On Wed, Sep 16, 2020 at 08:30:00PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> There's a subtle design flaw in the deferred log item code that can lead
> to pinning the log tail. Taking up the defer ops chain examples from
> the previous commit, we can get trapped in sequences like this:
>
> Caller hands us a transaction t0 with D0-D3 attached. The defer ops
> chain will look like the following if the transaction rolls succeed:
>
> t1: D0(t0), D1(t0), D2(t0), D3(t0)
> t2: d4(t1), d5(t1), D1(t0), D2(t0), D3(t0)
> t3: d5(t1), D1(t0), D2(t0), D3(t0)
> ...
> t9: d9(t7), D3(t0)
> t10: D3(t0)
> t11: d10(t10), d11(t10)
> t12: d11(t10)
>
> In transaction 9, we finish d9 and try to roll to t10 while holding onto
> an intent item for D3 that we logged in t0.
>
> The previous commit changed the order in which we place new defer ops in
> the defer ops processing chain to reduce the maximum chain length. Now
> make xfs_defer_finish_noroll capable of relogging the entire chain
> periodically so that we can always move the log tail forward. We do
> this every seven loops, having observed that while most chains never
> exceed seven items in length, the rest go far over that and seem to
> be involved in most of the stall problems.
>
> Callers are now required to ensure that the transaction reservation is
> large enough to handle logging done items and new intent items for the
> maximum possible chain length. Most callers are careful to keep the
> chain lengths low, so the overhead should be minimal.
>
> (Note that in the next patch we'll make it so that we only relog on
> demand, since 7 is an arbitrary number that I used here to get the basic
> mechanics working.)
>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
> fs/xfs/libxfs/xfs_defer.c | 30 ++++++++++++++++++++++++++++++
> fs/xfs/xfs_bmap_item.c | 25 +++++++++++++++++++++++++
> fs/xfs/xfs_extfree_item.c | 29 +++++++++++++++++++++++++++++
> fs/xfs/xfs_refcount_item.c | 27 +++++++++++++++++++++++++++
> fs/xfs/xfs_rmap_item.c | 27 +++++++++++++++++++++++++++
> fs/xfs/xfs_trace.h | 1 +
> fs/xfs/xfs_trans.h | 10 ++++++++++
> 7 files changed, 149 insertions(+)
>
>
> diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> index 84a70edd0da1..7938e4d3af90 100644
> --- a/fs/xfs/libxfs/xfs_defer.c
> +++ b/fs/xfs/libxfs/xfs_defer.c
> @@ -361,6 +361,28 @@ xfs_defer_cancel_list(
> }
> }
>
> +/*
> + * Prevent a log intent item from pinning the tail of the log by logging a
> + * done item to release the intent item; and then log a new intent item.
> + * The caller should provide a fresh transaction and roll it after we're done.
> + */
> +static int
> +xfs_defer_relog(
> + struct xfs_trans **tpp,
> + struct list_head *dfops)
> +{
> + struct xfs_defer_pending *dfp;
> +
> + ASSERT((*tpp)->t_flags & XFS_TRANS_PERM_LOG_RES);
> +
> + list_for_each_entry(dfp, dfops, dfp_list) {
> + 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);
> +}
> +
> /*
> * Log an intent-done item for the first pending intent, and finish the work
> * items.
> @@ -422,6 +444,7 @@ 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);
>
> @@ -447,6 +470,13 @@ xfs_defer_finish_noroll(
> if (error)
> 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;
> + }
> +
I thought the purpose of the push threshold detection was to avoid this
hardcoded relog logic..? On a quick look, I see the next patch
immediately replaces this. That should probably be refactored such that
the proper code is introduced from the start...
> dfp = list_first_entry(&dop_pending, struct xfs_defer_pending,
> dfp_list);
> error = xfs_defer_finish_one(*tp, dfp);
> diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c
> index 8461285a9dd9..2d4c60776714 100644
> --- a/fs/xfs/xfs_bmap_item.c
> +++ b/fs/xfs/xfs_bmap_item.c
> @@ -535,6 +535,30 @@ xfs_bui_item_match(
> return BUI_ITEM(lip)->bui_format.bui_id == intent_id;
> }
>
> +/* Relog an intent item to push the log tail forward. */
> +static struct xfs_log_item *
> +xfs_bui_item_relog(
> + struct xfs_log_item *intent,
> + struct xfs_trans *tp)
> +{
> + struct xfs_bud_log_item *budp;
> + struct xfs_bui_log_item *buip;
> + struct xfs_map_extent *extp;
> +
> + extp = BUI_ITEM(intent)->bui_format.bui_extents;
> +
> + tp->t_flags |= XFS_TRANS_DIRTY;
> + budp = xfs_trans_get_bud(tp, BUI_ITEM(intent));
> + set_bit(XFS_LI_DIRTY, &budp->bud_item.li_flags);
> +
> + buip = xfs_bui_init(tp->t_mountp);
> + memcpy(buip->bui_format.bui_extents, extp, sizeof(*extp));
> + atomic_set(&buip->bui_next_extent, 1);
I was a little confused by the hardcoded value, then I saw that the max
value is one. Since we have the extent array and ->bui_next_extent
fields, perhaps this would be more robust if coded to copy those such
that it wouldn't break if the max value changed..? E.g., memcpy() the maps *
bui_nextents and copy the next_extent index directly.
