Re: [Qemu-devel] [PATCH 1/2] trace: Add ftrace tracing backend

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Apr 03, 2013 at 03:30:58PM +0900, Eiichi Tsukata wrote:
> diff --git a/scripts/tracetool/backend/ftrace.py b/scripts/tracetool/backend/ftrace.py
> new file mode 100644
> index 0000000..e02f0ca
> --- /dev/null
> +++ b/scripts/tracetool/backend/ftrace.py
> @@ -0,0 +1,53 @@
> +#!/usr/bin/env python
> +# -*- coding: utf-8 -*-
> +
> +"""
> +Ftrace built-in backend.
> +"""
> +
> +__author__     = "Eiichi Tsukata <eiichi.tsukata.xh@hitachi.com>"
> +__copyright__  = "Copyright (C) 2013 Hitachi, Ltd."
> +__license__    = "GPL version 2 or (at your option) any later version"
> +
> +__maintainer__ = "Stefan Hajnoczi"
> +__email__      = "stefanha@linux.vnet.ibm.com"

My email is now stefanha@redhat.com.  I can fix this up when merging the
patch.

> +        out('static inline void trace_%(name)s(%(args)s)',
> +            '{',
> +            '    char ftrace_buf[MAX_TRACE_STRLEN];',
> +            '    int unused __attribute__ ((unused));'
> +            '    bool _state = trace_event_get_state(%(event_id)s);',
> +            '    if (_state) {',
> +            '        snprintf(ftrace_buf, MAX_TRACE_STRLEN,',
> +            '                 "%(name)s " %(fmt)s "\\n" %(argnames)s);',
> +            '        unused = write(trace_marker_fd, ftrace_buf,',
> +            '                       MAX_TRACE_STRLEN);',

Looking at kernel/trace/trace.c:tracing_mark_write() the kernel will
memcpy() the full number of bytes we give.

You could use the snprintf(3) return value (number of characters
written, excluding NUL byte).

I think that would waste less trace buffer space.

Stefan