[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, 178 insertions, 91 deletions

diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index 2d3c297259c2..7ed7425478de 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -57,7 +57,7 @@ xfs_trans_tail_ail(
         xfs_log_item_t  *lip;
 
         spin_lock(&mp->m_ail_lock);
-        lip = xfs_ail_min(&(mp->m_ail));
+        lip = xfs_ail_min(&(mp->m_ail.xa_ail));
         if (lip == NULL) {
                 lsn = (xfs_lsn_t)0;
         } else {
@@ -71,119 +71,185 @@ xfs_trans_tail_ail(
 /*
  * xfs_trans_push_ail
  *
- * This routine is called to move the tail of the AIL
- * forward.  It does this by trying to flush items in the AIL
- * whose lsns are below the given threshold_lsn.
+ * This routine is called to move the tail of the AIL forward.  It does this by
+ * trying to flush items in the AIL whose lsns are below the given
+ * threshold_lsn.
  *
- * The routine returns the lsn of the tail of the log.
+ * the push is run asynchronously in a separate thread, so we return the tail
+ * of the log right now instead of the tail after the push. This means we will
+ * either continue right away, or we will sleep waiting on the async thread to
+ * do it's work.
+ *
+ * We do this unlocked - we only need to know whether there is anything in the
+ * AIL at the time we are called. We don't need to access the contents of
+ * any of the objects, so the lock is not needed.
  */
-xfs_lsn_t
+void
 xfs_trans_push_ail(
         xfs_mount_t             *mp,
         xfs_lsn_t               threshold_lsn)
 {
-        xfs_lsn_t               lsn;
         xfs_log_item_t          *lip;
-        int                     gen;
-        int                     restarts;
-        int                     lock_result;
-        int                     flush_log;
 
-#define XFS_TRANS_PUSH_AIL_RESTARTS     1000
+        lip = xfs_ail_min(&mp->m_ail.xa_ail);
+        if (lip && !XFS_FORCED_SHUTDOWN(mp)) {
+                if (XFS_LSN_CMP(threshold_lsn, mp->m_ail.xa_target) > 0)
+                        xfsaild_wakeup(mp, threshold_lsn);
+        }
+}
+
+/*
+ * Return the item in the AIL with the current lsn.
+ * Return the current tree generation number for use
+ * in calls to xfs_trans_next_ail().
+ */
+STATIC xfs_log_item_t *
+xfs_trans_first_push_ail(
+        xfs_mount_t     *mp,
+        int             *gen,
+        xfs_lsn_t       lsn)
+{
+        xfs_log_item_t  *lip;
+
+        lip = xfs_ail_min(&(mp->m_ail.xa_ail));
+        *gen = (int)mp->m_ail.xa_gen;
+        if (lsn == 0)
+                return lip;
+
+        while (lip && (XFS_LSN_CMP(lip->li_lsn, lsn) < 0))
+                lip = lip->li_ail.ail_forw;
+
+        return lip;
+}
+
+/*
+ * Function that does the work of pushing on the AIL
+ */
+long
+xfsaild_push(
+        xfs_mount_t     *mp,
+        xfs_lsn_t       *last_lsn)
+{
+        long            tout = 1000; /* milliseconds */
+        xfs_lsn_t       last_pushed_lsn = *last_lsn;
+        xfs_lsn_t       target =  mp->m_ail.xa_target;
+        xfs_lsn_t       lsn;
+        xfs_log_item_t  *lip;
+        int             gen;
+        int             restarts;
+        int             flush_log, count, stuck;
+
+#define XFS_TRANS_PUSH_AIL_RESTARTS     10
 
         spin_lock(&mp->m_ail_lock);
-        lip = xfs_trans_first_ail(mp, &gen);
-        if (lip == NULL || XFS_FORCED_SHUTDOWN(mp)) {
+        lip = xfs_trans_first_push_ail(mp, &gen, *last_lsn);
+        if (!lip || XFS_FORCED_SHUTDOWN(mp)) {
                 /*
-                 * Just return if the AIL is empty.
+                 * AIL is empty or our push has reached the end.
                  */
                 spin_unlock(&mp->m_ail_lock);
-                return (xfs_lsn_t)0;
+                last_pushed_lsn = 0;
+                goto out;
         }
 
