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

From: Ingo Molnar
Date: Sat Feb 21 2015 - 13:40:00 EST

* Borislav Petkov <bp@xxxxxxxxx> wrote:

> On Sat, Feb 21, 2015 at 05:38:40PM +0100, Borislav Petkov wrote:
> > My assumption is that libc uses SSE for memcpy and thus the FPU will
> > be used. (I'll trace FPU-specific PMCs later to confirm).
> Ok, so I slapped a trace_printk() at the beginning of fpu_save_init()
> and did a kernel build once with default 3.19 and once with Andy's
> patch. So we end up with this count:
> default: 712000
> eager: 780000
> This would explain the very small difference in the
> performance counters data from the previous email.

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
that a bit too high?

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.


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