Re: [PATCH 04/11] sync: serialise per-superblock sync operations

[Jan Kara]

On Wed 31-07-13 14:15:43, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> When competing sync(2) calls walk the same filesystem, they need to
> walk the list of inodes on the superblock to find all the inodes
> that we need to wait for IO completion on. However, when multiple
> wait_sb_inodes() calls do this at the same time, they contend on the
> the inode_sb_list_lock and the contention causes system wide
> slowdowns. In effect, concurrent sync(2) calls can take longer and
> burn more CPU than if they were serialised.
> 
> Stop the worst of the contention by adding a per-sb mutex to wrap
> around wait_sb_inodes() so that we only execute one sync(2) IO
> completion walk per superblock superblock at a time and hence avoid
> contention being triggered by concurrent sync(2) calls.
  The patch looks OK. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  fs/fs-writeback.c  | 11 +++++++++++
>  fs/super.c         |  1 +
>  include/linux/fs.h |  2 ++
>  3 files changed, 14 insertions(+)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index ca66dc8..56272ec 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1207,6 +1207,15 @@ out_unlock_inode:
>  }
>  EXPORT_SYMBOL(__mark_inode_dirty);
>  
> +/*
> + * The @s_sync_lock is used to serialise concurrent sync operations
> + * to avoid lock contention problems with concurrent wait_sb_inodes() calls.
> + * Concurrent callers will block on the s_sync_lock rather than doing contending
> + * walks. The queueing maintains sync(2) required behaviour as all the IO that
> + * has been issued up to the time this function is enter is guaranteed to be
> + * completed by the time we have gained the lock and waited for all IO that is
> + * in progress regardless of the order callers are granted the lock.
> + */
>  static void wait_sb_inodes(struct super_block *sb)
>  {
>  	struct inode *inode, *old_inode = NULL;
> @@ -1217,6 +1226,7 @@ static void wait_sb_inodes(struct super_block *sb)
>  	 */
>  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
>  
> +	mutex_lock(&sb->s_sync_lock);
>  	spin_lock(&sb->s_inode_list_lock);
>  
>  	/*
> @@ -1258,6 +1268,7 @@ static void wait_sb_inodes(struct super_block *sb)
>  	}
>  	spin_unlock(&sb->s_inode_list_lock);
>  	iput(old_inode);
> +	mutex_unlock(&sb->s_sync_lock);
>  }
>  
>  /**
> diff --git a/fs/super.c b/fs/super.c
> index d4d753e..7f98fd6 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -200,6 +200,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
>  		s->s_bdi = &default_backing_dev_info;
>  		INIT_HLIST_NODE(&s->s_instances);
>  		INIT_HLIST_BL_HEAD(&s->s_anon);
> +		mutex_init(&s->s_sync_lock);
>  		INIT_LIST_HEAD(&s->s_inodes);
>  		spin_lock_init(&s->s_inode_list_lock);
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 923b465..971e8be 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1321,6 +1321,8 @@ struct super_block {
>  	/* Being remounted read-only */
>  	int s_readonly_remount;
>  
> +	struct mutex		s_sync_lock;	/* sync serialisation lock */
> +
>  	/*
>  	 * Keep the lru lists last in the structure so they always sit on their
>  	 * own individual cachelines.
> -- 
> 1.8.3.2
> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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