         XFS_STATS_INC(xs_push_ail);
 
         /*
          * While the item we are looking at is below the given threshold
-         * try to flush it out.  Make sure to limit the number of times
-         * we allow xfs_trans_next_ail() to restart scanning from the
-         * beginning of the list.  We'd like not to stop until we've at least
+         * try to flush it out. We'd like not to stop until we've at least
          * tried to push on everything in the AIL with an LSN less than
-         * the given threshold. However, we may give up before that if
-         * we realize that we've been holding the AIL lock for 'too long',
-         * blocking interrupts. Currently, too long is < 500us roughly.
+         * the given threshold.
+         *
+         * However, we will stop after a certain number of pushes and wait
+         * for a reduced timeout to fire before pushing further. This
+         * prevents use from spinning when we can't do anything or there is
+         * lots of contention on the AIL lists.
          */
-        flush_log = 0;
-        restarts = 0;
-        while (((restarts < XFS_TRANS_PUSH_AIL_RESTARTS) &&
-                (XFS_LSN_CMP(lip->li_lsn, threshold_lsn) < 0))) {
+        tout = 10;
+        lsn = lip->li_lsn;
+        flush_log = stuck = count = restarts = 0;
+        while ((XFS_LSN_CMP(lip->li_lsn, target) < 0)) {
+                int     lock_result;
                 /*
-                 * If we can lock the item without sleeping, unlock
-                 * the AIL lock and flush the item.  Then re-grab the
-                 * AIL lock so we can look for the next item on the
-                 * AIL.  Since we unlock the AIL while we flush the
-                 * item, the next routine may start over again at the
-                 * the beginning of the list if anything has changed.
-                 * That is what the generation count is for.
+                 * If we can lock the item without sleeping, unlock the AIL
+                 * lock and flush the item.  Then re-grab the AIL lock so we
+                 * can look for the next item on the AIL. List changes are
+                 * handled by the AIL lookup functions internally
                  *
-                 * If we can't lock the item, either its holder will flush
-                 * it or it is already being flushed or it is being relogged.
-                 * In any of these case it is being taken care of and we
-                 * can just skip to the next item in the list.
+                 * If we can't lock the item, either its holder will flush it
+                 * or it is already being flushed or it is being relogged.  In
+                 * any of these case it is being taken care of and we can just
+                 * skip to the next item in the list.
                  */
                 lock_result = IOP_TRYLOCK(lip);
+                spin_unlock(&mp->m_ail_lock);
                 switch (lock_result) {
-                      case XFS_ITEM_SUCCESS:
-                        spin_unlock(&mp->m_ail_lock);
+                case XFS_ITEM_SUCCESS:
                         XFS_STATS_INC(xs_push_ail_success);
                         IOP_PUSH(lip);
-                        spin_lock(&mp->m_ail_lock);
+                        last_pushed_lsn = lsn;
                         break;
 
-                      case XFS_ITEM_PUSHBUF:
-                        spin_unlock(&mp->m_ail_lock);
+                case XFS_ITEM_PUSHBUF:
                         XFS_STATS_INC(xs_push_ail_pushbuf);
-#ifdef XFSRACEDEBUG
-                        delay_for_intr();
-                        delay(300);
-#endif
-                        ASSERT(lip->li_ops->iop_pushbuf);
-                        ASSERT(lip);
                         IOP_PUSHBUF(lip);
-                        spin_lock(&mp->m_ail_lock);
+                        last_pushed_lsn = lsn;
                         break;
 
-                      case XFS_ITEM_PINNED:
+                case XFS_ITEM_PINNED:
                         XFS_STATS_INC(xs_push_ail_pinned);
+                        stuck++;
                         flush_log = 1;
                         break;
 
-                      case XFS_ITEM_LOCKED:
+                case XFS_ITEM_LOCKED:
                         XFS_STATS_INC(xs_push_ail_locked);
+                        last_pushed_lsn = lsn;
+                        stuck++;
                         break;
 
