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SubjectRe: [patch] introduce TRACE_EVENT_ABI (was Re: TRACE_EVENT_ABI ?)

* 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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