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

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Date:	Tue, 13 Oct 2009 17:26:50 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced
 ret with jmp

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.

> 
> 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?

-- Steve


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