xfs: add trace points for log forces

To enable easy tracing of the location of log forces and the
frequency of them via perf, add a pair of trace points to the log
force functions.  This will help debug where excessive log forces
are being issued from by simple perf commands like:

# ~/perf/perf top -e xfs:xfs_log_force -G -U

Which gives this sort of output:

Events: 141  xfs:xfs_log_force
-  100.00%  [kernel]  [k] xfs_log_force
   - xfs_log_force
        87.04% xfsaild
           kthread
           kernel_thread_helper
      - 12.87% xfs_buf_lock
           _xfs_buf_find
           xfs_buf_get
           xfs_trans_get_buf
           xfs_da_do_buf
           xfs_da_get_buf
           xfs_dir2_data_init
           xfs_dir2_leaf_addname
           xfs_dir_createname
           xfs_create
           xfs_vn_mknod
           xfs_vn_create
           vfs_create
           do_last.isra.41
           path_openat
           do_filp_open
           do_sys_open
           sys_open
           system_call_fastpath

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sig.com>

fs/xfs/xfs_log.c

@@ -2941,6 +2941,7 @@ xfs_log_force(
 {
 	int	error;
 
+	trace_xfs_log_force(mp, 0);
 	error = _xfs_log_force(mp, flags, NULL);
 	if (error)
 		xfs_warn(mp, "%s: error %d returned.", __func__, error);
@@ -3089,6 +3090,7 @@ xfs_log_force_lsn(
 {
 	int	error;
 
+	trace_xfs_log_force(mp, lsn);
 	error = _xfs_log_force_lsn(mp, lsn, flags, NULL);
 	if (error)
 		xfs_warn(mp, "%s: error %d returned.", __func__, error);

fs/xfs/xfs_trace.h

@@ -874,6 +874,22 @@ DECLARE_EVENT_CLASS(xfs_log_item_class,
 		  __print_flags(__entry->flags, "|", XFS_LI_FLAGS))
 )
 
+TRACE_EVENT(xfs_log_force,
+	TP_PROTO(struct xfs_mount *mp, xfs_lsn_t lsn),
+	TP_ARGS(mp, lsn),
+	TP_STRUCT__entry(
+		__field(dev_t, dev)
+		__field(xfs_lsn_t, lsn)
+	),
+	TP_fast_assign(
+		__entry->dev = mp->m_super->s_dev;
+		__entry->lsn = lsn;
+	),
+	TP_printk("dev %d:%d lsn 0x%llx",
+		  MAJOR(__entry->dev), MINOR(__entry->dev),
+		  __entry->lsn)
+)
+
 #define DEFINE_LOG_ITEM_EVENT(name) \
 DEFINE_EVENT(xfs_log_item_class, name, \
 	TP_PROTO(struct xfs_log_item *lip), \