Re: [PATCH 2/2] Debug - don't exhaustively check the AIL on every operation

[Lachlan McIlroy <lachlan@sgi.com>]

Looks good Dave.

There's lots of debug code bound by XFS_TRANS_DEBUG - should we be
enabling this in our QA?

David Chinner wrote:
> Checking the entire AIL on every insert and remove is
> prohibitively expensive - the sustained sequntial create rate
> on a single disk drops from about 1800/s to 60/s because of
> this checking resulting in the xfslogd becoming cpu bound.
> 
> By default on debug builds, only check the next and previous
> entries in the list to ensure they are ordered correctly.
> If you really want, define XFS_TRANS_DEBUG to use the old
> behaviour.
> 
> Signed-off-by: Dave Chinner <dgc@sgi.com>
> ---
>  fs/xfs/xfs_trans_ail.c |   37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)
> 
> Index: 2.6.x-xfs-new/fs/xfs/xfs_trans_ail.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/xfs_trans_ail.c	2007-11-22 10:34:01.564358689 +1100
> +++ 2.6.x-xfs-new/fs/xfs/xfs_trans_ail.c	2007-11-22 10:34:03.320134239 +1100
> @@ -34,9 +34,9 @@ STATIC xfs_log_item_t * xfs_ail_min(xfs_
>  STATIC xfs_log_item_t * xfs_ail_next(xfs_ail_entry_t *, xfs_log_item_t *);
>  
>  #ifdef DEBUG
> -STATIC void xfs_ail_check(xfs_ail_entry_t *);
> +STATIC void xfs_ail_check(xfs_ail_entry_t *, xfs_log_item_t *);
>  #else
> -#define	xfs_ail_check(a)
> +#define	xfs_ail_check(a,l)
>  #endif /* DEBUG */
>  
>  
> @@ -563,7 +563,7 @@ xfs_ail_insert(
>  	next_lip->li_ail.ail_forw = lip;
>  	lip->li_ail.ail_forw->li_ail.ail_back = lip;
>  
> -	xfs_ail_check(base);
> +	xfs_ail_check(base, lip);
>  	return;
>  }
>  
> @@ -577,12 +577,12 @@ xfs_ail_delete(
>  	xfs_log_item_t	*lip)
>  /* ARGSUSED */
>  {
> +	xfs_ail_check(base, lip);
>  	lip->li_ail.ail_forw->li_ail.ail_back = lip->li_ail.ail_back;
>  	lip->li_ail.ail_back->li_ail.ail_forw = lip->li_ail.ail_forw;
>  	lip->li_ail.ail_forw = NULL;
>  	lip->li_ail.ail_back = NULL;
>  
> -	xfs_ail_check(base);
>  	return lip;
>  }
>  
> @@ -626,13 +626,13 @@ xfs_ail_next(
>   */
>  STATIC void
>  xfs_ail_check(
> -	xfs_ail_entry_t *base)
> +	xfs_ail_entry_t *base,
> +	xfs_log_item_t	*lip)
>  {
> -	xfs_log_item_t	*lip;
>  	xfs_log_item_t	*prev_lip;
>  
> -	lip = base->ail_forw;
> -	if (lip == (xfs_log_item_t*)base) {
> +	prev_lip = base->ail_forw;
> +	if (prev_lip == (xfs_log_item_t*)base) {
>  		/*
>  		 * Make sure the pointers are correct when the list
>  		 * is empty.
> @@ -642,9 +642,27 @@ xfs_ail_check(
>  	}
>  
>  	/*
> +	 * Check the next and previous entries are valid.
> +	 */
> +	ASSERT((lip->li_flags & XFS_LI_IN_AIL) != 0);
> +	prev_lip = lip->li_ail.ail_back;
> +	if (prev_lip != (xfs_log_item_t*)base) {
> +		ASSERT(prev_lip->li_ail.ail_forw == lip);
> +		ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) <= 0);
> +	}
> +	prev_lip = lip->li_ail.ail_forw;
> +	if (prev_lip != (xfs_log_item_t*)base) {
> +		ASSERT(prev_lip->li_ail.ail_back == lip);
> +		ASSERT(XFS_LSN_CMP(prev_lip->li_lsn, lip->li_lsn) >= 0);
> +	}
> +
> +
> +#ifdef XFS_TRANS_DEBUG
> +	/*
>  	 * Walk the list checking forward and backward pointers,
>  	 * lsn ordering, and that every entry has the XFS_LI_IN_AIL
> -	 * flag set.
> +	 * flag set. This is really expensive, so only do it when
> +	 * specifically debugging the transaction subsystem.
>  	 */
>  	prev_lip = (xfs_log_item_t*)base;
>  	while (lip != (xfs_log_item_t*)base) {
> @@ -659,5 +677,6 @@ xfs_ail_check(
>  	}
>  	ASSERT(lip == (xfs_log_item_t*)base);
>  	ASSERT(base->ail_back == prev_lip);
> +#endif /* XFS_TRANS_DEBUG */
>  }
>  #endif /* DEBUG */
>