xfs: lockless per-ag lookups

When we start taking a reference to the per-ag for every cached
buffer in the system, kernel lockstat profiling on an 8-way create
workload shows the mp->m_perag_lock has higher acquisition rates
than the inode lock and has significantly more contention. That is,
it becomes the highest contended lock in the system.

The perag lookup is trivial to convert to lock-less RCU lookups
because perag structures never go away. Hence the only thing we need
to protect against is tree structure changes during a grow. This can
be done simply by replacing the locking in xfs_perag_get() with RCU
read locking. This removes the mp->m_perag_lock completely from this
path.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/xfs/xfs_ag.h b/fs/xfs/xfs_ag.h
index 4917d4eed4ed..51c42c202bf1 100644
--- a/fs/xfs/xfs_ag.h
+++ b/fs/xfs/xfs_ag.h
@@ -230,6 +230,9 @@ typedef struct xfs_perag {
         rwlock_t        pag_ici_lock;   /* incore inode lock */
         struct radix_tree_root pag_ici_root;    /* incore inode cache root */
         int             pag_ici_reclaimable;    /* reclaimable inodes */
+
+        /* for rcu-safe freeing */
+        struct rcu_head rcu_head;
 #endif
         int             pagb_count;     /* pagb slots in use */
 } xfs_perag_t;