linux-kernel - Re: [PATCH 17/22] cifs: Use alloc_ordered_workqueue() to create ordered workqueues

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Message-ID: <b6dfbc63d89715a1298117bc0afeb436.pc@manguebit.com>
Date:   Fri, 21 Apr 2023 15:38:57 -0300
From:   Paulo Alcantara <pc@...guebit.com>
To:     Tejun Heo <tj@...nel.org>, jiangshanlai@...il.com
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        Tejun Heo <tj@...nel.org>, Steve French <sfrench@...ba.org>,
        Paulo Alcantara <pc@....nz>,
        Ronnie Sahlberg <lsahlber@...hat.com>,
        Shyam Prasad N <sprasad@...rosoft.com>,
        Tom Talpey <tom@...pey.com>, linux-cifs@...r.kernel.org,
        samba-technical@...ts.samba.org
Subject: Re: [PATCH 17/22] cifs: Use alloc_ordered_workqueue() to create
 ordered workqueues

Tejun Heo <tj@...nel.org> writes:

> 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>
> Cc: Steve French <sfrench@...ba.org>
> Cc: Paulo Alcantara <pc@....nz>
> Cc: Ronnie Sahlberg <lsahlber@...hat.com>
> Cc: Shyam Prasad N <sprasad@...rosoft.com>
> Cc: Tom Talpey <tom@...pey.com>
> Cc: linux-cifs@...r.kernel.org
> Cc: samba-technical@...ts.samba.org
> ---
>  fs/cifs/dfs_cache.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Acked-by: Paulo Alcantara (SUSE) <pc@...guebit.com>