Re: [PATCH v4 2/3] xfs: introduce in-core global counter of allocbt blocks

[Brian Foster <bfoster@redhat.com>]

On Tue, Apr 27, 2021 at 09:15:09PM -0700, Darrick J. Wong wrote:
> On Fri, Apr 23, 2021 at 09:10:49AM -0400, Brian Foster wrote:
> > Introduce an in-core counter to track the sum of all allocbt blocks
> > used by the filesystem. This value is currently tracked per-ag via
> > the ->agf_btreeblks field in the AGF, which also happens to include
> > rmapbt blocks. A global, in-core count of allocbt blocks is required
> > to identify the subset of global ->m_fdblocks that consists of
> > unavailable blocks currently used for allocation btrees. To support
> > this calculation at block reservation time, construct a similar
> > global counter for allocbt blocks, populate it on first read of each
> > AGF and update it as allocbt blocks are used and released.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> >  fs/xfs/libxfs/xfs_alloc.c       | 12 ++++++++++++
> >  fs/xfs/libxfs/xfs_alloc_btree.c |  2 ++
> >  fs/xfs/xfs_mount.h              |  6 ++++++
> >  3 files changed, 20 insertions(+)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index aaa19101bb2a..144e2d68245c 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -3036,6 +3036,7 @@ xfs_alloc_read_agf(
> >  	struct xfs_agf		*agf;		/* ag freelist header */
> >  	struct xfs_perag	*pag;		/* per allocation group data */
> >  	int			error;
> > +	uint32_t		allocbt_blks;
> >  
> >  	trace_xfs_alloc_read_agf(mp, agno);
> >  
> > @@ -3066,6 +3067,17 @@ xfs_alloc_read_agf(
> >  		pag->pagf_refcount_level = be32_to_cpu(agf->agf_refcount_level);
> >  		pag->pagf_init = 1;
> >  		pag->pagf_agflreset = xfs_agfl_needs_reset(mp, agf);
> > +
> > +		/*
> > +		 * Update the global in-core allocbt block counter. Filter
> > +		 * rmapbt blocks from the on-disk counter because those are
> > +		 * managed by perag reservation.
> > +		 */
> > +		if (pag->pagf_btreeblks > be32_to_cpu(agf->agf_rmap_blocks)) {
> 
> As pointed out elsewhere in the thread, agf_rmap_blocks counts the total
> number of blocks in the rmapbt (whereas agf_btreeblks counts the number
> of non-root blocks in all three free space btrees).  Does this need a
> change?
> 
> 	int delta = (int)pag->pagf_btreeblks - (be32_to_cpu(...) - 1);
> 	if (delta > 0)
> 		atomic64_add(delta, &mp->m_allocbt_blks);
>

Hm yes, this makes more sense. Will fix and update the comment..

Brian
 
> --D
> 
> > +			allocbt_blks = pag->pagf_btreeblks -
> > +					be32_to_cpu(agf->agf_rmap_blocks);
> > +			atomic64_add(allocbt_blks, &mp->m_allocbt_blks);
> > +		}
> >  	}
> >  #ifdef DEBUG
> >  	else if (!XFS_FORCED_SHUTDOWN(mp)) {
> > diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
> > index 8e01231b308e..9f5a45f7baed 100644
> > --- a/fs/xfs/libxfs/xfs_alloc_btree.c
> > +++ b/fs/xfs/libxfs/xfs_alloc_btree.c
> > @@ -71,6 +71,7 @@ xfs_allocbt_alloc_block(
> >  		return 0;
> >  	}
> >  
> > +	atomic64_inc(&cur->bc_mp->m_allocbt_blks);
> >  	xfs_extent_busy_reuse(cur->bc_mp, cur->bc_ag.agno, bno, 1, false);
> >  
> >  	xfs_trans_agbtree_delta(cur->bc_tp, 1);
> > @@ -95,6 +96,7 @@ xfs_allocbt_free_block(
> >  	if (error)
> >  		return error;
> >  
> > +	atomic64_dec(&cur->bc_mp->m_allocbt_blks);
> >  	xfs_extent_busy_insert(cur->bc_tp, be32_to_cpu(agf->agf_seqno), bno, 1,
> >  			      XFS_EXTENT_BUSY_SKIP_DISCARD);
> >  	xfs_trans_agbtree_delta(cur->bc_tp, -1);
> > diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
> > index 81829d19596e..bb67274ee23f 100644
> > --- a/fs/xfs/xfs_mount.h
> > +++ b/fs/xfs/xfs_mount.h
> > @@ -170,6 +170,12 @@ typedef struct xfs_mount {
> >  	 * extents or anything related to the rt device.
> >  	 */
> >  	struct percpu_counter	m_delalloc_blks;
> > +	/*
> > +	 * Global count of allocation btree blocks in use across all AGs. Only
> > +	 * used when perag reservation is enabled. Helps prevent block
> > +	 * reservation from attempting to reserve allocation btree blocks.
> > +	 */
> > +	atomic64_t		m_allocbt_blks;
> >  
> >  	struct radix_tree_root	m_perag_tree;	/* per-ag accounting info */
> >  	spinlock_t		m_perag_lock;	/* lock for m_perag_tree */
> > -- 
> > 2.26.3
> > 
>