Merge tag 'xfs-buf-lockless-lookup-5.20' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs into xfs-5.20-mergeB

xfs: lockless buffer cache lookups

Current work to merge the XFS inode life cycle with the VFS inode
life cycle is finding some interesting issues. If we have a path
that hits buffer trylocks fairly hard (e.g. a non-blocking
background inode freeing function), we end up hitting massive
contention on the buffer cache hash locks:

-   92.71%     0.05%  [kernel]                  [k] xfs_inodegc_worker
   - 92.67% xfs_inodegc_worker
      - 92.13% xfs_inode_unlink
         - 91.52% xfs_inactive_ifree
            - 85.63% xfs_read_agi
               - 85.61% xfs_trans_read_buf_map
                  - 85.59% xfs_buf_read_map
                     - xfs_buf_get_map
                        - 85.55% xfs_buf_find
                           - 72.87% _raw_spin_lock
                              - do_raw_spin_lock
                                   71.86% __pv_queued_spin_lock_slowpath
                           - 8.74% xfs_buf_rele
                              - 7.88% _raw_spin_lock
                                 - 7.88% do_raw_spin_lock
                                      7.63% __pv_queued_spin_lock_slowpath
                           - 1.70% xfs_buf_trylock
                              - 1.68% down_trylock
                                 - 1.41% _raw_spin_lock_irqsave
                                    - 1.39% do_raw_spin_lock
                                         __pv_queued_spin_lock_slowpath
                           - 0.76% _raw_spin_unlock
                                0.75% do_raw_spin_unlock

This is basically hammering the pag->pag_buf_lock from lots of CPUs
doing trylocks at the same time. Most of the buffer trylock
operations ultimately fail after we've done the lookup, so we're
really hammering the buf hash lock whilst making no progress.

We can also see significant spinlock traffic on the same lock just
under normal operation when lots of tasks are accessing metadata
from the same AG, so let's avoid all this by creating a lookup fast
path which leverages the rhashtable's ability to do RCU protected
lookups.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

* tag 'xfs-buf-lockless-lookup-5.20' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs:
  xfs: lockless buffer lookup
  xfs: remove a superflous hash lookup when inserting new buffers
  xfs: reduce the number of atomic when locking a buffer after lookup
  xfs: merge xfs_buf_find() and xfs_buf_get_map()
  xfs: break up xfs_buf_find() into individual pieces
  xfs: rework xfs_buf_incore() API

1 files changed, 148 insertions, 115 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index bf4e60871068..6dac5583977f 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -295,6 +295,16 @@ xfs_buf_free_pages(
 }
 
 static void
+xfs_buf_free_callback(
+	struct callback_head	*cb)
+{
+	struct xfs_buf		*bp = container_of(cb, struct xfs_buf, b_rcu);
+
+	xfs_buf_free_maps(bp);
+	kmem_cache_free(xfs_buf_cache, bp);
+}
+
+static void
 xfs_buf_free(
 	struct xfs_buf		*bp)
 {
@@ -307,8 +317,7 @@ xfs_buf_free(
 	else if (bp->b_flags & _XBF_KMEM)
 		kmem_free(bp->b_addr);
 
-	xfs_buf_free_maps(bp);
-	kmem_cache_free(xfs_buf_cache, bp);
+	call_rcu(&bp->b_rcu, xfs_buf_free_callback);
 }
 
 static int
@@ -503,100 +512,45 @@ xfs_buf_hash_destroy(
 	rhashtable_destroy(&pag->pag_buf_hash);
 }
 
-/*
- * Look up a buffer in the buffer cache and return it referenced and locked
- * in @found_bp.
- *
- * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
- * cache.
- *
- * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
- * -EAGAIN if we fail to lock it.
- *
- * Return values are:
- *	-EFSCORRUPTED if have been supplied with an invalid address
- *	-EAGAIN on trylock failure
- *	-ENOENT if we fail to find a match and @new_bp was NULL
- *	0, with @found_bp:
- *		- @new_bp if we inserted it into the cache
- *		- the buffer we found and locked.
- */
 static int
-xfs_buf_find(
+xfs_buf_map_verify(
 	struct xfs_buftarg	*btp,
-	struct xfs_buf_map	*map,
-	int			nmaps,
-	xfs_buf_flags_t		flags,
-	struct xfs_buf		*new_bp,
-	struct xfs_buf		**found_bp)
+	struct xfs_buf_map	*map)
 {
-	struct xfs_perag	*pag;
-	struct xfs_buf		*bp;
-	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
 	xfs_daddr_t		eofs;
-	int			i;
-
-	*found_bp = NULL;
-
-	for (i = 0; i < nmaps; i++)
-		cmap.bm_len += map[i].bm_len;
 
 	/* Check for IOs smaller than the sector size / not sector aligned */
-	ASSERT(!(BBTOB(cmap.bm_len) < btp->bt_meta_sectorsize));
-	ASSERT(!(BBTOB(cmap.bm_bn) & (xfs_off_t)btp->bt_meta_sectormask));
+	ASSERT(!(BBTOB(map->bm_len) < btp->bt_meta_sectorsize));
+	ASSERT(!(BBTOB(map->bm_bn) & (xfs_off_t)btp->bt_meta_sectormask));
 
