Re: [PATCH 07/28] xfs: tail updates only need to occur when LSN changes

Darrick J. Wong
2019-11-04T23:18:45Z

On Fri, Nov 01, 2019 at 10:45:57AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.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@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Brian Foster <bfoster@redhat.com>

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