From: Dave Chinner <dchinner@xxxxxxxxxx>
In the situation where the log is full and the CIL has not recently
flushed, the AIL push threshold is throttled back to the where the
last write of the head of the log was completed. This is stored in
log->l_last_sync_lsn. Hence if the CIL holds > 25% of the log space
pinned by flushes and/or aggregation in progress, we can get the
situation where the head of the log lags a long way behind the
reservation grant head.
When this happens, the AIL push target is trimmed back from where
the reservation grant head wants to push the log tail to, back to
where the head of the log currently is. This means the push target
doesn't reach far enough into the log to actually move the tail
before the transaction reservation goes to sleep.
When the CIL push completes, it moves the log head forward such that
the AIL push target can now be moved, but that has no mechanism for
puhsing the log tail. Further, if the next tail movement of the log
is not large enough wake the waiter (i.e. still not enough space for
it to have a reservation granted), we don't wake anything up, and
hence we do not update the AIL push target to take into account the
head of the log moving and allowing the push target to be moved
forwards.
To avoid this particular condition, if we fail to wake the first
waiter on the grant head because we don't have enough space,
push on the AIL again. This will pick up any movement of the log
head and allow the push target to move forward due to completion of
CIL pushing.
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
fs/xfs/xfs_log.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index b159a9e9fef0..5e21450fb8f5 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -214,15 +214,36 @@ xlog_grant_head_wake(
{
struct xlog_ticket *tic;
int need_bytes;
+ bool woken_task = false;
list_for_each_entry(tic, &head->waiters, t_queue) {
+ /*
+ * The is a chance that the size of the CIL checkpoints in
+ * progress result at the last AIL push resulted in the log head
+ * (l_last_sync_lsn) not reflecting where the log head now is.
+ * Hence when we are woken here, it may be the head of the log
+ * that has moved rather than the tail. In that case, the tail
+ * didn't move and there won't be space available until the AIL
+ * push target is updated and the tail pushed again. If the AIL
+ * is already pushed to it's target, we will hang here until
+ * something else updates the AIL push target.
+ *
+ * Hence if there isn't space to wake the first waiter on the
+ * grant head, we need to push the AIL again to ensure the
+ * target reflects the current log tail and log head position
+ * before we wait for the tail to move again.
+ */
need_bytes = xlog_ticket_reservation(log, head, tic);
- if (*free_bytes < need_bytes)
+ if (*free_bytes < need_bytes) {
+ if (!woken_task)
+ xlog_grant_push_ail(log, need_bytes);
return false;
+ }
*free_bytes -= need_bytes;
trace_xfs_log_grant_wake_up(log, tic);
wake_up_process(tic->t_task);
+ woken_task = true;
}
return true;
--
2.23.0.rc1
