[XFS] 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.

SGI-PV: 972759
SGI-Modid: xfs-linux-melb:xfs-kern:30371a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 15025e01300a..f7c620ec6e69 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -219,12 +219,18 @@ extern void	xfs_icsb_sync_counters_flags(struct xfs_mount *, int);
 #define xfs_icsb_sync_counters_flags(mp, flags) do { } while (0)
 #endif
 
+typedef struct xfs_ail {
+        xfs_ail_entry_t         xa_ail;
+        uint                    xa_gen;
+        struct task_struct      *xa_task;
+        xfs_lsn_t               xa_target;
+} xfs_ail_t;
+
 typedef struct xfs_mount {
         struct super_block      *m_super;
         xfs_tid_t               m_tid;          /* next unused tid for fs */
         spinlock_t              m_ail_lock;     /* fs AIL mutex */
-        xfs_ail_entry_t         m_ail;          /* fs active log item list */
-        uint                    m_ail_gen;      /* fs AIL generation count */
+        xfs_ail_t               m_ail;          /* fs active log item list */
         xfs_sb_t                m_sb;           /* copy of fs superblock */
         spinlock_t              m_sb_lock;      /* sb counter lock */
         struct xfs_buf          *m_sb_bp;       /* buffer for superblock */