[XFS] Per iclog callback chain lock

Rather than use the icloglock for protecting the iclog completion callback
chain, use a new per-iclog lock so that walking the callback chain doesn't
require holding a global lock.

This reduces contention on the icloglock during transaction commit and log
I/O completion by reducing the number of times we need to hold the global
icloglock during these operations.

SGI-PV: 978729
SGI-Modid: xfs-linux-melb:xfs-kern:30770a
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

fs/xfs/xfs_log.c

@@ -397,12 +397,10 @@ xfs_log_notify(xfs_mount_t	  *mp,		/* mount of partition */
 	       void		  *iclog_hndl,	/* iclog to hang callback off */
 	       xfs_log_callback_t *cb)
 {
-	xlog_t *log = mp->m_log;
 	xlog_in_core_t	  *iclog = (xlog_in_core_t *)iclog_hndl;
 	int	abortflg;
 
-	cb->cb_next = NULL;
-	spin_lock(&log->l_icloglock);
+	spin_lock(&iclog->ic_callback_lock);
 	abortflg = (iclog->ic_state & XLOG_STATE_IOERROR);
 	if (!abortflg) {
 		ASSERT_ALWAYS((iclog->ic_state == XLOG_STATE_ACTIVE) ||
@@ -411,7 +409,7 @@ xfs_log_notify(xfs_mount_t	  *mp,		/* mount of partition */
 		*(iclog->ic_callback_tail) = cb;
 		iclog->ic_callback_tail = &(cb->cb_next);
 	}
-	spin_unlock(&log->l_icloglock);
+	spin_unlock(&iclog->ic_callback_lock);
 	return abortflg;
 }	/* xfs_log_notify */
 
@@ -1257,6 +1255,8 @@ xlog_alloc_log(xfs_mount_t	*mp,
 		iclog->ic_size = XFS_BUF_SIZE(bp) - log->l_iclog_hsize;
 		iclog->ic_state = XLOG_STATE_ACTIVE;
 		iclog->ic_log = log;
+		atomic_set(&iclog->ic_refcnt, 0);
+		spin_lock_init(&iclog->ic_callback_lock);
 		iclog->ic_callback_tail = &(iclog->ic_callback);
 		iclog->ic_datap = (char *)iclog->hic_data + log->l_iclog_hsize;
 
@@ -1987,7 +1987,7 @@ xlog_state_clean_log(xlog_t *log)
 		if (iclog->ic_state == XLOG_STATE_DIRTY) {
 			iclog->ic_state	= XLOG_STATE_ACTIVE;
 			iclog->ic_offset       = 0;
-			iclog->ic_callback	= NULL;   /* don't need to free */
+			ASSERT(iclog->ic_callback == NULL);
 			/*
 			 * If the number of ops in this iclog indicate it just
 			 * contains the dummy transaction, we can
@@ -2190,37 +2190,40 @@ xlog_state_do_callback(
 					be64_to_cpu(iclog->ic_header.h_lsn);
 				spin_unlock(&log->l_grant_lock);
 
-				/*
-				 * Keep processing entries in the callback list
-				 * until we come around and it is empty.  We
-				 * need to atomically see that the list is
-				 * empty and change the state to DIRTY so that
-				 * we don't miss any more callbacks being added.
-				 */
-				spin_lock(&log->l_icloglock);
 			} else {
+				spin_unlock(&log->l_icloglock);
 				ioerrors++;
 			}
-			cb = iclog->ic_callback;
 
+			/*
+			 * Keep processing entries in the callback list until
+			 * we come around and it is empty.  We need to
+			 * atomically see that the list is empty and change the
+			 * state to DIRTY so that we don't miss any more
+			 * callbacks being added.
+			 */
+			spin_lock(&iclog->ic_callback_lock);
+			cb = iclog->ic_callback;
 			while (cb) {
 				iclog->ic_callback_tail = &(iclog->ic_callback);
 				iclog->ic_callback = NULL;
-				spin_unlock(&log->l_icloglock);
+				spin_unlock(&iclog->ic_callback_lock);
 
 				/* perform callbacks in the order given */
 				for (; cb; cb = cb_next) {
 					cb_next = cb->cb_next;
 					cb->cb_func(cb->cb_arg, aborted);
 				}
-				spin_lock(&log->l_icloglock);
+				spin_lock(&iclog->ic_callback_lock);
 				cb = iclog->ic_callback;
 			}
 
 			loopdidcallbacks++;
 			funcdidcallbacks++;
 
+			spin_lock(&log->l_icloglock);
 			ASSERT(iclog->ic_callback == NULL);
+			spin_unlock(&iclog->ic_callback_lock);
 			if (!(iclog->ic_state & XLOG_STATE_IOERROR))
 				iclog->ic_state = XLOG_STATE_DIRTY;
 

fs/xfs/xfs_log_priv.h

@@ -324,6 +324,19 @@ typedef struct xlog_rec_ext_header {
  * - ic_offset is the current number of bytes written to in this iclog.
  * - ic_refcnt is bumped when someone is writing to the log.
  * - ic_state is the state of the iclog.
+ *
+ * Because of cacheline contention on large machines, we need to separate
+ * various resources onto different cachelines. To start with, make the
+ * structure cacheline aligned. The following fields can be contended on
+ * by independent processes:
+ *
+ *	- ic_callback_*
+ *	- ic_refcnt
+ *	- fields protected by the global l_icloglock
+ *
+ * so we need to ensure that these fields are located in separate cachelines.
+ * We'll put all the read-only and l_icloglock fields in the first cacheline,
+ * and move everything else out to subsequent cachelines.
  */
 typedef struct xlog_iclog_fields {
 	sv_t			ic_forcesema;
@@ -332,18 +345,23 @@ typedef struct xlog_iclog_fields {
 	struct xlog_in_core	*ic_prev;
 	struct xfs_buf		*ic_bp;
 	struct log		*ic_log;
-	xfs_log_callback_t	*ic_callback;
-	xfs_log_callback_t	**ic_callback_tail;
-#ifdef XFS_LOG_TRACE
-	struct ktrace		*ic_trace;
-#endif
 	int			ic_size;
 	int			ic_offset;
-	atomic_t		ic_refcnt;
 	int			ic_bwritecnt;
 	ushort_t		ic_state;
 	char			*ic_datap;	/* pointer to iclog data */
-} xlog_iclog_fields_t;
+#ifdef XFS_LOG_TRACE
+	struct ktrace		*ic_trace;
+#endif
+
+	/* Callback structures need their own cacheline */
+	spinlock_t		ic_callback_lock ____cacheline_aligned_in_smp;
+	xfs_log_callback_t	*ic_callback;
+	xfs_log_callback_t	**ic_callback_tail;
+
+	/* reference counts need their own cacheline */
+	atomic_t		ic_refcnt ____cacheline_aligned_in_smp;
+} xlog_iclog_fields_t ____cacheline_aligned_in_smp;
 
 typedef union xlog_in_core2 {
 	xlog_rec_header_t	hic_header;
@@ -366,6 +384,7 @@ typedef struct xlog_in_core {
 #define	ic_bp		hic_fields.ic_bp
 #define	ic_log		hic_fields.ic_log
 #define	ic_callback	hic_fields.ic_callback
+#define	ic_callback_lock hic_fields.ic_callback_lock
 #define	ic_callback_tail hic_fields.ic_callback_tail
 #define	ic_trace	hic_fields.ic_trace
 #define	ic_size		hic_fields.ic_size