Date:   Mon, 4 Nov 2019 15:18:34 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/28] xfs: tail updates only need to occur when LSN
 changes

On Fri, Nov 01, 2019 at 10:45:57AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@...hat.com>
> 
> We currently wake anything waiting on the log tail to move whenever
> the log item at the tail of the log is removed. Historically this
> was fine behaviour because there were very few items at any given
> LSN. But with delayed logging, there may be thousands of items at
> any given LSN, and we can't move the tail until they are all gone.
> 
> Hence if we are removing them in near tail-first order, we might be
> waking up processes waiting on the tail LSN to change (e.g. log
> space waiters) repeatedly without them being able to make progress.
> This also occurs with the new sync push waiters, and can result in
> thousands of spurious wakeups every second when under heavy direct
> reclaim pressure.
> 
> To fix this, check that the tail LSN has actually changed on the
> AIL before triggering wakeups. This will reduce the number of
> spurious wakeups when doing bulk AIL removal and make this code much
> more efficient.
> 
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Reviewed-by: Brian Foster <bfoster@...hat.com>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com>

--D

> ---
>  fs/xfs/xfs_inode_item.c | 18 ++++++++++----
>  fs/xfs/xfs_trans_ail.c  | 52 ++++++++++++++++++++++++++++-------------
>  fs/xfs/xfs_trans_priv.h |  4 ++--
>  3 files changed, 51 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index ab12e526540a..79ffe6dff115 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -731,19 +731,27 @@ xfs_iflush_done(
>  	 * holding the lock before removing the inode from the AIL.
>  	 */
>  	if (need_ail) {
> -		bool			mlip_changed = false;
> +		xfs_lsn_t	tail_lsn = 0;
>  
>  		/* this is an opencoded batch version of xfs_trans_ail_delete */
>  		spin_lock(&ailp->ail_lock);
>  		list_for_each_entry(blip, &tmp, li_bio_list) {
>  			if (INODE_ITEM(blip)->ili_logged &&
> -			    blip->li_lsn == INODE_ITEM(blip)->ili_flush_lsn)
> -				mlip_changed |= xfs_ail_delete_one(ailp, blip);
> -			else {
> +			    blip->li_lsn == INODE_ITEM(blip)->ili_flush_lsn) {
> +				/*
> +				 * xfs_ail_update_finish() only cares about the
> +				 * lsn of the first tail item removed, any
> +				 * others will be at the same or higher lsn so
> +				 * we just ignore them.
> +				 */
> +				xfs_lsn_t lsn = xfs_ail_delete_one(ailp, blip);
> +				if (!tail_lsn && lsn)
> +					tail_lsn = lsn;
> +			} else {
>  				xfs_clear_li_failed(blip);
>  			}
>  		}
> -		xfs_ail_update_finish(ailp, mlip_changed);
> +		xfs_ail_update_finish(ailp, tail_lsn);
>  	}
>  
>  	/*
> diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> index 656819523bbd..685a21cd24c0 100644
> --- a/fs/xfs/xfs_trans_ail.c
> +++ b/fs/xfs/xfs_trans_ail.c
> @@ -108,17 +108,25 @@ xfs_ail_next(
>   * We need the AIL lock in order to get a coherent read of the lsn of the last
>   * item in the AIL.
>   */
> +static xfs_lsn_t
> +__xfs_ail_min_lsn(
> +	struct xfs_ail		*ailp)
> +{
> +	struct xfs_log_item	*lip = xfs_ail_min(ailp);
> +
> +	if (lip)
> +		return lip->li_lsn;
> +	return 0;
> +}
> +
>  xfs_lsn_t
>  xfs_ail_min_lsn(
>  	struct xfs_ail		*ailp)
>  {
> -	xfs_lsn_t		lsn = 0;
> -	struct xfs_log_item	*lip;
> +	xfs_lsn_t		lsn;
>  
>  	spin_lock(&ailp->ail_lock);
> -	lip = xfs_ail_min(ailp);
> -	if (lip)
