diff mbox series

bcachefs: Use alloc_ordered_workqueue() to create ordered workqueues

Message ID ZcF2JaEuwMdg9kYi@slm.duckdns.org
State New
Headers
Series bcachefs: Use alloc_ordered_workqueue() to create ordered workqueues |

Commit Message

Tejun Heo Feb. 5, 2024, 11:58 p.m. UTC 
  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: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Brian Foster <bfoster@redhat.com>
---
 fs/bcachefs/btree_update_interior.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Kent Overstreet Feb. 6, 2024, 12:10 a.m. UTC | #1 
On Mon, Feb 05, 2024 at 01:58:29PM -1000, 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: Kent Overstreet <kent.overstreet@linux.dev>
> Cc: Brian Foster <bfoster@redhat.com>
> ---
>  fs/bcachefs/btree_update_interior.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/fs/bcachefs/btree_update_interior.c
> +++ b/fs/bcachefs/btree_update_interior.c
> @@ -2484,7 +2484,7 @@ void bch2_fs_btree_interior_update_init_
>  int bch2_fs_btree_interior_update_init(struct bch_fs *c)
>  {
>  	c->btree_interior_update_worker =
> -		alloc_workqueue("btree_update", WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> +		alloc_ordered_workqueue("btree_update", WQ_MEM_RECLAIM);
>  	if (!c->btree_interior_update_worker)
>  		return -BCH_ERR_ENOMEM_btree_interior_update_worker_init;
>  

This one doesn't actually need to be ordered - if anything, we might
want to bump up max_active.

Tejun, can you drop this patch? If you're trying to get rid of all
WQ_UNBOUND with max_active=1 workqueues for your own auditing, either
you or I could change that to 8.
Tejun Heo Feb. 6, 2024, 12:13 a.m. UTC | #2 
Hello, Kent.

On Mon, Feb 05, 2024 at 07:10:25PM -0500, Kent Overstreet wrote:
> This one doesn't actually need to be ordered - if anything, we might
> want to bump up max_active.
> 
> Tejun, can you drop this patch? If you're trying to get rid of all

Will drop it.

> WQ_UNBOUND with max_active=1 workqueues for your own auditing, either
> you or I could change that to 8.

Yeah, can you please either add a comment saying that it doesn't require
ordered execution or bump max_active?

Thanks.
diff mbox series

Patch

--- a/fs/bcachefs/btree_update_interior.c
+++ b/fs/bcachefs/btree_update_interior.c
@@ -2484,7 +2484,7 @@  void bch2_fs_btree_interior_update_init_
 int bch2_fs_btree_interior_update_init(struct bch_fs *c)
 {
 	c->btree_interior_update_worker =
-		alloc_workqueue("btree_update", WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
+		alloc_ordered_workqueue("btree_update", WQ_MEM_RECLAIM);
 	if (!c->btree_interior_update_worker)
 		return -BCH_ERR_ENOMEM_btree_interior_update_worker_init;