 	/*
 	 * Corrupted block numbers can get through to here, unfortunately, so we
 	 * have to check that the buffer falls within the filesystem bounds.
 	 */
 	eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
-	if (cmap.bm_bn < 0 || cmap.bm_bn >= eofs) {
+	if (map->bm_bn < 0 || map->bm_bn >= eofs) {
 		xfs_alert(btp->bt_mount,
 			  "%s: daddr 0x%llx out of range, EOFS 0x%llx",
-			  __func__, cmap.bm_bn, eofs);
+			  __func__, map->bm_bn, eofs);
 		WARN_ON(1);
 		return -EFSCORRUPTED;
 	}
-
-	pag = xfs_perag_get(btp->bt_mount,
-			    xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));
-
-	spin_lock(&pag->pag_buf_lock);
-	bp = rhashtable_lookup_fast(&pag->pag_buf_hash, &cmap,
-				    xfs_buf_hash_params);
-	if (bp) {
-		atomic_inc(&bp->b_hold);
-		goto found;
-	}
-
-	/* No match found */
-	if (!new_bp) {
-		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
-		spin_unlock(&pag->pag_buf_lock);
-		xfs_perag_put(pag);
-		return -ENOENT;
-	}
-
-	/* the buffer keeps the perag reference until it is freed */
-	new_bp->b_pag = pag;
-	rhashtable_insert_fast(&pag->pag_buf_hash, &new_bp->b_rhash_head,
-			       xfs_buf_hash_params);
-	spin_unlock(&pag->pag_buf_lock);
-	*found_bp = new_bp;
 	return 0;
+}
 
-found:
-	spin_unlock(&pag->pag_buf_lock);
-	xfs_perag_put(pag);
-
-	if (!xfs_buf_trylock(bp)) {
-		if (flags & XBF_TRYLOCK) {
-			xfs_buf_rele(bp);
-			XFS_STATS_INC(btp->bt_mount, xb_busy_locked);
+static int
+xfs_buf_find_lock(
+	struct xfs_buf          *bp,
+	xfs_buf_flags_t		flags)
+{
+	if (flags & XBF_TRYLOCK) {
+		if (!xfs_buf_trylock(bp)) {
+			XFS_STATS_INC(bp->b_mount, xb_busy_locked);
 			return -EAGAIN;
 		}
+	} else {
 		xfs_buf_lock(bp);
-		XFS_STATS_INC(btp->bt_mount, xb_get_locked_waited);
+		XFS_STATS_INC(bp->b_mount, xb_get_locked_waited);
 	}
 
 	/*
@@ -609,57 +563,59 @@ found:
 		bp->b_flags &= _XBF_KMEM | _XBF_PAGES;
 		bp->b_ops = NULL;
 	}
-
-	trace_xfs_buf_find(bp, flags, _RET_IP_);
-	XFS_STATS_INC(btp->bt_mount, xb_get_locked);
-	*found_bp = bp;
 	return 0;
 }
 
-struct xfs_buf *
-xfs_buf_incore(
-	struct xfs_buftarg	*target,
-	xfs_daddr_t		blkno,
-	size_t			numblks,
-	xfs_buf_flags_t		flags)
+static inline int
+xfs_buf_lookup(
+	struct xfs_perag	*pag,
+	struct xfs_buf_map	*map,
+	xfs_buf_flags_t		flags,
+	struct xfs_buf		**bpp)
 {
-	struct xfs_buf		*bp;
+	struct xfs_buf          *bp;
 	int			error;
-	DEFINE_SINGLE_BUF_MAP(map, blkno, numblks);
 
-	error = xfs_buf_find(target, &map, 1, flags, NULL, &bp);
-	if (error)
-		return NULL;
-	return bp;
+	rcu_read_lock();
+	bp = rhashtable_lookup(&pag->pag_buf_hash, map, xfs_buf_hash_params);
+	if (!bp || !atomic_inc_not_zero(&bp->b_hold)) {
+		rcu_read_unlock();
+		return -ENOENT;
+	}
+	rcu_read_unlock();
+
+	error = xfs_buf_find_lock(bp, flags);
+	if (error) {
+		xfs_buf_rele(bp);
+		return error;
+	}
+
+	trace_xfs_buf_find(bp, flags, _RET_IP_);
+	*bpp = bp;
+	return 0;
 }
 
 /*
- * Assembles a buffer covering the specified range. The code is optimised for
- * cache hits, as metadata intensive workloads will see 3 orders of magnitude
- * more hits than misses.
+ * Insert the new_bp into the hash table. This consumes the perag reference
+ * taken for the lookup regardless of the result of the insert.
  */
-int
-xfs_buf_get_map(
-	struct xfs_buftarg	*target,
+static int
+xfs_buf_find_insert(
+	struct xfs_buftarg	*btp,
+	struct xfs_perag	*pag,
+	struct xfs_buf_map	*cmap,
 	struct xfs_buf_map	*map,
 	int			nmaps,
 	xfs_buf_flags_t		flags,
 	struct xfs_buf		**bpp)
 {
-	struct xfs_buf		*bp;
 	struct xfs_buf		*new_bp;
+	struct xfs_buf		*bp;
 	int			error;
 
