqueue single-threaded to enforce
> +			 * Make the link workqueue ordered to enforce
>  			 * serialization.
>  			 */
>  			ppd->link_wq =
> -				alloc_workqueue(
> +				alloc_ordered_workqueue(
>  				    "hfi_link_%d_%d",
> -				    WQ_SYSFS | WQ_MEM_RECLAIM | WQ_UNBOUND,
> -				    1, /* max_active */
> +				    WQ_SYSFS | WQ_MEM_RECLAIM,
>  				    dd->unit, pidx);
>  			if (!ppd->link_wq)
>  				goto wq_error;

Seems OK to me.

Acked-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>