Date: Wed, 23 Sep 2009 17:38:34 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Christoph Hellwig <hch@infradead.org>,
       Arjan van de Ven <arjan@infradead.org>, linux-kernel@vger.kernel.org,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch] introduce TRACE_EVENT_ABI (was Re: TRACE_EVENT_ABI ?)
Message-ID: <20090923153834.GA6105@elte.hu>
References: <20090923124340.GA6093@infradead.org> <1253714632.4498.3.camel@frodo>
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* Steven Rostedt <rostedt@goodmis.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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