On Mon, Sep 21, 2020 at 08:59:58AM -0400, Brian Foster wrote:
> On Thu, Sep 17, 2020 at 07:22:33PM -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.
> >
> > The decision to relog an intent item is made based on whether or not the
> > intent was added to the current checkpoint. If so, the checkpoint is
> > still open and there's no point in relogging. Otherwise, the old
> > checkpoint is closed and we relog the intent to add it to the current
> > one.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> > ---
> ...
> > diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> > index 84a70edd0da1..288649ca5821 100644
> > --- a/fs/xfs/libxfs/xfs_defer.c
> > +++ b/fs/xfs/libxfs/xfs_defer.c
> ...
> > @@ -361,6 +362,40 @@ 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) {
> > + /*
> > + * If the log intent item for this deferred op is in a
> > + * different checkpoint, relog it to keep the log tail moving
> > + * forward. We're ok with this being racy because an incorrect
> > + * decision means we'll be a little slower at pushing the tail.
> > + */
> > + if (dfp->dfp_intent == NULL ||
> > + xfs_log_item_in_current_chkpt(dfp->dfp_intent))
> > + continue;
>
> I'm curious why we'd change over to using much more crude relog logic
> from what was in the previous version..? Hmm.. I see that Dave suggested
> this approach in v1, but that was in reply to the hardcoded 'if
> (!(++nr_rolls % 7))' thing which was immediately removed in the next
> patch. Was that missed in the review feedback?
No, I got confused and thought Dave wanted me to replace all that logic
with the single call to _in_current_chkpt. I had wondered, since it
seemed to me that we only need to relog intents if they've passed out of
the current checkpoint /and/ we've used up a lot of log space, but...
> I can certainly see favoring this logic over the roll count thing, but
> doing this over the push logic doesn't make a lot of sense to me. The
> logic isn't doesn't seem any more simple, we're calling a log helper to
> do some LSN checking in either case, but the resulting behavior seems
> far less predictable when we consider external factors such as workload
> (fsyncs?), log size, size of the op chain, etc. It also looks like we're
> now far more likely to relog the entire chain over and over in certain
> cases (as it grows) as opposed to just the intents that have been
> sitting around waiting to be processed and pinning the tail of the log.
>
> ISTM that the ideal approach is to 1. use the push logic to determine if
> an item needs relogging in the first place and then 2. use this sort of
> checkpoint checking logic to filter out subsequent relogs once an intent
> has been relogged and committed to the CIL, but has not yet checkpointed
> to the log (and so hasn't moved on disk)... hm?
...yeah, it looks like we're in agreement, at least. I'll re-add the
threshold checking. :)
--D
>
> Brian
>
> > +
> > + trace_xfs_defer_relog_intent((*tpp)->t_mountp, dfp);
> > + dfp->dfp_intent = xfs_trans_item_relog(dfp->dfp_intent, *tpp);
> > + }
> > +
> > + if ((*tpp)->t_flags & XFS_TRANS_DIRTY)
> > + return xfs_defer_trans_roll(tpp);
> > + return 0;
> > +}
> > +
> > /*
> > * Log an intent-done item for the first pending intent, and finish the work
> > * items.
> > @@ -447,6 +482,11 @@ xfs_defer_finish_noroll(
> > if (error)
> > goto out_shutdown;
> >
> > + /* Every few rolls we relog all the intent items. */
> > + error = xfs_defer_relog(tp, &dop_pending);
> > + if (error)
> > + goto out_shutdown;
> > +
> > 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 008436ef5bce..6317fdf4a3a0 100644
> > --- a/fs/xfs/xfs_bmap_item.c
> > +++ b/fs/xfs/xfs_bmap_item.c
> > @@ -532,6 +532,32 @@ 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;
> > + unsigned int count;
> > +
> > + count = BUI_ITEM(intent)->bui_format.bui_nextents;
> > + 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, count * sizeof(*extp));
> > + atomic_set(&buip->bui_next_extent, count);
> > + 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,
> > @@ -539,6 +565,7 @@ 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 337fae015bca..30a53f3913d1 100644
> > --- a/fs/xfs/xfs_extfree_item.c
> > +++ b/fs/xfs/xfs_extfree_item.c
> > @@ -642,6 +642,34 @@ 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,
> > @@ -649,6 +677,7 @@ 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 c78412755b8a..cf0205fdc607 100644
> > --- a/fs/xfs/xfs_refcount_item.c
> > +++ b/fs/xfs/xfs_refcount_item.c
> > @@ -560,6 +560,32 @@ 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,
> > @@ -567,6 +593,7 @@ 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 c1a1fd62ca74..3237243d375d 100644
> > --- a/fs/xfs/xfs_rmap_item.c
> > +++ b/fs/xfs/xfs_rmap_item.c
> > @@ -583,6 +583,32 @@ 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,
> > @@ -590,6 +616,7 @@ 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__ */
> >
>
