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

[Eiichi Tsukata]

Hello Stefan,

Thank you for reviewing my patch.

(2013/04/08 23:10), Stefan Hajnoczi wrote:
> 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<address@hidden>"
> > +__copyright__  = "Copyright (C) 2013 Hitachi, Ltd."
> > +__license__    = "GPL version 2 or (at your option) any later version"
> > +
> > +__maintainer__ = "Stefan Hajnoczi"
> > +__email__      = "address@hidden"
>
> My email is now address@hidden  I can fix this up when merging the
> patch.

I see, I'll fix it up.

> > +        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.

Sounds good.
I'll use snprintf(3) return value in V2 patch and send it soon.

Eiichi

> Stefan
> .