Re: [PATCH v3 3/4] xfs: randomly fall back to near mode lookup algorithm in debug mode

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Aug 15, 2019 at 08:55:37AM -0400, Brian Foster wrote:
> The last block scan is the dominant near mode allocation algorithm
> for a newer filesystem with fewer, large free extents. Add debug
> mode logic to randomly fall back to lookup mode to improve
> regression test coverage.

How about just using an errortag since the new sysfs interface lets
testcases / admins control the frequency?

--D

> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_alloc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 7753b61ba532..d550aa5597bf 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -1266,6 +1266,7 @@ xfs_alloc_ag_vextent_near(
>  	int			i;
>  	xfs_agblock_t		bno;
>  	xfs_extlen_t		len;
> +	bool			lastblock;
>  
>  	/* handle unitialized agbno range so caller doesn't have to */
>  	if (!args->min_agbno && !args->max_agbno)
> @@ -1291,7 +1292,12 @@ xfs_alloc_ag_vextent_near(
>  	 * Otherwise run the optimized lookup search algorithm from the current
>  	 * location to the end of the tree.
>  	 */
> -	if (xfs_btree_islastblock(acur.cnt, 0)) {
> +	lastblock = xfs_btree_islastblock(acur.cnt, 0);
> +#ifdef DEBUG
> +	if (lastblock)
> +		lastblock = prandom_u32() & 1;
> +#endif
> +	if (lastblock) {
>  		int	j;
>  
>  		trace_xfs_alloc_cur_lastblock(args);
> -- 
> 2.20.1
>