(Looking further, I see that the other intent relog handlers follow that
pattern presumably because they allow a count > 1. So IOW, I'd just
follow that same pattern here.)
Brian
> + xfs_trans_add_item(tp, &buip->bui_item);
> + set_bit(XFS_LI_DIRTY, &buip->bui_item.li_flags);
> + return &buip->bui_item;
> +}
> +
> static const struct xfs_item_ops xfs_bui_item_ops = {
> .iop_size = xfs_bui_item_size,
> .iop_format = xfs_bui_item_format,
> @@ -542,6 +566,7 @@ static const struct xfs_item_ops xfs_bui_item_ops = {
> .iop_release = xfs_bui_item_release,
> .iop_recover = xfs_bui_item_recover,
> .iop_match = xfs_bui_item_match,
> + .iop_relog = xfs_bui_item_relog,
> };
>
> /*
> diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c
> index f544ec007c12..7c3eebfd9064 100644
> --- a/fs/xfs/xfs_extfree_item.c
> +++ b/fs/xfs/xfs_extfree_item.c
> @@ -648,6 +648,34 @@ xfs_efi_item_match(
> return EFI_ITEM(lip)->efi_format.efi_id == intent_id;
> }
>
> +/* Relog an intent item to push the log tail forward. */
> +static struct xfs_log_item *
> +xfs_efi_item_relog(
> + struct xfs_log_item *intent,
> + struct xfs_trans *tp)
> +{
> + struct xfs_efd_log_item *efdp;
> + struct xfs_efi_log_item *efip;
> + struct xfs_extent *extp;
> + unsigned int count;
> +
> + count = EFI_ITEM(intent)->efi_format.efi_nextents;
> + extp = EFI_ITEM(intent)->efi_format.efi_extents;
> +
> + tp->t_flags |= XFS_TRANS_DIRTY;
> + efdp = xfs_trans_get_efd(tp, EFI_ITEM(intent), count);
> + efdp->efd_next_extent = count;
> + memcpy(efdp->efd_format.efd_extents, extp, count * sizeof(*extp));
> + set_bit(XFS_LI_DIRTY, &efdp->efd_item.li_flags);
> +
> + efip = xfs_efi_init(tp->t_mountp, count);
> + memcpy(efip->efi_format.efi_extents, extp, count * sizeof(*extp));
> + atomic_set(&efip->efi_next_extent, count);
> + xfs_trans_add_item(tp, &efip->efi_item);
> + set_bit(XFS_LI_DIRTY, &efip->efi_item.li_flags);
> + return &efip->efi_item;
> +}
> +
> static const struct xfs_item_ops xfs_efi_item_ops = {
> .iop_size = xfs_efi_item_size,
> .iop_format = xfs_efi_item_format,
> @@ -655,6 +683,7 @@ static const struct xfs_item_ops xfs_efi_item_ops = {
> .iop_release = xfs_efi_item_release,
> .iop_recover = xfs_efi_item_recover,
> .iop_match = xfs_efi_item_match,
> + .iop_relog = xfs_efi_item_relog,
> };
>
> /*
> diff --git a/fs/xfs/xfs_refcount_item.c b/fs/xfs/xfs_refcount_item.c
> index 30e579b5857d..9af7099e2a86 100644
> --- a/fs/xfs/xfs_refcount_item.c
> +++ b/fs/xfs/xfs_refcount_item.c
> @@ -566,6 +566,32 @@ xfs_cui_item_match(
> return CUI_ITEM(lip)->cui_format.cui_id == intent_id;
> }
>
> +/* Relog an intent item to push the log tail forward. */
> +static struct xfs_log_item *
> +xfs_cui_item_relog(
> + struct xfs_log_item *intent,
> + struct xfs_trans *tp)
> +{
> + struct xfs_cud_log_item *cudp;
> + struct xfs_cui_log_item *cuip;
> + struct xfs_phys_extent *extp;
> + unsigned int count;
> +
> + count = CUI_ITEM(intent)->cui_format.cui_nextents;
> + extp = CUI_ITEM(intent)->cui_format.cui_extents;
> +
> + tp->t_flags |= XFS_TRANS_DIRTY;
> + cudp = xfs_trans_get_cud(tp, CUI_ITEM(intent));
> + set_bit(XFS_LI_DIRTY, &cudp->cud_item.li_flags);
> +
> + cuip = xfs_cui_init(tp->t_mountp, count);
> + memcpy(cuip->cui_format.cui_extents, extp, count * sizeof(*extp));
> + atomic_set(&cuip->cui_next_extent, count);
