RE: [EXT] [PATCH 06/22] net: thunderx: Use alloc_ordered_workqueue() to create ordered workqueues

Sunil Kovvuri Goutham
2023-04-21T06:19:48Z

> -----Original Message-----
> From: Tejun Heo <htejun@gmail.com> On Behalf Of Tejun Heo
> Sent: Friday, April 21, 2023 8:21 AM
> To: jiangshanlai@gmail.com
> Cc: linux-kernel@vger.kernel.org; kernel-team@meta.com; Tejun Heo
> <tj@kernel.org>; Sunil Kovvuri Goutham <sgoutham@marvell.com>; David S.
> Miller <davem@davemloft.net>; Eric Dumazet <edumazet@google.com>; Jakub
> Kicinski <kuba@kernel.org>; Paolo Abeni <pabeni@redhat.com>; linux-arm-
> kernel@lists.infradead.org; netdev@vger.kernel.org
> Subject: [EXT] [PATCH 06/22] net: thunderx: Use alloc_ordered_workqueue() to
> create ordered workqueues
>
> External Email
>
> ----------------------------------------------------------------------
> 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: Sunil Goutham <sgoutham@marvell.com>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: netdev@vger.kernel.org
> ---
> drivers/net/ethernet/cavium/thunder/thunder_bgx.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/net/ethernet/cavium/thunder/thunder_bgx.c
> b/drivers/net/ethernet/cavium/thunder/thunder_bgx.c
> index 7eb2ddbe9bad..a317feb8decb 100644
> --- a/drivers/net/ethernet/cavium/thunder/thunder_bgx.c
> +++ b/drivers/net/ethernet/cavium/thunder/thunder_bgx.c
> @@ -1126,8 +1126,7 @@ static int bgx_lmac_enable(struct bgx *bgx, u8
> lmacid)
> }
>
> poll:
> - lmac->check_link = alloc_workqueue("check_link", WQ_UNBOUND |
> - WQ_MEM_RECLAIM, 1);
> + lmac->check_link = alloc_ordered_workqueue("check_link",
> +WQ_MEM_RECLAIM);
> if (!lmac->check_link)
> return -ENOMEM;
> INIT_DELAYED_WORK(&lmac->dwork, bgx_poll_for_link);
> --
> 2.40.0

Reviewed-by: Sunil Goutham <sgoutham@marvell.com>