linux-kernel - Re: [PATCH 2/2] ext4: Convert instrumentation from markers to tracepoints

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Date:	Mon, 13 Apr 2009 23:37:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Theodore Ts'o <tytso@....edu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] ext4: Convert instrumentation from markers to
	tracepoints


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

> Signed-off-by: "Theodore Ts'o" <tytso@....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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