[PATCH 3/4] xfs: expose the log push threshold

Darrick J. Wong
2020-09-29T17:44:38Z

Separate the computation of the log push threshold and the push logic in
xlog_grant_push_ail. This enables higher level code to determine (for
example) that it is holding on to a logged intent item and the log is so
busy that it is more than 75% full. In that case, it would be desirable
to move the log item towards the head to release the tail, which we will
cover in the next patch.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
---
fs/xfs/xfs_log.c | 40 ++++++++++++++++++++++++++++++----------
fs/xfs/xfs_log.h | 2 ++
2 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 7a4ba408a3a2..fa2d05e65ff1 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1475,14 +1475,14 @@ 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 that we'd need to push the log tail towards in order to have
+ * (a) enough on-disk log space to log the number of bytes specified, (b) at
+ * least 25% of the log space free, and (c) at least 256 blocks free. If the
+ * log free space already meets all three thresholds, this function returns
+ * NULLCOMMITLSN.
*/
-STATIC void
-xlog_grant_push_ail(
+xfs_lsn_t
+xlog_grant_push_threshold(
struct xlog *log,
int need_bytes)
{
@@ -1508,7 +1508,7 @@ 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 +1528,33 @@ xlog_grant_push_ail(
if (XFS_LSN_CMP(threshold_lsn, last_sync_lsn) > 0)
threshold_lsn = last_sync_lsn;

+ return threshold_lsn;
+}
+
+/*
+ * Push the tail of the log if we need to do so to maintain the free log space
+ * thresholds set out by xlog_grant_push_threshold. 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);
}

/*
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__ */