Re: [GIT PULL] perf fixes

[Arjan van de Ven]

On 6/22/2012 4:18 PM, Steven Rostedt wrote:
> How so? It's not a C function call (like the -finstrument-functions
> produces). It's an assembly function call. The only differences between
> having ftrace enabled and ftrace disabled with -mfentry is that you get
> a 5 byte nop at the start of each traceable function. Sure, it might put
> a little pressure on the icache, but from the benchmarks I've run, the
> impact has all been within the noise.
> 
> I've been told that it doesn't even hurt the pipeline. But I've Cc'd hpa
> and Arjan for their comments. How much impact does a 5 byte nop at the
> start of each function really have on the normal operations of the
> kernel?
> 

if it's truely an official nop, it will take decoder bandwidth obviously
(which can decode 3 to 4 instructions per cycle, depending on the
brand/model of CPU and the total size in bytes of these instructions).
likewise, at the end of the out of order pipeline, NOPs may take a
retirement slot (again 3 to 4 instructions per cycle)

icache is there as well, and if the NOP actually changes cpu flags (some
of the less fortunate ones do) that can create a false data dependency.


I would also worry about the compiler being able to inline a function
containing one of these, but that's a compiler thing, not a CPU type of
thing.

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