From e176579e70118ed7cfdb60f963628fe0ca771f3d Mon Sep 17 00:00:00 2001 From: Dave Chinner Date: Wed, 22 Sep 2010 10:47:20 +1000 Subject: [PATCH] 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 Reviewed-by: Christoph Hellwig Reviewed-by: Alex Elder --- fs/xfs/linux-2.6/xfs_sync.c | 6 +++--- fs/xfs/xfs_ag.h | 3 +++ fs/xfs/xfs_mount.c | 25 +++++++++++++++++-------- 3 files changed, 23 insertions(+), 11 deletions(-) diff --git a/fs/xfs/linux-2.6/xfs_sync.c b/fs/xfs/linux-2.6/xfs_sync.c index 81976ffed7d6..3a1d229b4784 100644 --- a/fs/xfs/linux-2.6/xfs_sync.c +++ b/fs/xfs/linux-2.6/xfs_sync.c @@ -150,17 +150,17 @@ xfs_inode_ag_iter_next_pag( int found; int ref; - spin_lock(&mp->m_perag_lock); + rcu_read_lock(); found = radix_tree_gang_lookup_tag(&mp->m_perag_tree, (void **)&pag, *first, 1, tag); if (found <= 0) { - spin_unlock(&mp->m_perag_lock); + rcu_read_unlock(); return NULL; } *first = pag->pag_agno + 1; /* open coded pag reference increment */ ref = atomic_inc_return(&pag->pag_ref); - spin_unlock(&mp->m_perag_lock); + rcu_read_unlock(); trace_xfs_perag_get_reclaim(mp, pag->pag_agno, ref, _RET_IP_); } else { pag = xfs_perag_get(mp, *first); 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; diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c index 00c7a876807d..14fc6e9e1816 100644 --- a/fs/xfs/xfs_mount.c +++ b/fs/xfs/xfs_mount.c @@ -199,6 +199,8 @@ xfs_uuid_unmount( /* * Reference counting access wrappers to the perag structures. + * Because we never free per-ag structures, the only thing we + * have to protect against changes is the tree structure itself. */ struct xfs_perag * xfs_perag_get(struct xfs_mount *mp, xfs_agnumber_t agno) @@ -206,13 +208,13 @@ xfs_perag_get(struct xfs_mount *mp, xfs_agnumber_t agno) struct xfs_perag *pag; int ref = 0; - spin_lock(&mp->m_perag_lock); + rcu_read_lock(); pag = radix_tree_lookup(&mp->m_perag_tree, agno); if (pag) { ASSERT(atomic_read(&pag->pag_ref) >= 0); ref = atomic_inc_return(&pag->pag_ref); } - spin_unlock(&mp->m_perag_lock); + rcu_read_unlock(); trace_xfs_perag_get(mp, agno, ref, _RET_IP_); return pag; } @@ -227,10 +229,18 @@ xfs_perag_put(struct xfs_perag *pag) trace_xfs_perag_put(pag->pag_mount, pag->pag_agno, ref, _RET_IP_); } +STATIC void +__xfs_free_perag( + struct rcu_head *head) +{ + struct xfs_perag *pag = container_of(head, struct xfs_perag, rcu_head); + + ASSERT(atomic_read(&pag->pag_ref) == 0); + kmem_free(pag); +} + /* - * Free up the resources associated with a mount structure. Assume that - * the structure was initially zeroed, so we can tell which fields got - * initialized. + * Free up the per-ag resources associated with the mount structure. */ STATIC void xfs_free_perag( @@ -242,10 +252,9 @@ xfs_free_perag( for (agno = 0; agno < mp->m_sb.sb_agcount; agno++) { spin_lock(&mp->m_perag_lock); pag = radix_tree_delete(&mp->m_perag_tree, agno); - ASSERT(pag); - ASSERT(atomic_read(&pag->pag_ref) == 0); spin_unlock(&mp->m_perag_lock); - kmem_free(pag); + ASSERT(pag); + call_rcu(&pag->rcu_head, __xfs_free_perag); } } -- 2.39.5