From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:32838)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1UPCmo-0003jw-4W
	for qemu-devel@nongnu.org; Mon, 08 Apr 2013 10:11:13 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1UPCmi-0003e1-1W
	for qemu-devel@nongnu.org; Mon, 08 Apr 2013 10:11:06 -0400
Received: from mail-we0-x232.google.com ([2a00:1450:400c:c03::232]:58280)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1UPCmh-0003dh-Ps
	for qemu-devel@nongnu.org; Mon, 08 Apr 2013 10:10:59 -0400
Received: by mail-we0-f178.google.com with SMTP id z53so4497162wey.37
	for <qemu-devel@nongnu.org>; Mon, 08 Apr 2013 07:10:58 -0700 (PDT)
Date: Mon, 8 Apr 2013 16:10:54 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130408141054.GE4429@stefanha-thinkpad.redhat.com>
References: <1364970659-5715-1-git-send-email-eiichi.tsukata.xh@hitachi.com>
	<1364970659-5715-2-git-send-email-eiichi.tsukata.xh@hitachi.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1364970659-5715-2-git-send-email-eiichi.tsukata.xh@hitachi.com>
Subject: Re: [Qemu-devel] [PATCH 1/2] trace: Add ftrace tracing backend
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Eiichi Tsukata <eiichi.tsukata.xh@hitachi.com>
Cc: qemu-devel@nongnu.org

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