Re: [PATCH 3/3] xfs: set WQ_SYSFS on all workqueues

["Darrick J. Wong" <djwong@kernel.org>]

On Sun, Jan 24, 2021 at 09:54:54AM +0000, Christoph Hellwig wrote:
> >  	log->l_ioend_workqueue = alloc_workqueue("xfs-log/%s",
> > -			WQ_MEM_RECLAIM | WQ_FREEZABLE | WQ_HIGHPRI, 0,
> > -			mp->m_super->s_id);
> > +			WQ_SYSFS | WQ_MEM_RECLAIM | WQ_FREEZABLE | WQ_HIGHPRI,
> > +			0, mp->m_super->s_id);
> 
> This is just used for log I/O completions which are effectlively single
> thread.  I don't see any reason to adjust the parameters here.
> 
> >  	if (!log->l_ioend_workqueue)
> >  		goto out_free_iclog;
> >  
> > diff --git a/fs/xfs/xfs_mru_cache.c b/fs/xfs/xfs_mru_cache.c
> > index a06661dac5be..b6dab34e361d 100644
> > --- a/fs/xfs/xfs_mru_cache.c
> > +++ b/fs/xfs/xfs_mru_cache.c
> > @@ -294,7 +294,7 @@ int
> >  xfs_mru_cache_init(void)
> >  {
> >  	xfs_mru_reap_wq = alloc_workqueue("xfs_mru_cache",
> > -				WQ_MEM_RECLAIM|WQ_FREEZABLE, 1);
> > +				WQ_SYSFS | WQ_MEM_RECLAIM | WQ_FREEZABLE, 1);
> >  	if (!xfs_mru_reap_wq)
> >  		return -ENOMEM;
> 
> This one also hasn't ever been something we tune, so I don't think there
> is a good case for enabling WQ_SYSFS.

Yeah, the only ones I want to push for (and hence document) are
quotacheck, background blockgc, and (in 5.13) background inode
inactivation.

> I've stopped here.  I think we should have a good use case for making
> workqueues show up in sysfs based on that we:
> 
>  a) have resons to adjust them ever
>  b) actually having them easily discoverable and documented for adminds
>     to tune

TBH I think the only workqueues we really ought to expose publicly are
the unbound ones, since they represent kernel threads that can log
transactions, and hence are known to have a performance impact that
sysadmins could tune reasonably.

Dave suggests exposing them all on a debug kernel, of course. :)

--D