On Wed, 8 Apr 2026 19:20:17 +0800
Li Chen <[email protected]> wrote:
> Commit-time fast commit snapshots run under jbd2_journal_lock_updates(),
> so it is useful to quantify the time spent with updates locked and to
> understand why snapshotting can fail.
>
> Add a new tracepoint, ext4_fc_lock_updates, reporting the time spent in
> the updates-locked window along with the number of snapshotted inodes
> and ranges. Record the first snapshot failure reason in a stable snap_err
> field for tooling.
>
[..]
> @@ -1338,13 +1375,13 @@ static int ext4_fc_perform_commit(journal_t *journal)
> if (ret)
> return ret;
>
> -
> ret = ext4_fc_alloc_snapshot_inodes(sb, &inodes, &inodes_size);
> if (ret)
> return ret;
>
> /* Step 4: Mark all inodes as being committed. */
> jbd2_journal_lock_updates(journal);
> + lock_start = ktime_get();
ktime_get() is rather quick but if you care about micro-optimizations, you
could have:
if (trace_ext4_fc_lock_updates_enabled())
lock_start = ktime_get();
else
lock_start = 0;
> /*
> * The journal is now locked. No more handles can start and all the
> * previous handles are now drained. Snapshotting happens in this
> @@ -1358,8 +1395,15 @@ static int ext4_fc_perform_commit(journal_t *journal)
> }
> ext4_fc_unlock(sb, alloc_ctx);
>
> - ret = ext4_fc_snapshot_inodes(journal, inodes, inodes_size);
> + ret = ext4_fc_snapshot_inodes(journal, inodes, inodes_size,
> + &snap_inodes, &snap_ranges, &snap_err);
> jbd2_journal_unlock_updates(journal);
> + if (trace_ext4_fc_lock_updates_enabled()) {
if (trace_ext4_fc_lock_updates_enabled() && lock_start) {
But feel free to ignore this if the overhead of always calling ktime_get()
is not an issue.
> + locked_ns = ktime_to_ns(ktime_sub(ktime_get(), lock_start));
> + trace_ext4_fc_lock_updates(sb, commit_tid, locked_ns,
> + snap_inodes, snap_ranges, ret,
> + snap_err);
> + }
> kvfree(inodes);
> if (ret)
> return ret;
> @@ -1564,7 +1608,7 @@ int ext4_fc_commit(journal_t *journal, tid_t commit_tid)
> journal_ioprio = EXT4_DEF_JOURNAL_IOPRIO;
> set_task_ioprio(current, journal_ioprio);
> fc_bufs_before = (sbi->s_fc_bytes + bsize - 1) / bsize;
> - ret = ext4_fc_perform_commit(journal);
> + ret = ext4_fc_perform_commit(journal, commit_tid);
> if (ret < 0) {
> if (ret == -EAGAIN || ret == -E2BIG || ret == -ECANCELED)
> status = EXT4_FC_STATUS_INELIGIBLE;
> diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
> index f493642cf121..7028a28316fa 100644
> --- a/include/trace/events/ext4.h
> +++ b/include/trace/events/ext4.h
> @@ -107,6 +107,26 @@ TRACE_DEFINE_ENUM(EXT4_FC_REASON_VERITY);
> TRACE_DEFINE_ENUM(EXT4_FC_REASON_MOVE_EXT);
> TRACE_DEFINE_ENUM(EXT4_FC_REASON_MAX);
>
> +#undef EM
> +#undef EMe
> +#define EM(a) TRACE_DEFINE_ENUM(EXT4_FC_SNAP_ERR_##a);
> +#define EMe(a) TRACE_DEFINE_ENUM(EXT4_FC_SNAP_ERR_##a);
> +
> +#define TRACE_SNAP_ERR \
> + EM(NONE) \
> + EM(ES_MISS) \
> + EM(ES_DELAYED) \
> + EM(ES_OTHER) \
> + EM(INODES_CAP) \
> + EM(RANGES_CAP) \
> + EM(NOMEM) \
> + EMe(INODE_LOC)
> +
> +TRACE_SNAP_ERR
> +
> +#undef EM
> +#undef EMe
> +
> #define show_fc_reason(reason)
> \
> __print_symbolic(reason, \
> { EXT4_FC_REASON_XATTR, "XATTR"}, \
> @@ -2818,6 +2838,47 @@ TRACE_EVENT(ext4_fc_commit_stop,
> __entry->num_fc_ineligible, __entry->nblks_agg, __entry->tid)
> );
>
> +#define EM(a) { EXT4_FC_SNAP_ERR_##a, #a },
> +#define EMe(a) { EXT4_FC_SNAP_ERR_##a, #a }
> +
> +TRACE_EVENT(ext4_fc_lock_updates,
> + TP_PROTO(struct super_block *sb, tid_t commit_tid, u64 locked_ns,
> + unsigned int nr_inodes, unsigned int nr_ranges, int err,
> + int snap_err),
> +
> + TP_ARGS(sb, commit_tid, locked_ns, nr_inodes, nr_ranges, err, snap_err),
> +
> + TP_STRUCT__entry(/* entry */
> + __field(dev_t, dev)
> + __field(tid_t, tid)
> + __field(u64, locked_ns)
> + __field(unsigned int, nr_inodes)
> + __field(unsigned int, nr_ranges)
> + __field(int, err)
> + __field(int, snap_err)
> + ),
> +
> + TP_fast_assign(/* assign */
> + __entry->dev = sb->s_dev;
> + __entry->tid = commit_tid;
> + __entry->locked_ns = locked_ns;
> + __entry->nr_inodes = nr_inodes;
> + __entry->nr_ranges = nr_ranges;
> + __entry->err = err;
> + __entry->snap_err = snap_err;
> + ),
> +
> + TP_printk("dev %d,%d tid %u locked_ns %llu nr_inodes %u nr_ranges %u
> err %d snap_err %s",
> + MAJOR(__entry->dev), MINOR(__entry->dev), __entry->tid,
> + __entry->locked_ns, __entry->nr_inodes, __entry->nr_ranges,
> + __entry->err, __print_symbolic(__entry->snap_err,
> + TRACE_SNAP_ERR))
> +);
> +
> +#undef EM
> +#undef EMe
> +#undef TRACE_SNAP_ERR
> +
> #define FC_REASON_NAME_STAT(reason) \
> show_fc_reason(reason), \
> __entry->fc_ineligible_rc[reason]
As for the rest:
Reviewed-by: Steven Rostedt (Google) <[email protected]>
[ Please add this reviewed-by to any new versions so I remember I already
looked at it. ]
Thanks,
-- Steve
