xfs: grant heads track byte counts, not LSNs

The grant heads in the log track the space reserved in the log for
running transactions. They do this by tracking how far ahead of the
tail that the reservation has reached, and the units for doing this
are {cycle,bytes} for the reserve head rather than {cycle,blocks}
which are normal used by LSNs.

This is annoyingly complex because we have to split, crack and
combined these tuples for any calculation we do to determine log
space and targets. This is computationally expensive as well as
difficult to do atomically and locklessly, as well as limiting the
size of the log to 2^32 bytes.

Really, though, all the grant heads are tracking is how much space
is currently available for use in the log. We can track this as a
simply byte count - we just don't care what the actual physical
location in the log the head and tail are at, just how much space we
have remaining before the head and tail overlap.

So, convert the grant heads to track the byte reservations that are
active rather than the current (cycle, offset) tuples. This means an
empty log has zero bytes consumed, and a full log is when the
reservations reach the size of the log minus the space consumed by
the AIL.

This greatly simplifies the accounting and checks for whether there
is space available. We no longer need to crack or combine LSNs to
determine how much space the log has left, nor do we need to look at
the head or tail of the log to determine how close to full we are.

There is, however, a complexity that needs to be handled. We know
how much space is being tracked in the AIL now via log->l_tail_space
and the log tickets track active reservations and return the unused
portions to the grant heads when ungranted.  Unfortunately, we don't
track the used portion of the grant, so when we transfer log items
from the CIL to the AIL, the space accounted to the grant heads is
transferred to the log tail space.  Hence when we move the AIL head
forwards on item insert, we have to remove that space from the grant
heads.

We also remove the xlog_verify_grant_tail() debug function as it is
no longer useful. The check it performs has been racy since delayed
logging was introduced, but now it is clearly only detecting false
positives so remove it.

The result of this substantially simpler accounting algorithm is an
increase in sustained transaction rate from ~1.3 million
transactions/s to ~1.9 million transactions/s with no increase in
CPU usage. We also remove the 32 bit space limitation on the grant
heads, which will allow us to increase the journal size beyond 2GB
in future.

Note that this renames the sysfs files exposing the log grant space
now that the values are exported in bytes.  This allows xfstests
to auto-detect the old or new ABI.

[hch: move xlog_grant_sub_space out of line,
      update the xlog_grant_{add,sub}_space prototypes,
      rename the sysfs files to allow auto-detection in xfstests]

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

1 files changed, 3 insertions, 30 deletions

diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 0838c57ca8ac..b8778a4fd6b6 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -544,36 +544,6 @@ xlog_assign_atomic_lsn(atomic64_t *lsn, uint cycle, uint block)
 }
 
 /*
- * When we crack the grant head, we sample it first so that the value will not
- * change while we are cracking it into the component values. This means we
- * will always get consistent component values to work from.
- */
-static inline void
-xlog_crack_grant_head_val(int64_t val, int *cycle, int *space)
-{
-	*cycle = val >> 32;
-	*space = val & 0xffffffff;
-}
-
-static inline void
-xlog_crack_grant_head(atomic64_t *head, int *cycle, int *space)
-{
-	xlog_crack_grant_head_val(atomic64_read(head), cycle, space);
-}
-
-static inline int64_t
-xlog_assign_grant_head_val(int cycle, int space)
-{
-	return ((int64_t)cycle << 32) | space;
-}
-
-static inline void
-xlog_assign_grant_head(atomic64_t *head, int cycle, int space)
-{
-	atomic64_set(head, xlog_assign_grant_head_val(cycle, space));
-}
-
-/*
  * Committed Item List interfaces
  */
 int	xlog_cil_init(struct xlog *log);
@@ -639,6 +609,9 @@ xlog_lsn_sub(
 	return (uint64_t)log->l_logsize - BBTOB(lo_block - hi_block);
 }
 
+void xlog_grant_return_space(struct xlog *log, xfs_lsn_t old_head,
+		xfs_lsn_t new_head);
+
 /*
  * The LSN is valid so long as it is behind the current LSN. If it isn't, this
  * means that the next log record that includes this metadata could have a