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 <fdmanana@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Leo Martins <loemra.dev@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index 0864700f76e0..8ad7a2d76c1d 100644
--- a/include/trace/events/btrfs.h
+++ b/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,