Re: [PATCH 3/3] xfs: use the log grant push threshold to decide if we're going to relog deferred items

[Brian Foster <bfoster@redhat.com>]

On Wed, Sep 16, 2020 at 08:30:09PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Now that we've landed a means for the defer ops manager to ask log items
> to relog themselves to move the log tail forward, we can improve how we
> decide when to relog so that we're not just using an arbitrary hardcoded
> value.
> 
> The XFS log has "push threshold", which tells us how far we'd have to
> move the log tail forward to keep 25% of the ondisk log space available.
> We use this threshold to decide when to force defer ops chains to relog
> themselves.  This avoids unnecessary relogging (which adds extra steps
> to metadata updates) while helping us to avoid pinning the tail.
> 
> A better algorithm would be to relog only when we detect that the time
> required to move the tail forward is greater than the time remaining
> before all the log space gets used up, but letting the upper levels
> drive the relogging means that it is difficult to coordinate relogging
> the lowest LSN'd intents first.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---

FYI, the commit log doesn't match the git branch referenced in the cover
letter.

>  fs/xfs/libxfs/xfs_defer.c |   32 +++++++++++++++++++++++++-------
>  fs/xfs/xfs_log.c          |   41 +++++++++++++++++++++++++++++++----------
>  fs/xfs/xfs_log.h          |    2 ++
>  3 files changed, 58 insertions(+), 17 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> index 7938e4d3af90..97ec36f32a0a 100644
> --- a/fs/xfs/libxfs/xfs_defer.c
> +++ b/fs/xfs/libxfs/xfs_defer.c
> @@ -17,6 +17,7 @@
>  #include "xfs_inode_item.h"
>  #include "xfs_trace.h"
>  #include "xfs_icache.h"
> +#include "xfs_log.h"
>  
>  /*
>   * Deferred Operations in XFS
> @@ -372,15 +373,35 @@ xfs_defer_relog(
>  	struct list_head		*dfops)
>  {
>  	struct xfs_defer_pending	*dfp;
> +	xfs_lsn_t			threshold_lsn;
>  
>  	ASSERT((*tpp)->t_flags & XFS_TRANS_PERM_LOG_RES);
>  
> +	/*
> +	 * Figure out where we need the tail to be in order to maintain the
> +	 * minimum required free space in the log.
> +	 */
> +	threshold_lsn = xlog_grant_push_threshold((*tpp)->t_mountp->m_log, 0);
> +	if (threshold_lsn == NULLCOMMITLSN)
> +		return 0;
> +
>  	list_for_each_entry(dfp, dfops, dfp_list) {
> +		/*
> +		 * If the log intent item for this deferred op is behind the
> +		 * threshold, we're running out of space and need to relog it
> +		 * to release the tail.
> +		 */
> +		if (dfp->dfp_intent == NULL ||

Any reason the NULL check isn't in the previous patch?

> +		    XFS_LSN_CMP(dfp->dfp_intent->li_lsn, threshold_lsn) < 0)
> +			continue;
> +

Logic looks backwards, we should relog (not skip) if li_lsn is within
the threshold, right?

>  		trace_xfs_defer_relog_intent((*tpp)->t_mountp, dfp);
>  		dfp->dfp_intent = xfs_trans_item_relog(dfp->dfp_intent, *tpp);
>  	}
>  
> -	return xfs_defer_trans_roll(tpp);
> +	if ((*tpp)->t_flags & XFS_TRANS_DIRTY)
> +		return xfs_defer_trans_roll(tpp);

I suspect this churn is eliminated if this code uses the threshold logic
from the start..