-	*bpp = NULL;
-	error = xfs_buf_find(target, map, nmaps, flags, NULL, &bp);
-	if (!error)
-		goto found;
-	if (error != -ENOENT)
-		return error;
-
-	error = _xfs_buf_alloc(target, map, nmaps, flags, &new_bp);
+	error = _xfs_buf_alloc(btp, map, nmaps, flags, &new_bp);
 	if (error)
-		return error;
+		goto out_drop_pag;
 
 	/*
 	 * For buffers that fit entirely within a single page, first attempt to
@@ -674,18 +630,94 @@ xfs_buf_get_map(
 			goto out_free_buf;
 	}
 
-	error = xfs_buf_find(target, map, nmaps, flags, new_bp, &bp);
-	if (error)
+	spin_lock(&pag->pag_buf_lock);
+	bp = rhashtable_lookup_get_insert_fast(&pag->pag_buf_hash,
+			&new_bp->b_rhash_head, xfs_buf_hash_params);
+	if (IS_ERR(bp)) {
+		error = PTR_ERR(bp);
+		spin_unlock(&pag->pag_buf_lock);
 		goto out_free_buf;
+	}
+	if (bp) {
+		/* found an existing buffer */
+		atomic_inc(&bp->b_hold);
+		spin_unlock(&pag->pag_buf_lock);
+		error = xfs_buf_find_lock(bp, flags);
+		if (error)
+			xfs_buf_rele(bp);
+		else
+			*bpp = bp;
+		goto out_free_buf;
+	}
+
+	/* The new buffer keeps the perag reference until it is freed. */
+	new_bp->b_pag = pag;
+	spin_unlock(&pag->pag_buf_lock);
+	*bpp = new_bp;
+	return 0;
+
+out_free_buf:
+	xfs_buf_free(new_bp);
+out_drop_pag:
+	xfs_perag_put(pag);
+	return error;
+}
+
+/*
+ * Assembles a buffer covering the specified range. The code is optimised for
+ * cache hits, as metadata intensive workloads will see 3 orders of magnitude
+ * more hits than misses.
+ */
+int
+xfs_buf_get_map(
+	struct xfs_buftarg	*btp,
+	struct xfs_buf_map	*map,
+	int			nmaps,
+	xfs_buf_flags_t		flags,
+	struct xfs_buf		**bpp)
+{
+	struct xfs_perag	*pag;
+	struct xfs_buf		*bp = NULL;
+	struct xfs_buf_map	cmap = { .bm_bn = map[0].bm_bn };
+	int			error;
+	int			i;
 
-	if (bp != new_bp)
-		xfs_buf_free(new_bp);
+	for (i = 0; i < nmaps; i++)
+		cmap.bm_len += map[i].bm_len;
 
-found:
+	error = xfs_buf_map_verify(btp, &cmap);
+	if (error)
+		return error;
+
+	pag = xfs_perag_get(btp->bt_mount,
+			    xfs_daddr_to_agno(btp->bt_mount, cmap.bm_bn));
+
+	error = xfs_buf_lookup(pag, &cmap, flags, &bp);
+	if (error && error != -ENOENT)
+		goto out_put_perag;
+
+	/* cache hits always outnumber misses by at least 10:1 */
+	if (unlikely(!bp)) {
+		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
+
+		if (flags & XBF_INCORE)
+			goto out_put_perag;
+
+		/* xfs_buf_find_insert() consumes the perag reference. */
+		error = xfs_buf_find_insert(btp, pag, &cmap, map, nmaps,
+				flags, &bp);
+		if (error)
+			return error;
+	} else {
+		XFS_STATS_INC(btp->bt_mount, xb_get_locked);
+		xfs_perag_put(pag);
+	}
+
+	/* We do not hold a perag reference anymore. */
 	if (!bp->b_addr) {
 		error = _xfs_buf_map_pages(bp, flags);
 		if (unlikely(error)) {
-			xfs_warn_ratelimited(target->bt_mount,
+			xfs_warn_ratelimited(btp->bt_mount,
 				"%s: failed to map %u pages", __func__,
 				bp->b_page_count);
 			xfs_buf_relse(bp);
@@ -700,12 +732,13 @@ found:
 	if (!(flags & XBF_READ))
 		xfs_buf_ioerror(bp, 0);
 
-	XFS_STATS_INC(target->bt_mount, xb_get);
+	XFS_STATS_INC(btp->bt_mount, xb_get);
 	trace_xfs_buf_get(bp, flags, _RET_IP_);
 	*bpp = bp;
 	return 0;
-out_free_buf:
-	xfs_buf_free(new_bp);
+
+out_put_perag:
+	xfs_perag_put(pag);
 	return error;
 }