[PATCH 20/34] xfs: remove all the inodes on a buffer from the AIL in bulk

[Alex Elder]

On Tue, 2010-12-21 at 18:29 +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> When inode buffer IO completes, usually all of the inodes are removed from the
> AIL. This involves processing them one at a time and taking the AIL lock once
> for every inode. When all CPUs are processing inode IO completions, this causes
> excessive amount sof contention on the AIL lock.
> 
> Instead, change the way we process inode IO completion in the buffer
> IO done callback. Allow the inode IO done callback to walk the list
> of IO done callbacks and pull all the inodes off the buffer in one
> go and then process them as a batch.
> 
> Once all the inodes for removal are collected, take the AIL lock
> once and do a bulk removal operation to minimise traffic on the AIL
> lock.
> 
> Signed-off-by: Dave Chinner <dchinner at redhat.com>
> Reviewed-by: Christoph Hellwig <hch at lst.de>

One question, below.		-Alex

. . .

> @@ -861,28 +910,37 @@ xfs_iflush_done(
>  	 * the lock since it's cheaper, and then we recheck while
>  	 * holding the lock before removing the inode from the AIL.
>  	 */
> -	if (iip->ili_logged && lip->li_lsn == iip->ili_flush_lsn) {
> +	if (need_ail) {
> +		struct xfs_log_item *log_items[need_ail];

What's the worst-case value of need_ail we might see here?