Re: [PATCH 8/8] xfs: push the grant head when the log head moves forward

Darrick J. Wong
2019-09-05T16:00:14Z

On Thu, Sep 05, 2019 at 06:47:17PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> When the log fills up, we can get into the state where the
> outstanding items in the CIL being committed and aggregated are
> larger than the range that the reservation grant head tail pushing
> will attempt to clean. This can result in the tail pushing range
> being trimmed back to the the log head (l_last_sync_lsn) and so
> may not actually move the push target at all.
>
> When the iclogs associated with the CIL commit finally land, the
> log head moves forward, and this removes the restriction on the AIL
> push target. However, if we already have transactions sleeping on
> the grant head, and there's nothing in the AIL still to flush from
> the current push target, then nothing will move the tail of the log
> and trigger a log reservation wakeup.
>
> Hence the there is nothing that will trigger xlog_grant_push_ail()
> to recalculate the AIL push target and start pushing on the AIL
> again to write back the metadata objects that pin the tail of the
> log and hence free up space and allow the transaction reservations
> to be woken and make progress.
>
> Hence we need to push on the grant head when we move the log head
> forward, as this may be the only trigger we have that can move the
> AIL push target forwards in this situation.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Seems reasonable to me.

There's two unfortunate twists for applying this series -- there won't be
any new for-next trees from Stephen Rothwell until Sept. 30th, which
means we (XFS developers) are all pretty much on our own for testing
this in the xfs for-next branch.

The second twist of course is that I'm leaving Friday afternoon for a
vacation. That means either (a) everything passes muster, I fix the
comment nits, and push this into xfs for-next before I go; (b) there are
deeper review comments and so this waits until I return on the 16th; or
(c) I guess Dave could tack it on for-next himself when the patches are
ready since he still has commit access. ;)

Either way this probably means a separate pull request for the log fixes
during the second week of the merge window. Thoughts/flames?

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
> fs/xfs/xfs_log.c | 72 +++++++++++++++++++++++++++++++-----------------
> 1 file changed, 47 insertions(+), 25 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index bef314361bc4..f90765af6916 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -2648,6 +2648,46 @@ xlog_get_lowest_lsn(
> return lowest_lsn;
> }
>
> +/*
> + * Completion of a iclog IO does not imply that a transaction has completed, as
> + * transactions can be large enough to span many iclogs. We cannot change the
> + * tail of the log half way through a transaction as this may be the only
> + * transaction in the log and moving the tail to point to the middle of it
> + * will prevent recovery from finding the start of the transaction. Hence we
> + * should only update the last_sync_lsn if this iclog contains transaction
> + * completion callbacks on it.
> + *
> + * We have to do this before we drop the icloglock to ensure we are the only one
> + * that can update it.
> + *
> + * If we are moving the last_sync_lsn forwards, we also need to ensure we kick
> + * the reservation grant head pushing. This is due to the fact that the push
> + * target is bound by the current last_sync_lsn value. Hence if we have a large
> + * amount of log space bound up in this committing transaction then the
> + * last_sync_lsn value may be the limiting factor preventing tail pushing from
> + * freeing space in the log. Hence once we've updated the last_sync_lsn we
> + * should push the AIL to ensure the push target (and hence the grant head) is
> + * no longer bound by the old log head location and can move forwards and make
> + * progress again.
> + */
> +static void
> +xlog_state_set_callback(
> + struct xlog *log,
> + struct xlog_in_core *iclog,
> + xfs_lsn_t header_lsn)
> +{
> + iclog->ic_state = XLOG_STATE_CALLBACK;
> +
> + ASSERT(XFS_LSN_CMP(atomic64_read(&log->l_last_sync_lsn),
> + header_lsn) <= 0);
> +
> + if (list_empty_careful(&iclog->ic_callbacks))
> + return;
> +
> + atomic64_set(&log->l_last_sync_lsn, header_lsn);
> + xlog_grant_push_ail(log, 0);
> +}
> +
> /*
> * Return true if we need to stop processing, false to continue to the next
> * iclog. The caller will need to run callbacks if the iclog is returned in the
> @@ -2661,6 +2701,7 @@ xlog_state_iodone_process_iclog(
> bool *ioerror)
> {
> xfs_lsn_t lowest_lsn;
> + xfs_lsn_t header_lsn;
>
> /* Skip all iclogs in the ACTIVE & DIRTY states */
> if (iclog->ic_state & (XLOG_STATE_ACTIVE | XLOG_STATE_DIRTY))
> @@ -2700,34 +2741,15 @@ xlog_state_iodone_process_iclog(
> * callbacks) see the above if.
> *
> * We will do one more check here to see if we have chased our tail
> - * around.
> + * around. If this is not the lowest lsn iclog, then we will leave it
> + * for another completion to process.
> */
> + header_lsn = be64_to_cpu(iclog->ic_header.h_lsn);
> lowest_lsn = xlog_get_lowest_lsn(log);
> - if (lowest_lsn &&
> - XFS_LSN_CMP(lowest_lsn, be64_to_cpu(iclog->ic_header.h_lsn)) < 0)
> - return false; /* Leave this iclog for another thread */
> -
> - iclog->ic_state = XLOG_STATE_CALLBACK;
> -
> - /*
> - * Completion of a iclog IO does not imply that a transaction has
> - * completed, as transactions can be large enough to span many iclogs.
> - * We cannot change the tail of the log half way through a transaction
> - * as this may be the only transaction in the log and moving th etail to
> - * point to the middle of it will prevent recovery from finding the
> - * start of the transaction. Hence we should only update the
> - * last_sync_lsn if this iclog contains transaction completion callbacks
> - * on it.
> - *
> - * We have to do this before we drop the icloglock to ensure we are the
> - * only one that can update it.
> - */
> - ASSERT(XFS_LSN_CMP(atomic64_read(&log->l_last_sync_lsn),
> - be64_to_cpu(iclog->ic_header.h_lsn)) <= 0);
> - if (!list_empty_careful(&iclog->ic_callbacks))
> - atomic64_set(&log->l_last_sync_lsn,
> - be64_to_cpu(iclog->ic_header.h_lsn));
> + if (lowest_lsn && XFS_LSN_CMP(lowest_lsn, header_lsn) < 0)
> + return false;
>
> + xlog_state_set_callback(log, iclog, header_lsn);
> return false;
>
> }
> --
> 2.23.0.rc1
>