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

[Steven Rostedt]

On Sat, 9 Aug 2008, Andi Kleen wrote:

> Steven Rostedt <rostedt@xxxxxxxxxxx> 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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