Re: [RFC PATCH] xfs: implement per-inode writeback completion

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Mar 28, 2019 at 10:08:59AM -0400, Brian Foster wrote:
> On Tue, Mar 26, 2019 at 08:05:50PM -0700, Darrick J. Wong wrote:
> > Hi folks,
> > 
> > Here's a quick patchset reworking writeback ioend completion processing
> > into per-inode completion queues so that we don't have a thundering herd
> > of unwritten/cow completion kworkers contending for the ILOCK.  The
> > second patch will also combine adjacent ioends when possible to reduce
> > the overhead further.  Let me know what you think. :)
> > 
> 
> I assume the issue is that you get a bunch of ioends associated with COW
> remaps or whatever and workqueue parallelism leads to a bunch contending
> on inode locks because they happen to be associated with the same inode.

Yes, as outlined in the thread "xlog_grant_head_wait deadlocks on
high-rolling transactions?"

> Hence, it's more efficient to have a single work item per inode, let
> that task complete as many ioends as available for the inode and let the
> workqueue spread around workers for different inodes. Sounds like a nice
> idea to me. Is there a workload that measurably benefits from this or is
> this just based on observation?

If you consider 'xfstests' to be my primary workload, then yes, it
reduces developer deadlock triage time considerably. :)

Uhh but more seriously, on kvm hosts that use pagecache-as-diskcache(!)
the number of kworkers spirals insanely when someone issue a cache flush
(a.k.a. fsync) because they're all contending with each other and the
host writeback slows to a crawl if it doesn't just fall over dead.

> A couple small nits..
> 
> > --D
> > ---
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Restructure the buffered writeback completion code to use a single work
> > item per inode, since it's pointless to awaken a thundering herd of
> > threads to contend on a single inode's ILOCK.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/xfs_aops.c   |   48 +++++++++++++++++++++++++++++++++++++-----------
> >  fs/xfs/xfs_aops.h   |    1 -
> >  fs/xfs/xfs_icache.c |    3 +++
> >  fs/xfs/xfs_inode.h  |    7 +++++++
> >  4 files changed, 47 insertions(+), 12 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > index 3619e9e8d359..f7a9bb661826 100644
> > --- a/fs/xfs/xfs_aops.c
> > +++ b/fs/xfs/xfs_aops.c
> > @@ -234,11 +234,9 @@ xfs_setfilesize_ioend(
> >   * IO write completion.
> >   */
> >  STATIC void
> > -xfs_end_io(
> > -	struct work_struct *work)
> > +xfs_end_ioend(
> > +	struct xfs_ioend	*ioend)
> >  {
> > -	struct xfs_ioend	*ioend =
> > -		container_of(work, struct xfs_ioend, io_work);
> >  	struct xfs_inode	*ip = XFS_I(ioend->io_inode);
> >  	xfs_off_t		offset = ioend->io_offset;
> >  	size_t			size = ioend->io_size;
> > @@ -278,19 +276,48 @@ xfs_end_io(
> >  	xfs_destroy_ioend(ioend, error);
> >  }
> >  
> > +/* Finish all pending io completions. */
> > +void
> > +xfs_end_io(
> > +	struct work_struct	*work)
> > +{
> > +	struct xfs_inode	*ip;
> > +	struct xfs_ioend	*ioend;
> > +	struct list_head	completion_list;
> > +	unsigned long		flags;
> > +
> > +	ip = container_of(work, struct xfs_inode, i_iodone_work);
> > +
> > +	spin_lock_irqsave(&ip->i_iodone_lock, flags);
> > +	list_replace_init(&ip->i_iodone_list, &completion_list);
> > +	spin_unlock_irqrestore(&ip->i_iodone_lock, flags);
> > +
> > +	while (!list_empty(&completion_list)) {
> > +		ioend = list_first_entry(&completion_list, struct xfs_ioend,
> > +				io_list);
> > +		list_del_init(&ioend->io_list);
> > +		xfs_end_ioend(ioend);
> > +	}
> > +}
> > +
> >  STATIC void
> >  xfs_end_bio(
> >  	struct bio		*bio)
> >  {
> >  	struct xfs_ioend	*ioend = bio->bi_private;
> > -	struct xfs_mount	*mp = XFS_I(ioend->io_inode)->i_mount;
> > +	struct xfs_inode	*ip = XFS_I(ioend->io_inode);
> > +	struct xfs_mount	*mp = ip->i_mount;
> > +	unsigned long		flags;
> >  
> >  	if (ioend->io_fork == XFS_COW_FORK ||
> > -	    ioend->io_state == XFS_EXT_UNWRITTEN)
> > -		queue_work(mp->m_unwritten_workqueue, &ioend->io_work);
> > -	else if (ioend->io_append_trans)
> > -		queue_work(mp->m_data_workqueue, &ioend->io_work);
> > -	else
> > +	    ioend->io_state == XFS_EXT_UNWRITTEN ||
> > +	    ioend->io_append_trans != NULL) {
> > +		spin_lock_irqsave(&ip->i_iodone_lock, flags);
> > +		if (list_empty(&ip->i_iodone_list))
> > +			queue_work(mp->m_unwritten_workqueue, &ip->i_iodone_work);
> > +		list_add_tail(&ioend->io_list, &ip->i_iodone_list);
> > +		spin_unlock_irqrestore(&ip->i_iodone_lock, flags);
> > +	} else
> 
> Ok, so there's no longer a need for multiple ioend workqueues because
> all such operations are going to be processed by the single work
> execution. Perhaps we should rename m_unwritten_workqueue or just use
> m_data_workqueue since it happens to have a more generic name. We can
> also kill off the other wq either way since it appears to be unused.

