Re: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Steven Rostedt (rostedt@goodmis.org) wrote:
> On Tue, 2009-10-13 at 17:21 -0400, Mathieu Desnoyers wrote:
> 
> > > What it was:
> > > 
> > > call function
> > >   function:
> > >   call mcount
> > >      mcount:
> > >      call ftrace_entry
> > 
> > Can we manage to change this call
> 
> Note, that call jumps to C code.
> 
> > 
> > >        ftrace_entry:
> > >        mess up with return code of caller
> > >        ret
> > 
> > .. and this ret for 2 jmp instructions too ?
> 
> The code is all in C, and it too calls functions. Not sure where this
> helps out any. The ret here matches their calls. Thus the prediction
> will work.
> 

Oh, OK. I thought the callback was in assembly. That's a bit more work
than I thought.

> > 
> > Given that we have no choice but to kill call/ret prediction logic, I
> > think it might be good to try to use this logic as little as possible
> > (by favoring jmp jmp over call/ret when the return target is invariant).
> > 
> > That's just an idea, benchmarks could prove me right/wrong.
> 
> I don't see how this would help. And I'm not about to waste time
> experimenting. What's the rational?
> 

The idea is that call/ret are fast when predictions are right. In this
case, I wonder if the fact that we trash the call/ret prediction (even
if this happens after the paired call/ret) would have an impact on
balanced call/ret in the tracing code path. I doubt so, but who knows.

Probably not worth spending much time on this. It would just have been
nice to try if the implementation would have been trivial.

Thanks,

Mathieu

> -- Steve
> 
> 

-- 
Mathieu Desnoyers
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