Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

From: Borislav Petkov
Date: Sat Feb 21 2015 - 14:16:36 EST

On Sat, Feb 21, 2015 at 07:39:52PM +0100, Ingo Molnar wrote:
> So the workload improved by ~600,000 usecs, and there's
> 68,000 less calls, so it saved 8.8 usecs per call. Isn't

I think you mean more calls. The eager measurement has more calls. Let
me do some primitive math:

def =(234.681331200 / 712000)*10^6 = 329.60861123595505000000 microsecs/call
eager=(234.066525648 / 780000)*10^6 = 300.08528929230769000000 microsecs/call

diff is 29.52332194364736000000 microsecs speedup per call which could
explain the cost of CR0.TS serialization semantics in the lazy mode.

> that a bit too high?

Now, is 29 microseconds too high? I'm not sure this is even correct and
not some noise interfering.

> I'd sleep a lot better if we had some runtime debug flag to
> be able to do run-to-run comparisons on the same booted up
> kernel, or so.

Let me take a look whether we could so some knob... The nice thing is,
code uses use_eager_fpu() to check stuff so we should be able to clear
the cpufeature flag.


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