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SubjectRe: [PATCH 2/2] ext4: Convert instrumentation from markers to tracepoints

* Theodore Ts'o <tytso@mit.edu> wrote:

> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> ---
> fs/ext4/ext4.h | 22 +--
> fs/ext4/ext4_i.h | 12 -
> fs/ext4/fsync.c | 8 +-
> fs/ext4/ialloc.c | 17 +-
> fs/ext4/inode.c | 79 ++---
> fs/ext4/mballoc.c | 84 ++---
> fs/ext4/mballoc.h | 68 +----
> fs/ext4/super.c | 6 +-
> include/linux/ext4_trace_types.h | 108 ++++++
> include/trace/ext4.h | 8 +
> include/trace/ext4_event_types.h | 690 +++++++++++++++++++++++++++++++++++++
> include/trace/trace_event_types.h | 1 +
> include/trace/trace_events.h | 1 +
> 13 files changed, 886 insertions(+), 218 deletions(-)
> create mode 100644 include/linux/ext4_trace_types.h
> create mode 100644 include/trace/ext4.h
> create mode 100644 include/trace/ext4_event_types.h

Really nice stuff!

A general, high-level observation: it is visible that the conversion
to TRACE_EVENTS() does increase the total linecount, but it also
removes code from the most often used source code areas
(fs/ext4/*.c) - and using an intuitive function-call-alike
single-line tracepoint there.
I'd call that a positive effect.

It would still be nice to compact the 690 lines of (out of line)
ext4_event_types.h definitions some more - and i think we'll be able
to do it without affecting the usage sites.

OTOH, even those bits look fairly readable, and there's a fair
amount of bang-for-bucks ratio - these tracepoints give us:

- integrated tracepoints
- zero-copy and per-cpu splice() based tracing
- binary tracing without printf overhead
- structured logging records exposed under /debug/tracing/events
- trace events embedded in function tracer output and other plugins
- user-defined, per tracepoint filter expressions

... so even if we were to keep the 690 lines of overhead with no
further compacting, it still looks like a good deal to me, all
things considered.

Ingo


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