Re: [PATCH 1/3] x86: Move msr accesses out of line

Andi Kleen
2015-02-25T18:20:37Z

> Still, I wondered, so I ran me a little test. Note that I used a
> serializing instruction (LOCK XCHG) because WRMSR is too.

WRMSR has a lot of uops internally unlike LOCK XCHG, so I expect it
will mostly overlap with what it does. I'll run some benchmarks on
this today.

Also we do quite a few RDMSRs, which are not necessarily
serializing.

> I see a ~14 cycle difference between the inline and noinline version.
>
> If I substitute the LOCK XCHG with XADD, I get to 1,5 cycles in
> difference, so clearly there is some magic happening, but serializing
> instructions wreck it.
>
> Anybody can explain how such RSP deps get magiced away?

On Intel Core (since Yonah), the CPU frontend has a special
stack tracker that avoids these dependencies.

See 2.3.2.5 in the optimization manual

Also BTW just from tracing MSRs there is a lot of optimization
potential. Will send some patches later.

-Andi