function-graph/x86: Replace unbalanced ret with jmp

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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.

This patch replaces the ret with a jmp to keep the calls and ret
balanced. I tested this on one box and it showed a 1.7% increase in
performance. Another box only showed a small 0.3% increase. But no
box that I tested this on showed a decrease in performance by
making this change.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20091013203425.042034383@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

...

This commit is contained in:

Steven Rostedt 2009-10-13 16:33:50 -04:00 • committed by Ingo Molnar

parent 1bac0497ef

commit 194ec34184

2 changed files with 5 additions and 8 deletions

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7

arch/x86/kernel/entry_32.S

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6

arch/x86/kernel/entry_64.S

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