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

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Message-ID: <20091013210208.GB5135@nowhere>
Date:	Tue, 13 Oct 2009 23:02:10 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced
	ret with jmp

On Tue, Oct 13, 2009 at 04:33:50PM -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> The function graph tracer replaces the return address with a hook to
> trace the exit of the function call. This hook will finish by returning
> to the real location the function should return to.
> 
> But the current implementation uses a ret to jump to the real return
> location. This causes a imbalance between calls and ret. That is
> the original function does a call, the ret goes to the handler
> and then the handler does a ret without a matching call.
> 
> Although the function graph tracer itself still breaks the branch
> predictor by replacing the original ret, by using a second ret and
> causing an imbalance, it breaks the predictor even more.



I have troubles to understand by it breaks the predictor, especially
since there is not conditional branch in return_to_handler.
But still I don't understand why a ret would break more the branch
prediction than a jmp.

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