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SubjectRe: [PATCH 0/5] ftrace: to kill a daemon

On Sat, 9 Aug 2008, Andi Kleen wrote:

> Steven Rostedt <rostedt@goodmis.org> writes:
>
> > I'm stubborn, I want to get it right _and_ keep it fast.
>
> For me it would seem better to just not use two part 5 byte nops
> instead of adding such hacks. I doubt there are that many of them
> anyways. I bet you won't be able to measure any difference between the
> different nop types in any macro benchmark.

I wish we had a true 5 byte nop. The alternative is a jmp 0, which is
measurable. This is replacing mcount from a kernel compile with the -pg
option. With a basic build (not counting modules), I have over 15,000
locations that are turned into these 5 byte nops.

# objdump -dr vmlinux.o | grep mcount |wc
15152 45489 764924

If we use the jmp 0, then yes, we will see the overhead. The double nop
that is used for 5 bytes, is significantly better than the jump.

-- Steve



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