Re: [PATCH 01/22] powerpc, workqueue: Use alloc_ordered_workqueue() to create ordered workqueues

Michael Ellerman
2023-04-21T05:20:21Z

Tejun Heo <tj@kernel.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@kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Nathan Lynch <nathanl@linux.ibm.com>
> Cc: linuxppc-dev@lists.ozlabs.org
> ---
> arch/powerpc/kernel/tau_6xx.c | 2 +-
> arch/powerpc/platforms/pseries/dlpar.c | 3 +--
> 2 files changed, 2 insertions(+), 3 deletions(-)

Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)

> diff --git a/arch/powerpc/kernel/tau_6xx.c b/arch/powerpc/kernel/tau_6xx.c
> index 828d0f4106d2..cba6dd15de3b 100644
> --- a/arch/powerpc/kernel/tau_6xx.c
> +++ b/arch/powerpc/kernel/tau_6xx.c
> @@ -200,7 +200,7 @@ static int __init TAU_init(void)
> tau_int_enable = IS_ENABLED(CONFIG_TAU_INT) &&
> !strcmp(cur_cpu_spec->platform, "ppc750");
>
> - tau_workq = alloc_workqueue("tau", WQ_UNBOUND, 1);
> + tau_workq = alloc_ordered_workqueue("tau", 0);
> if (!tau_workq)
> return -ENOMEM;
>
> diff --git a/arch/powerpc/platforms/pseries/dlpar.c b/arch/powerpc/platforms/pseries/dlpar.c
> index 75ffdbcd2865..e9117b03807e 100644
> --- a/arch/powerpc/platforms/pseries/dlpar.c
> +++ b/arch/powerpc/platforms/pseries/dlpar.c
> @@ -564,8 +564,7 @@ int __init dlpar_workqueue_init(void)
> if (pseries_hp_wq)
> return 0;
>
> - pseries_hp_wq = alloc_workqueue("pseries hotplug workqueue",
> - WQ_UNBOUND, 1);
> + pseries_hp_wq = alloc_ordered_workqueue("pseries hotplug workqueue", 0);
>
> return pseries_hp_wq ? 0 : -ENOMEM;
> }

The change log of commit 9054619ef54a ("powerpc/pseries: Add pseries
hotplug workqueue") makes it fairly clear that this code does explicitly
want an ordered queue.

cheers