<nod> I'll add a patch to kill off one of the workqueues.

> >  		xfs_destroy_ioend(ioend, blk_status_to_errno(bio->bi_status));
> >  }
> >  
> > @@ -594,7 +621,6 @@ xfs_alloc_ioend(
> >  	ioend->io_inode = inode;
> >  	ioend->io_size = 0;
> >  	ioend->io_offset = offset;
> > -	INIT_WORK(&ioend->io_work, xfs_end_io);
> >  	ioend->io_append_trans = NULL;
> >  	ioend->io_bio = bio;
> >  	return ioend;
> > diff --git a/fs/xfs/xfs_aops.h b/fs/xfs/xfs_aops.h
> > index 6c2615b83c5d..f62b03186c62 100644
> > --- a/fs/xfs/xfs_aops.h
> > +++ b/fs/xfs/xfs_aops.h
> > @@ -18,7 +18,6 @@ struct xfs_ioend {
> >  	struct inode		*io_inode;	/* file being written to */
> >  	size_t			io_size;	/* size of the extent */
> >  	xfs_off_t		io_offset;	/* offset in the file */
> > -	struct work_struct	io_work;	/* xfsdatad work queue */
> >  	struct xfs_trans	*io_append_trans;/* xact. for size update */
> >  	struct bio		*io_bio;	/* bio being built */
> >  	struct bio		io_inline_bio;	/* MUST BE LAST! */
> > diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> > index 245483cc282b..e70e7db29026 100644
> > --- a/fs/xfs/xfs_icache.c
> > +++ b/fs/xfs/xfs_icache.c
> > @@ -70,6 +70,9 @@ xfs_inode_alloc(
> >  	ip->i_flags = 0;
> >  	ip->i_delayed_blks = 0;
> >  	memset(&ip->i_d, 0, sizeof(ip->i_d));
> > +	INIT_WORK(&ip->i_iodone_work, xfs_end_io);
> > +	INIT_LIST_HEAD(&ip->i_iodone_list);
> > +	spin_lock_init(&ip->i_iodone_lock);
> >  
> >  	return ip;
> >  }
> > diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> > index e62074a5257c..88239c2dd824 100644
> > --- a/fs/xfs/xfs_inode.h
> > +++ b/fs/xfs/xfs_inode.h
> > @@ -57,6 +57,11 @@ typedef struct xfs_inode {
> >  
> >  	/* VFS inode */
> >  	struct inode		i_vnode;	/* embedded VFS inode */
> > +
> > +	/* pending io completions */
> > +	spinlock_t		i_iodone_lock;
> > +	struct work_struct	i_iodone_work;
> > +	struct list_head	i_iodone_list;
> 
> A nit but I guess I'd name these with ioend instead of iodone. The
> latter kind of confuses with buffer iodone callbacks (to me).

<nod> Will do.

--D

> Brian
> 
> >  } xfs_inode_t;
> >  
> >  /* Convert from vfs inode to xfs inode */
> > @@ -503,4 +508,6 @@ bool xfs_inode_verify_forks(struct xfs_inode *ip);
> >  int xfs_iunlink_init(struct xfs_perag *pag);
> >  void xfs_iunlink_destroy(struct xfs_perag *pag);
> >  
> > +void xfs_end_io(struct work_struct *work);
> > +
> >  #endif	/* __XFS_INODE_H__ */