Re: [PATCH 26/45] xfs: log ticket region debug is largely useless

Brian Foster
2021-03-16T14:56:33Z

On Fri, Mar 05, 2021 at 04:11:24PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> xlog_tic_add_region() is used to trace the regions being added to a
> log ticket to provide information in the situation where a ticket
> reservation overrun occurs. The information gathered is stored int
> the ticket, and dumped if xlog_print_tic_res() is called.
>
> For a front end struct xfs_trans overrun, the ticket only contains
> reservation tracking information - the ticket is never handed to the
> log so has no regions attached to it. The overrun debug information in this
> case comes from xlog_print_trans(), which walks the items attached
> to the transaction and dumps their attached formatted log vectors
> directly. It also dumps the ticket state, but that only contains
> reservation accounting and nothing else. Hence xlog_print_tic_res()
> never dumps region or overrun information from this path.
>
> xlog_tic_add_region() is actually called from xlog_write(), which
> means it is being used to track the regions seen in a
> CIL checkpoint log vector chain. In looking at CIL behaviour
> recently, I've seen 32MB checkpoints regularly exceed 250,000
> regions in the LV chain. The log ticket debug code can track *15*
> regions. IOWs, if there is a ticket overrun in the CIL code, the
> ticket region tracking code is going to be completely useless for
> determining what went wrong. The only thing it can tell us is how
> much of an overrun occurred, and we really don't need extra debug
> information in the log ticket to tell us that.
>
> Indeed, the main place we call xlog_tic_add_region() is also adding
> up the number of regions and the space used so that xlog_write()
> knows how much will be written to the log. This is exactly the same
> information that log ticket is storing once we take away the useless
> region tracking array. Hence xlog_tic_add_region() is not useful,
> but can be called 250,000 times a CIL push...
>
> Just strip all that debug "information" out of the of the log ticket
> and only have it report reservation space information when an
> overrun occurs. This also reduces the size of a log ticket down by
> about 150 bytes...
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> fs/xfs/xfs_log.c | 107 +++---------------------------------------
> fs/xfs/xfs_log_priv.h | 17 -------
> 2 files changed, 6 insertions(+), 118 deletions(-)
>
...
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 7f601c1c9f45..8ee6a5f74396 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -139,16 +139,6 @@ enum xlog_iclog_state {
> /* Ticket reservation region accounting */
> #define XLOG_TIC_LEN_MAX 15
>

This is unused now.

> -/*
> - * Reservation region
> - * As would be stored in xfs_log_iovec but without the i_addr which
> - * we don't care about.
> - */
> -typedef struct xlog_res {
> - uint r_len; /* region length :4 */
> - uint r_type; /* region's transaction type :4 */
> -} xlog_res_t;
> -
> typedef struct xlog_ticket {
> struct list_head t_queue; /* reserve/write queue */
> struct task_struct *t_task; /* task that owns this ticket */
> @@ -159,13 +149,6 @@ typedef struct xlog_ticket {
> char t_ocnt; /* original count : 1 */
> char t_cnt; /* current count : 1 */
> char t_flags; /* properties of reservation : 1 */
> -
> - /* reservation array fields */
> - uint t_res_num; /* num in array : 4 */
> - uint t_res_num_ophdrs; /* num op hdrs : 4 */

I'm curious why we wouldn't want to retain the ophdr count..? That's
managed separately from the _add_region() bits and provides some info on
the total number of vectors, etc. Otherwise looks reasonable.

Brian

> - uint t_res_arr_sum; /* array sum : 4 */
> - uint t_res_o_flow; /* sum overflow : 4 */
> - xlog_res_t t_res_arr[XLOG_TIC_LEN_MAX]; /* array of res : 8 * 15 */
> } xlog_ticket_t;
>
> /*
> --
> 2.28.0
>