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Move AIL pushing into it's own thread

When many hundreds to thousands of threads all try to do simultaneous
transactions and the log is in a tail-pushing situation (i.e. full),
we can get multiple threads walking the AIL list and contending on
the AIL lock.

The AIL push is, in effect, a simple I/O dispatch algorithm complicated
by the ordering constraints placed on it by the transaction subsystem.
It really does not need multiple threads to push on it - even when
only a single CPU is pushing the AIL, it can push the I/O out far faster
that pretty much any disk subsystem can handle.

So, to avoid contention problems stemming from multiple list walkers,
move the list walk off into another thread and simply provide a "target"
to push to. When a thread requires a push, it sets the target and wakes
the push thread, then goes to sleep waiting for the required amount
of space to become available in the log.

This mechanism should also be a lot fairer under heavy load as the
waiters will queue in arrival order, rather than queuing in "who completed
a push first" order.

Also, by moving the pushing to a separate thread we can do more effectively
overload detection and prevention as we can keep context from loop iteration
to loop iteration. That is, we can push only part of the list each loop and not
have to loop back to the start of the list every time we run. This should
also help by reducing the number of items we try to lock and/or push items
that we cannot move.

Note that this patch is not intended to solve the inefficiencies in the
AIL structure and the associated issues with extremely large list contents.
That needs to be addresses separately; parallel access would cause problems
to any new structure as well, so I'm only aiming to isolate the structure
from unbounded parallelism here.
Merge of xfs-linux-melb:xfs-kern:30371a by kenmcd.

  Define private AIl threading interface functions.

/*
 * Copyright (c) 2000,2002,2005 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#ifndef __XFS_TRANS_PRIV_H__
#define	__XFS_TRANS_PRIV_H__

struct xfs_log_item;
struct xfs_log_item_desc;
struct xfs_mount;
struct xfs_trans;

/*
 * From xfs_trans_item.c
 */
struct xfs_log_item_desc	*xfs_trans_add_item(struct xfs_trans *,
					    struct xfs_log_item *);
void				xfs_trans_free_item(struct xfs_trans *,
					    struct xfs_log_item_desc *);
struct xfs_log_item_desc	*xfs_trans_find_item(struct xfs_trans *,
					     struct xfs_log_item *);
struct xfs_log_item_desc	*xfs_trans_first_item(struct xfs_trans *);
struct xfs_log_item_desc	*xfs_trans_next_item(struct xfs_trans *,
					     struct xfs_log_item_desc *);
void				xfs_trans_free_items(struct xfs_trans *, int);
void				xfs_trans_unlock_items(struct xfs_trans *,
							xfs_lsn_t);
void				xfs_trans_free_busy(xfs_trans_t *tp);
xfs_log_busy_slot_t		*xfs_trans_add_busy(xfs_trans_t *tp,
						    xfs_agnumber_t ag,
						    xfs_extlen_t idx);

/*
 * From xfs_trans_ail.c
 */
void			xfs_trans_update_ail(struct xfs_mount *mp,
				     struct xfs_log_item *lip, xfs_lsn_t lsn)
				     __releases(mp->m_ail_lock);
void			xfs_trans_delete_ail(struct xfs_mount *mp,
				     struct xfs_log_item *lip)
				     __releases(mp->m_ail_lock);
struct xfs_log_item	*xfs_trans_first_ail(struct xfs_mount *, int *);
struct xfs_log_item	*xfs_trans_next_ail(struct xfs_mount *,
				     struct xfs_log_item *, int *, int *);


/*
 * AIL push thread support
 */
long	xfsaild_push(struct xfs_mount *, xfs_lsn_t *);
void	xfsaild_wakeup(struct xfs_mount *, xfs_lsn_t);
int	xfsaild_start(struct xfs_mount *);
void	xfsaild_stop(struct xfs_mount *);

#endif	/* __XFS_TRANS_PRIV_H__ */