Re: [PATCH 1/8] xfs: push the AIL in xlog_grant_head_wake

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Sep 06, 2019 at 10:05:46AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> 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@redhat.com>
> ---
>  fs/xfs/xfs_log.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index b159a9e9fef0..c2862b1a2780 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -214,15 +214,42 @@ 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

"There is a chance..."

Other than that,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> +		 * progress at the last AIL push target calculation resulted in
> +		 * limiting the target to the log head (l_last_sync_lsn) at the
> +		 * time. This may not reflect where the log head is now as the
> +		 * CIL checkpoints may have completed.
> +		 *
> +		 * Hence when we are woken here, it may be that the head of the
> +		 * log that has moved rather than the tail. As the tail didn't
> +		 * move, there still won't be space available for the
> +		 * reservation we require.  However, if the AIL has already
> +		 * pushed to the target defined by the old log head location, we
> +		 * will hang here waiting for something else to update the AIL
> +		 * push target.
> +		 *
> +		 * Therefore, 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 both 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
>