Re: [PATCH 6/8] btrfs: rework wake_all_tickets

[Nikolay Borisov <nborisov@suse.com>]

On 16.08.19 г. 17:19 ч., Josef Bacik wrote:
> Now that we no longer partially fill tickets we need to rework
> wake_all_tickets to call btrfs_try_to_wakeup_tickets() in order to see
> if any subsequent tickets are able to be satisfied.  If our tickets_id
> changes we know something happened and we can keep flushing.
> 
> Also if we find a ticket that is smaller than the first ticket in our
> queue then we want to retry the flushing loop again in case
> may_commit_transaction() decides we could satisfy the ticket by
> committing the transaction.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/space-info.c | 34 +++++++++++++++++++++++++++-------
>  1 file changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index 8a1c7ada67cb..bd485be783b8 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -676,19 +676,39 @@ static inline int need_do_async_reclaim(struct btrfs_fs_info *fs_info,
>  		!test_bit(BTRFS_FS_STATE_REMOUNTING, &fs_info->fs_state));
>  }
>  
> -static bool wake_all_tickets(struct list_head *head)
> +static bool wake_all_tickets(struct btrfs_fs_info *fs_info,
> +			     struct btrfs_space_info *space_info)
>  {
>  	struct reserve_ticket *ticket;
> +	u64 tickets_id = space_info->tickets_id;
> +	u64 first_ticket_bytes = 0;
> +
> +	while (!list_empty(&space_info->tickets) &&
> +	       tickets_id == space_info->tickets_id) {
> +		ticket = list_first_entry(&space_info->tickets,
> +					  struct reserve_ticket, list);
> +
> +		/*
> +		 * may_commit_transaction will avoid committing the transaction
> +		 * if it doesn't feel like the space reclaimed by the commit
> +		 * would result in the ticket succeeding.  However if we have a
> +		 * smaller ticket in the queue it may be small enough to be
> +		 * satisified by committing the transaction, so if any
> +		 * subsequent ticket is smaller than the first ticket go ahead
> +		 * and send us back for another loop through the enospc flushing
> +		 * code.
> +		 */
> +		if (first_ticket_bytes == 0)
> +			first_ticket_bytes = ticket->bytes;
> +		else if (first_ticket_bytes > ticket->bytes)
> +			return true;
>  
> -	while (!list_empty(head)) {
> -		ticket = list_first_entry(head, struct reserve_ticket, list);
>  		list_del_init(&ticket->list);
>  		ticket->error = -ENOSPC;
>  		wake_up(&ticket->wait);
> -		if (ticket->bytes != ticket->orig_bytes)
> -			return true;
> +		btrfs_try_to_wakeup_tickets(fs_info, space_info);

So the change in this logic is directly related to the implementation of
btrfs_try_to_wakeup_tickets. Because when we fail and remove a ticket in
this function we give a chance that the next ticket *could* be
satisfied. But how well does that work in practice, given you fail
normal prio tickets here, whereas btrfs_try_to_wakeup_tickets first
checks the prio ticket. So even if you are failing normal ticket but
there is one unsatifiable prio ticket that won't really change anything.

>  	}
> -	return false;
> +	return (tickets_id != space_info->tickets_id);
>  }
>  
>  /*
> @@ -756,7 +776,7 @@ static void btrfs_async_reclaim_metadata_space(struct work_struct *work)
>  		if (flush_state > COMMIT_TRANS) {
>  			commit_cycles++;
>  			if (commit_cycles > 2) {
> -				if (wake_all_tickets(&space_info->tickets)) {
> +				if (wake_all_tickets(fs_info, space_info)) {
>  					flush_state = FLUSH_DELAYED_ITEMS_NR;
>  					commit_cycles--;
>  				} else {
>