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

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Date:	Wed, 25 Feb 2015 19:20:32 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86: Move msr accesses out of line

> 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
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