-                      case XFS_ITEM_FLUSHING:
+                case XFS_ITEM_FLUSHING:
                         XFS_STATS_INC(xs_push_ail_flushing);
+                        last_pushed_lsn = lsn;
+                        stuck++;
                         break;
 
-                      default:
+                default:
                         ASSERT(0);
                         break;
                 }
 
-                lip = xfs_trans_next_ail(mp, lip, &gen, &restarts);
-                if (lip == NULL) {
+                spin_lock(&mp->m_ail_lock);
+                /* should we bother continuing? */
+                if (XFS_FORCED_SHUTDOWN(mp))
+                        break;
+                ASSERT(mp->m_log);
+
+                count++;
+
+                /*
+                 * Are there too many items we can't do anything with?
+                 * If we we are skipping too many items because we can't flush
+                 * them or they are already being flushed, we back off and
+                 * given them time to complete whatever operation is being
+                 * done. i.e. remove pressure from the AIL while we can't make
+                 * progress so traversals don't slow down further inserts and
+                 * removals to/from the AIL.
+                 *
+                 * The value of 100 is an arbitrary magic number based on
+                 * observation.
+                 */
+                if (stuck > 100)
                         break;
-                }
-                if (XFS_FORCED_SHUTDOWN(mp)) {
-                        /*
-                         * Just return if we shut down during the last try.
-                         */
-                        spin_unlock(&mp->m_ail_lock);
-                        return (xfs_lsn_t)0;
-                }
 
+                lip = xfs_trans_next_ail(mp, lip, &gen, &restarts);
+                if (lip == NULL)
+                        break;
+                if (restarts > XFS_TRANS_PUSH_AIL_RESTARTS)
+                        break;
+                lsn = lip->li_lsn;
         }
+        spin_unlock(&mp->m_ail_lock);
 
         if (flush_log) {
                 /*
@@ -191,22 +257,35 @@ xfs_trans_push_ail(
                  * push out the log so it will become unpinned and
                  * move forward in the AIL.
                  */
-                spin_unlock(&mp->m_ail_lock);
                 XFS_STATS_INC(xs_push_ail_flush);
                 xfs_log_force(mp, (xfs_lsn_t)0, XFS_LOG_FORCE);
-                spin_lock(&mp->m_ail_lock);
         }
 
-        lip = xfs_ail_min(&(mp->m_ail));
-        if (lip == NULL) {
-                lsn = (xfs_lsn_t)0;
-        } else {
-                lsn = lip->li_lsn;
+        /*
+         * We reached the target so wait a bit longer for I/O to complete and
+         * remove pushed items from the AIL before we start the next scan from
+         * the start of the AIL.
+         */
+        if ((XFS_LSN_CMP(lsn, target) >= 0)) {
+                tout += 20;
+                last_pushed_lsn = 0;
+        } else if ((restarts > XFS_TRANS_PUSH_AIL_RESTARTS) ||
+                   (count && ((stuck * 100) / count > 90))) {
+                /*
+                 * Either there is a lot of contention on the AIL or we
+                 * are stuck due to operations in progress. "Stuck" in this
+                 * case is defined as >90% of the items we tried to push
+                 * were stuck.
+                 *
+                 * Backoff a bit more to allow some I/O to complete before
+                 * continuing from where we were.
+                 */
+                tout += 10;
         }
-
-        spin_unlock(&mp->m_ail_lock);
-        return lsn;
-}       /* xfs_trans_push_ail */
+out:
+        *last_lsn = last_pushed_lsn;
+        return tout;
+}       /* xfsaild_push */
 
 
 /*
@@ -247,7 +326,7 @@ xfs_trans_unlocked_item(
          * the call to xfs_log_move_tail() doesn't do anything if there's
          * not enough free space to wake people up so we're safe calling it.
          */
-        min_lip = xfs_ail_min(&mp->m_ail);
+        min_lip = xfs_ail_min(&mp->m_ail.xa_ail);
 
         if (min_lip == lip)
                 xfs_log_move_tail(mp, 1);
@@ -279,7 +358,7 @@ xfs_trans_update_ail(
         xfs_log_item_t          *dlip=NULL;
         xfs_log_item_t          *mlip;  /* ptr to minimum lip */
 
