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

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Date:	Sat, 21 Feb 2015 19:39:52 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs


* Borislav Petkov <bp@...en8.de> 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.

Thanks,

	Ingo
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