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btrfs: add tracepoint for search slot restart tracking

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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>
This commit is contained in:
Leo Martins
2026-02-26 01:51:08 -08:00
committed by David Sterba
parent f9a48549a1
commit cc970d21c4
2 changed files with 32 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
fs/btrfs/ctree.c
View File

@@ -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;

24
include/trace/events/btrfs.h
View File

@@ -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,