On Fri, 24 Apr 2009, Andrew Morton wrote:

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> On Fri, 24 Apr 2009 22:51:03 -0400 (EDT) Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > In the old -rt patch series, we had trace points scattered all over the 
> > kernel. This was the original "event tracer". It was low overhead and can 
> > still give a good overview of the system when the function tracer was too 
> > much data. Yes, we solved many issues in -rt because of the event tracer. 
> 
> Sure, tracers can be useful.  The ext3 tracer I did way back when
> maintained a 32-element trace buffer inside each buffer_head and then would
> emit that trace when an assertion failed against that buffer_head, so
> you can see the last 32 things which happened to that bh.  It would
> have been nigh impossible to fix many of the things which were fixed
> without that facility.  (I doubt, incidentally, whether ftrace can do
> this sort of data-centric tracing?).

Not currently, but be careful what you say to me. I might implement it ;-)

Remember what happened when Ingo hypothetically asked me if it would be 
possible to trace all branches.

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> 
> But I never merged it into Linux.  Some of the tracepoints are in there
> (grep TRACE fs/ext3/*.c) but the core was kept out-of-tree.

 grep TRACE fs/ext3/*.c |wc
     79     308    5010

Some?

This is exactly my point. We must still be frugal about what trace 
points gets into the kernel. But I think places that give a good idea of 
the events that are happening (stategically placed) can help the general 
community.

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> 
> > BTW, you work for Google,
> 
> Hey, it's the other way round ;)

lol

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> 
> > doesn't google claim to have some magical 
> > 20-some tracepoints that is all they need? Could you give us a hint to 
> > what and where they are?
> 
> Wouldn't have a clue.  Jiaying Zhang should be able to find out.
> 

OK, maybe I'll ping her next week.

Thanks,

-- Steve

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