btrfs: add tracepoint for search slot restart tracking

Add a btrfs_search_slot_restart tracepoint that fires at each restart
site in btrfs_search_slot(), recording the root, tree level, and
reason for the restart. This enables tracking search slot restarts
which contribute to COW amplification under memory pressure.

The four restart reasons are:
 - write_lock: insufficient write lock level, need to restart with
   higher lock
 - setup_nodes: node setup returned -EAGAIN
 - slot_zero: insertion at slot 0 requires higher write lock level
 - read_block: read_block_for_search returned -EAGAIN (block not
   cached or lock contention)

COW counts are already tracked by the existing trace_btrfs_cow_block()
tracepoint. The per-restart-site tracepoint avoids counter overhead
in the critical path when tracepoints are disabled, and provides
richer per-event information that bpftrace scripts can aggregate into
counts, histograms, and per-root breakdowns.

Reviewed-by: Filipe Manana <[email protected]>
Reviewed-by: Boris Burkov <[email protected]>
Signed-off-by: Leo Martins <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: loemraw

fs/btrfs/ctree.c

@@ -2102,6 +2102,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 			    p->nodes[level + 1])) {
 				write_lock_level = level + 1;
 				btrfs_release_path(p);
+				trace_btrfs_search_slot_restart(root, level, "write_lock");
 				goto again;
 			}
 
@@ -2164,8 +2165,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		p->slots[level] = slot;
 		ret2 = setup_nodes_for_search(trans, root, p, b, level, ins_len,
 					      &write_lock_level);
-		if (ret2 == -EAGAIN)
+		if (ret2 == -EAGAIN) {
+			trace_btrfs_search_slot_restart(root, level, "setup_nodes");
 			goto again;
+		}
 		if (ret2) {
 			ret = ret2;
 			goto done;
@@ -2181,6 +2184,7 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		if (slot == 0 && ins_len && write_lock_level < level + 1) {
 			write_lock_level = level + 1;
 			btrfs_release_path(p);
+			trace_btrfs_search_slot_restart(root, level, "slot_zero");
 			goto again;
 		}
 
@@ -2194,8 +2198,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		}
 
 		ret2 = read_block_for_search(root, p, &b, slot, key);
-		if (ret2 == -EAGAIN && !p->nowait)
+		if (ret2 == -EAGAIN && !p->nowait) {
+			trace_btrfs_search_slot_restart(root, level, "read_block");
 			goto again;
+		}
 		if (ret2) {
 			ret = ret2;
 			goto done;

include/trace/events/btrfs.h

@@ -1113,6 +1113,30 @@ TRACE_EVENT(btrfs_cow_block,
 		  __entry->cow_level)
 );
 
+TRACE_EVENT(btrfs_search_slot_restart,
+
+	TP_PROTO(const struct btrfs_root *root, int level,
+		 const char *reason),
+
+	TP_ARGS(root, level, reason),
+
+	TP_STRUCT__entry_btrfs(
+		__field(	u64,	root_objectid		)
+		__field(	int,	level			)
+		__string(	reason,	reason			)
+	),
+
+	TP_fast_assign_btrfs(root->fs_info,
+		__entry->root_objectid	= btrfs_root_id(root);
+		__entry->level		= level;
+		__assign_str(reason);
+	),
+
+	TP_printk_btrfs("root=%llu(%s) level=%d reason=%s",
+		  show_root_type(__entry->root_objectid),
+		  __entry->level, __get_str(reason))
+);
+
 TRACE_EVENT(btrfs_space_reservation,
 
 	TP_PROTO(const struct btrfs_fs_info *fs_info, const char *type, u64 val,