> -		lsn = lip->li_lsn;
> +	lsn = __xfs_ail_min_lsn(ailp);
>  	spin_unlock(&ailp->ail_lock);
>  
>  	return lsn;
> @@ -681,11 +689,12 @@ xfs_ail_push_all_sync(
>  void
>  xfs_ail_update_finish(
>  	struct xfs_ail		*ailp,
> -	bool			do_tail_update) __releases(ailp->ail_lock)
> +	xfs_lsn_t		old_lsn) __releases(ailp->ail_lock)
>  {
>  	struct xfs_mount	*mp = ailp->ail_mount;
>  
> -	if (!do_tail_update) {
> +	/* if the tail lsn hasn't changed, don't do updates or wakeups. */
> +	if (!old_lsn || old_lsn == __xfs_ail_min_lsn(ailp)) {
>  		spin_unlock(&ailp->ail_lock);
>  		return;
>  	}
> @@ -730,7 +739,7 @@ xfs_trans_ail_update_bulk(
>  	xfs_lsn_t		lsn) __releases(ailp->ail_lock)
>  {
>  	struct xfs_log_item	*mlip;
> -	int			mlip_changed = 0;
> +	xfs_lsn_t		tail_lsn = 0;
>  	int			i;
>  	LIST_HEAD(tmp);
>  
> @@ -745,9 +754,10 @@ xfs_trans_ail_update_bulk(
>  				continue;
>  
>  			trace_xfs_ail_move(lip, lip->li_lsn, lsn);
> +			if (mlip == lip && !tail_lsn)
> +				tail_lsn = lip->li_lsn;
> +
>  			xfs_ail_delete(ailp, lip);
> -			if (mlip == lip)
> -				mlip_changed = 1;
>  		} else {
>  			trace_xfs_ail_insert(lip, 0, lsn);
>  		}
> @@ -758,15 +768,23 @@ xfs_trans_ail_update_bulk(
>  	if (!list_empty(&tmp))
>  		xfs_ail_splice(ailp, cur, &tmp, lsn);
>  
> -	xfs_ail_update_finish(ailp, mlip_changed);
> +	xfs_ail_update_finish(ailp, tail_lsn);
>  }
>  
> -bool
> +/*
> + * Delete one log item from the AIL.
> + *
> + * If this item was at the tail of the AIL, return the LSN of the log item so
> + * that we can use it to check if the LSN of the tail of the log has moved
> + * when finishing up the AIL delete process in xfs_ail_update_finish().
> + */
> +xfs_lsn_t
>  xfs_ail_delete_one(
>  	struct xfs_ail		*ailp,
>  	struct xfs_log_item	*lip)
>  {
>  	struct xfs_log_item	*mlip = xfs_ail_min(ailp);
> +	xfs_lsn_t		lsn = lip->li_lsn;
>  
>  	trace_xfs_ail_delete(lip, mlip->li_lsn, lip->li_lsn);
>  	xfs_ail_delete(ailp, lip);
> @@ -774,7 +792,9 @@ xfs_ail_delete_one(
>  	clear_bit(XFS_LI_IN_AIL, &lip->li_flags);
>  	lip->li_lsn = 0;
>  
> -	return mlip == lip;
> +	if (mlip == lip)
> +		return lsn;
> +	return 0;
>  }
>  
>  /**
> @@ -805,7 +825,7 @@ xfs_trans_ail_delete(
>  	int			shutdown_type)
>  {
>  	struct xfs_mount	*mp = ailp->ail_mount;
> -	bool			need_update;
> +	xfs_lsn_t		tail_lsn;
>  
>  	if (!test_bit(XFS_LI_IN_AIL, &lip->li_flags)) {
>  		spin_unlock(&ailp->ail_lock);
> @@ -818,8 +838,8 @@ xfs_trans_ail_delete(
>  		return;
>  	}
>  
> -	need_update = xfs_ail_delete_one(ailp, lip);
> -	xfs_ail_update_finish(ailp, need_update);
> +	tail_lsn = xfs_ail_delete_one(ailp, lip);
> +	xfs_ail_update_finish(ailp, tail_lsn);
>  }
>  
>  int
> diff --git a/fs/xfs/xfs_trans_priv.h b/fs/xfs/xfs_trans_priv.h
> index 64ffa746730e..35655eac01a6 100644
> --- a/fs/xfs/xfs_trans_priv.h
> +++ b/fs/xfs/xfs_trans_priv.h
> @@ -91,8 +91,8 @@ xfs_trans_ail_update(
>  	xfs_trans_ail_update_bulk(ailp, NULL, &lip, 1, lsn);
>  }
>  
> -bool xfs_ail_delete_one(struct xfs_ail *ailp, struct xfs_log_item *lip);
> -void xfs_ail_update_finish(struct xfs_ail *ailp, bool do_tail_update)
> +xfs_lsn_t xfs_ail_delete_one(struct xfs_ail *ailp, struct xfs_log_item *lip);
> +void xfs_ail_update_finish(struct xfs_ail *ailp, xfs_lsn_t old_lsn)
>  			__releases(ailp->ail_lock);
>  void xfs_trans_ail_delete(struct xfs_ail *ailp, struct xfs_log_item *lip,
>  		int shutdown_type);
> -- 
> 2.24.0.rc0
> 

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