Re: [PATCH 08/13] btrfs: Use alloc_ordered_workqueue() to create ordered workqueues

[Tejun Heo]

Hello, David.

Thanks for taking a look.

On Tue, May 09, 2023 at 04:53:32PM +0200, David Sterba wrote:
> > diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> > index 59ea049fe7ee..32d08aed88b6 100644
> > --- a/fs/btrfs/disk-io.c
> > +++ b/fs/btrfs/disk-io.c
> > @@ -2217,7 +2217,7 @@ static int btrfs_init_workqueues(struct btrfs_fs_info *fs_info)
> >  	fs_info->qgroup_rescan_workers =
> >  		btrfs_alloc_workqueue(fs_info, "qgroup-rescan", flags, 1, 0);
> >  	fs_info->discard_ctl.discard_workers =
> > -		alloc_workqueue("btrfs_discard", WQ_UNBOUND | WQ_FREEZABLE, 1);
> > +		alloc_ordered_workqueue("btrfs_discard", WQ_FREEZABLE);
> >  
> >  	if (!(fs_info->workers && fs_info->hipri_workers &&
> >  	      fs_info->delalloc_workers && fs_info->flush_workers &&
> 
> I think there are a few more conversions missing. There's a local flags
> variable in btrfs_init_workqueues
> 
> 2175 static int btrfs_init_workqueues(struct btrfs_fs_info *fs_info)
> 2176 {
> 2177         u32 max_active = fs_info->thread_pool_size;
> 2178         unsigned int flags = WQ_MEM_RECLAIM | WQ_FREEZABLE | WQ_UNBOUND;
> 
> And used like
> 
> 2194         fs_info->fixup_workers =
> 2195                 btrfs_alloc_workqueue(fs_info, "fixup", flags, 1, 0);
> 
> 2213         fs_info->qgroup_rescan_workers =
> 2214                 btrfs_alloc_workqueue(fs_info, "qgroup-rescan", flags, 1, 0);

Right you are.

> WQ_UNBOUND is not mentioned explicitliy like for the "btrfs_discard"
> workqueue.  Patch v2 did the switch in btrfs_alloc_workqueue according
> to the max_active/limit_active parameter but this would affect all
> queues and not all of them require to be ordered.

The thresh mechanism which auto adjusts max active means that the workqueues
allocated btrfs_alloc_workqueue() can't be ordered, right? When thresh is
smaller than DFT_THRESHOLD, the mechanism is disabled but that looks like an
optimization.

> In btrfs_resize_thread_pool the workqueue_set_max_active is called
> directly or indirectly so this can set the max_active to a user-defined
> mount option. Could this be a problem or trigger a warning? This would
> lead to max_active==1 + WQ_UNBOUND.

That's not a problem. The only thing we need to make sure is that the
workqueues which actually *must* be ordered use alloc_ordered_workqueue() as
they won't be implicitly treated as ordered in the future.

* The current patch converts two - fs_info->discard_ctl.discard_workers and
  scrub_workers when @is_dev_replace is set. Do they actually need to be
  ordered?

* As you pointed out, fs_info->fixup_workers and
  fs_info->qgroup_rescan_workers are also currently implicitly ordered. Do
  they actually need to be ordered?

Thanks.

-- 
tejun