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

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Message-ID: <CAK1hOcMa78F+pGA2Egp+L_wzn7QkTt-AfgyoYz740=zvM4w3WQ@mail.gmail.com>
Date:	Tue, 24 Feb 2015 20:15:58 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Oleg Nesterov <oleg@...hat.com>, Rik van Riel <riel@...hat.com>,
	X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

On Fri, Feb 20, 2015 at 7:58 PM, Andy Lutomirski <luto@...capital.net> wrote:
> We have eager and lazy fpu modes, introduced in:
>
> 304bceda6a18 x86, fpu: use non-lazy fpu restore for processors supporting xsave
>
> The result is rather messy.  There are two code paths in almost all of the
> FPU code, and only one of them (the eager case) is tested frequently, since
> most kernel developers have new enough hardware that we use eagerfpu.
>
> It seems that, on any remotely recent hardware, eagerfpu is a win:
> glibc uses SSE2, so laziness is probably overoptimistic, and, in any
> case, manipulating TS is far slower that saving and restoring the full
> state.
>
> To try to shake out any latent issues on old hardware, this changes
> the default to eager on all CPUs.  If no performance or functionality
> problems show up, a subsequent patch could remove lazy mode entirely.

I'm a big fan of simplifying things, but.

SIMD registers were growing in x86, and they are going to grow again,
this time four-fold in Intel MIC:
from sixteen 256-bit registers to thirty two 512-bit registers.

That's 2 kbytes of data. Just moving this data out to/from memory
will take some time.

And some people talk about 1024-bit registers already...

Let's not completely remove lazy FPU saving code just yet.
Maybe we'll be forced to reinstate it.
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