linux-kernel - Re: [patch] introduce TRACE_EVENT_ABI (was Re: TRACE_EVENT

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Date:	Wed, 23 Sep 2009 17:38:34 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] introduce TRACE_EVENT_ABI (was Re: TRACE_EVENT_ABI ?)

* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 2009-09-23 at 08:43 -0400, Christoph Hellwig wrote:

> > I'm not sure we can support any as an ABI yet.  The text format 
> > seems to volatile in general - not just the output of the individual 
> > trace events but also the common file format, and for the binary 
> > format we need to figure a good way to tag the output yet.  Also 
> > when we define one as one ABI we should make very clear what that 
> > means, e.g. does it have to stay exactly as is?  Or can we add new 
> > fields but not remove old one?
> 
> I had this discussion with people in Portland. We seem to agree that 
> this should just lock the old fields in, but you can add new ones at 
> the end.

Yeah, that's the sanest approach i was thinking about when i suggested 
TRACE_EVENT_ABI() to Arjan and Peter.

The raw record is opaque, comes with a length field and goes into the 
ring-buffer so it's nicely extensible. Existing bits shouldnt change.

User-space that relies on a record can define a structure of that and 
copy that over from the ring-buffer - and ignore any new bits.

What i'd also like to see is the use of typical ABI-safe type fields in 
the trace definitions themselves: u8, u32, u64, etc. 'long' is obviously 
not good. Could we do some automation for that perhaps? I.e. emit a 
warning (boot time or so) if TRACE_EVENT_ABI() is used with unsafe type 
fields.

( Endianness is another detail, if perf.data is shipped to a
  different-endian system. Best is probably to define a new perf.data
  attribute extension with the endianness of the generator system
  included. That way the perf.data parser can convert endianness if it 
  wants/needs to. )

	Ingo
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