> + xfs_trans_add_item(tp, &cuip->cui_item);
> + set_bit(XFS_LI_DIRTY, &cuip->cui_item.li_flags);
> + return &cuip->cui_item;
> +}
> +
> static const struct xfs_item_ops xfs_cui_item_ops = {
> .iop_size = xfs_cui_item_size,
> .iop_format = xfs_cui_item_format,
> @@ -573,6 +599,7 @@ static const struct xfs_item_ops xfs_cui_item_ops = {
> .iop_release = xfs_cui_item_release,
> .iop_recover = xfs_cui_item_recover,
> .iop_match = xfs_cui_item_match,
> + .iop_relog = xfs_cui_item_relog,
> };
>
> /*
> diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c
> index 34dd6b02bdc8..535b0358e0aa 100644
> --- a/fs/xfs/xfs_rmap_item.c
> +++ b/fs/xfs/xfs_rmap_item.c
> @@ -589,6 +589,32 @@ xfs_rui_item_match(
> return RUI_ITEM(lip)->rui_format.rui_id == intent_id;
> }
>
> +/* Relog an intent item to push the log tail forward. */
> +static struct xfs_log_item *
> +xfs_rui_item_relog(
> + struct xfs_log_item *intent,
> + struct xfs_trans *tp)
> +{
> + struct xfs_rud_log_item *rudp;
> + struct xfs_rui_log_item *ruip;
> + struct xfs_map_extent *extp;
> + unsigned int count;
> +
> + count = RUI_ITEM(intent)->rui_format.rui_nextents;
> + extp = RUI_ITEM(intent)->rui_format.rui_extents;
> +
> + tp->t_flags |= XFS_TRANS_DIRTY;
> + rudp = xfs_trans_get_rud(tp, RUI_ITEM(intent));
> + set_bit(XFS_LI_DIRTY, &rudp->rud_item.li_flags);
> +
> + ruip = xfs_rui_init(tp->t_mountp, count);
> + memcpy(ruip->rui_format.rui_extents, extp, count * sizeof(*extp));
> + atomic_set(&ruip->rui_next_extent, count);
> + xfs_trans_add_item(tp, &ruip->rui_item);
> + set_bit(XFS_LI_DIRTY, &ruip->rui_item.li_flags);
> + return &ruip->rui_item;
> +}
> +
> static const struct xfs_item_ops xfs_rui_item_ops = {
> .iop_size = xfs_rui_item_size,
> .iop_format = xfs_rui_item_format,
> @@ -596,6 +622,7 @@ static const struct xfs_item_ops xfs_rui_item_ops = {
> .iop_release = xfs_rui_item_release,
> .iop_recover = xfs_rui_item_recover,
> .iop_match = xfs_rui_item_match,
> + .iop_relog = xfs_rui_item_relog,
> };
>
> /*
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index a3a35a2d8ed9..362c155be525 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -2533,6 +2533,7 @@ DEFINE_DEFER_PENDING_EVENT(xfs_defer_create_intent);
> DEFINE_DEFER_PENDING_EVENT(xfs_defer_cancel_list);
> DEFINE_DEFER_PENDING_EVENT(xfs_defer_pending_finish);
> DEFINE_DEFER_PENDING_EVENT(xfs_defer_pending_abort);
> +DEFINE_DEFER_PENDING_EVENT(xfs_defer_relog_intent);
>
> #define DEFINE_BMAP_FREE_DEFERRED_EVENT DEFINE_PHYS_EXTENT_DEFERRED_EVENT
> DEFINE_BMAP_FREE_DEFERRED_EVENT(xfs_bmap_free_defer);
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 995c1513693c..e838e8327510 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -78,6 +78,8 @@ struct xfs_item_ops {
> int (*iop_recover)(struct xfs_log_item *lip,
> struct xfs_defer_capture **dfcp);
> bool (*iop_match)(struct xfs_log_item *item, uint64_t id);
> + struct xfs_log_item *(*iop_relog)(struct xfs_log_item *intent,
> + struct xfs_trans *tp);
> };
>
> /*
> @@ -239,4 +241,12 @@ void xfs_trans_buf_copy_type(struct xfs_buf *dst_bp,
>
> extern kmem_zone_t *xfs_trans_zone;
>
> +static inline struct xfs_log_item *
> +xfs_trans_item_relog(
> + struct xfs_log_item *lip,
> + struct xfs_trans *tp)
> +{
> + return lip->li_ops->iop_relog(lip, tp);
> +}
> +
> #endif /* __XFS_TRANS_H__ */
>