> +	return 0;
>  }
>  
>  /*
> @@ -444,7 +465,6 @@ xfs_defer_finish_noroll(
>  	struct xfs_trans		**tp)
>  {
>  	struct xfs_defer_pending	*dfp;
> -	unsigned int			nr_rolls = 0;
>  	int				error = 0;
>  	LIST_HEAD(dop_pending);
>  
> @@ -471,11 +491,9 @@ xfs_defer_finish_noroll(
>  			goto out_shutdown;
>  
>  		/* Every few rolls we relog all the intent items. */
> -		if (!(++nr_rolls % 7)) {
> -			error = xfs_defer_relog(tp, &dop_pending);
> -			if (error)
> -				goto out_shutdown;
> -		}
> +		error = xfs_defer_relog(tp, &dop_pending);
> +		if (error)
> +			goto out_shutdown;
>  
>  		dfp = list_first_entry(&dop_pending, struct xfs_defer_pending,
>  				       dfp_list);
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index ad0c69ee8947..62c9e0aaa7df 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1475,14 +1475,15 @@ xlog_commit_record(
>  }
>  
>  /*
> - * Push on the buffer cache code if we ever use more than 75% of the on-disk
> - * log space.  This code pushes on the lsn which would supposedly free up
> - * the 25% which we want to leave free.  We may need to adopt a policy which
> - * pushes on an lsn which is further along in the log once we reach the high
> - * water mark.  In this manner, we would be creating a low water mark.
> + * Compute the LSN push target needed to push on the buffer cache code if we
> + * ever use more than 75% of the on-disk log space.  This code pushes on the
> + * lsn which would supposedly free up the 25% which we want to leave free.  We
> + * may need to adopt a policy which pushes on an lsn which is further along in
> + * the log once we reach the high water mark.  In this manner, we would be
> + * creating a low water mark.
>   */
> -STATIC void
> -xlog_grant_push_ail(
> +xfs_lsn_t
> +xlog_grant_push_threshold(
>  	struct xlog	*log,
>  	int		need_bytes)
>  {
> @@ -1508,7 +1509,7 @@ xlog_grant_push_ail(
>  	free_threshold = max(free_threshold, (log->l_logBBsize >> 2));
>  	free_threshold = max(free_threshold, 256);
>  	if (free_blocks >= free_threshold)
> -		return;
> +		return NULLCOMMITLSN;
>  
>  	xlog_crack_atomic_lsn(&log->l_tail_lsn, &threshold_cycle,
>  						&threshold_block);
> @@ -1528,13 +1529,33 @@ xlog_grant_push_ail(
>  	if (XFS_LSN_CMP(threshold_lsn, last_sync_lsn) > 0)
>  		threshold_lsn = last_sync_lsn;
>  
> +	return threshold_lsn;
> +}
> +
> +/*
> + * Push on the buffer cache code if we ever use more than 75% of the on-disk
> + * log space.  This code pushes on the lsn which would supposedly free up
> + * the 25% which we want to leave free.  We may need to adopt a policy which
> + * pushes on an lsn which is further along in the log once we reach the high
> + * water mark.  In this manner, we would be creating a low water mark.
> + */
> +STATIC void
> +xlog_grant_push_ail(
> +	struct xlog	*log,
> +	int		need_bytes)
> +{
> +	xfs_lsn_t	threshold_lsn;
> +
> +	threshold_lsn = xlog_grant_push_threshold(log, need_bytes);
> +	if (threshold_lsn == NULLCOMMITLSN || XLOG_FORCED_SHUTDOWN(log))
> +		return;
> +
>  	/*
>  	 * Get the transaction layer to kick the dirty buffers out to
>  	 * disk asynchronously. No point in trying to do this if
>  	 * the filesystem is shutting down.
>  	 */
> -	if (!XLOG_FORCED_SHUTDOWN(log))
> -		xfs_ail_push(log->l_ailp, threshold_lsn);
> +	xfs_ail_push(log->l_ailp, threshold_lsn);
>  }

Separate refactoring patch for the xfs_log.c bits, please.

Brian

>  
>  /*
> diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h
> index 1412d6993f1e..58c3fcbec94a 100644
> --- a/fs/xfs/xfs_log.h
> +++ b/fs/xfs/xfs_log.h
> @@ -141,4 +141,6 @@ void	xfs_log_quiesce(struct xfs_mount *mp);
>  bool	xfs_log_check_lsn(struct xfs_mount *, xfs_lsn_t);
>  bool	xfs_log_in_recovery(struct xfs_mount *);
>  
> +xfs_lsn_t xlog_grant_push_threshold(struct xlog *log, int need_bytes);
> +
>  #endif	/* __XFS_LOG_H__ */
>