quota: Reduce contention on dq_data_lock

dq_data_lock is currently used to protect all modifications of quota
accounting information, consistency of quota accounting on the inode,
and dquot pointers from inode. As a result contention on the lock can be
pretty heavy.

Reduce the contention on the lock by protecting quota accounting
information by a new dquot->dq_dqb_lock and consistency of quota
accounting with inode usage by inode->i_lock.

This change reduces time to create 500000 files on ext4 on ramdisk by 50
different processes in separate directories by 6% when user quota is
turned on. When those 50 processes belong to 50 different users, the
improvement is about 9%.

Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/quota.h b/include/linux/quota.h
index eccc1cb6274e..074123975595 100644
--- a/include/linux/quota.h
+++ b/include/linux/quota.h
@@ -298,12 +298,13 @@ struct dquot {
         struct list_head dq_free;       /* Free list element */
         struct list_head dq_dirty;      /* List of dirty dquots */
         struct mutex dq_lock;           /* dquot IO lock */
+        spinlock_t dq_dqb_lock;         /* Lock protecting dq_dqb changes */
         atomic_t dq_count;              /* Use count */
         struct super_block *dq_sb;      /* superblock this applies to */
         struct kqid dq_id;              /* ID this applies to (uid, gid, projid) */
         loff_t dq_off;                  /* Offset of dquot on disk */
         unsigned long dq_flags;         /* See DQ_* */
-        struct mem_dqblk dq_dqb;        /* Diskquota usage */
+        struct mem_dqblk dq_dqb;        /* Diskquota usage [dq_dqb_lock] */
 };
 
 /* Operations which must be implemented by each quota format */