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

Darrick J. Wong
2020-09-22T15:51:30Z

On Thu, Sep 17, 2020 at 11:28:29AM -0400, Brian Foster wrote:
> 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?

No. Now that I've squashed all these patches together, all the wonky
churn should be eliminated.

> > + 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?

Oops, fixed now.

> > 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.

Ok, fixed.

--D

> 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__ */
> >
>