Messages in this thread | | | Date | Wed, 14 Oct 2009 00:26:45 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp |
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On Tue, Oct 13, 2009 at 05:12:46PM -0400, Steven Rostedt wrote: > On Tue, 2009-10-13 at 23:02 +0200, Frederic Weisbecker wrote: > > On Tue, Oct 13, 2009 at 04:33:50PM -0400, Steven Rostedt wrote: > > > From: Steven Rostedt <srostedt@redhat.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. > > Calls are branch prediction jumps. Which associates the "ret" with the > call. As it approaches the ret, it starts to receive the code after the > call. > > But this is stack order. Every call should hit one ret. But with the > original code, we break this stack. We have one call and two rets. Which > means that the branch prediction will also get messed up with the > previous stored call.
Oh, ok I got it. Thanks.
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