-        ailp = &(mp->m_ail);
+        ailp = &(mp->m_ail.xa_ail);
         mlip = xfs_ail_min(ailp);
 
         if (lip->li_flags & XFS_LI_IN_AIL) {
@@ -292,10 +371,10 @@ xfs_trans_update_ail(
         lip->li_lsn = lsn;
 
         xfs_ail_insert(ailp, lip);
-        mp->m_ail_gen++;
+        mp->m_ail.xa_gen++;
 
         if (mlip == dlip) {
-                mlip = xfs_ail_min(&(mp->m_ail));
+                mlip = xfs_ail_min(&(mp->m_ail.xa_ail));
                 spin_unlock(&mp->m_ail_lock);
                 xfs_log_move_tail(mp, mlip->li_lsn);
         } else {
@@ -330,7 +409,7 @@ xfs_trans_delete_ail(
         xfs_log_item_t          *mlip;
 
         if (lip->li_flags & XFS_LI_IN_AIL) {
-                ailp = &(mp->m_ail);
+                ailp = &(mp->m_ail.xa_ail);
                 mlip = xfs_ail_min(ailp);
                 dlip = xfs_ail_delete(ailp, lip);
                 ASSERT(dlip == lip);
@@ -338,10 +417,10 @@ xfs_trans_delete_ail(
 
                 lip->li_flags &= ~XFS_LI_IN_AIL;
                 lip->li_lsn = 0;
-                mp->m_ail_gen++;
+                mp->m_ail.xa_gen++;
 
                 if (mlip == dlip) {
-                        mlip = xfs_ail_min(&(mp->m_ail));
+                        mlip = xfs_ail_min(&(mp->m_ail.xa_ail));
                         spin_unlock(&mp->m_ail_lock);
                         xfs_log_move_tail(mp, (mlip ? mlip->li_lsn : 0));
                 } else {
@@ -379,10 +458,10 @@ xfs_trans_first_ail(
 {
         xfs_log_item_t  *lip;
 
-        lip = xfs_ail_min(&(mp->m_ail));
-        *gen = (int)mp->m_ail_gen;
+        lip = xfs_ail_min(&(mp->m_ail.xa_ail));
+        *gen = (int)mp->m_ail.xa_gen;
 
-        return (lip);
+        return lip;
 }
 
 /*
@@ -402,11 +481,11 @@ xfs_trans_next_ail(
         xfs_log_item_t  *nlip;
 
         ASSERT(mp && lip && gen);
-        if (mp->m_ail_gen == *gen) {
-                nlip = xfs_ail_next(&(mp->m_ail), lip);
+        if (mp->m_ail.xa_gen == *gen) {
+                nlip = xfs_ail_next(&(mp->m_ail.xa_ail), lip);
         } else {
-                nlip = xfs_ail_min(&(mp->m_ail));
-                *gen = (int)mp->m_ail_gen;
+                nlip = xfs_ail_min(&(mp->m_ail).xa_ail);
+                *gen = (int)mp->m_ail.xa_gen;
                 if (restarts != NULL) {
                         XFS_STATS_INC(xs_push_ail_restarts);
                         (*restarts)++;
@@ -431,12 +510,20 @@ xfs_trans_next_ail(
 /*
  * Initialize the doubly linked list to point only to itself.
  */
-void
+int
 xfs_trans_ail_init(
         xfs_mount_t     *mp)
 {
-        mp->m_ail.ail_forw = (xfs_log_item_t*)&(mp->m_ail);
-        mp->m_ail.ail_back = (xfs_log_item_t*)&(mp->m_ail);
+        mp->m_ail.xa_ail.ail_forw = (xfs_log_item_t*)&mp->m_ail.xa_ail;
+        mp->m_ail.xa_ail.ail_back = (xfs_log_item_t*)&mp->m_ail.xa_ail;
+        return xfsaild_start(mp);
+}
+
+void
+xfs_trans_ail_destroy(
+        xfs_mount_t     *mp)
+{
+        xfsaild_stop(mp);
 }
 
 /*