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

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Message-ID: <20150223145929.GA10342@redhat.com>
Date:	Mon, 23 Feb 2015 15:59:29 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Rik van Riel <riel@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

On 02/20, Andy Lutomirski 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.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/kernel/xsave.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/xsave.c b/arch/x86/kernel/xsave.c
> index 0de1fae2bdf0..1928c8f34ce5 100644
> --- a/arch/x86/kernel/xsave.c
> +++ b/arch/x86/kernel/xsave.c
> @@ -637,8 +637,8 @@ static void __init xstate_enable_boot_cpu(void)
>  	prepare_fx_sw_frame();
>  	setup_init_fpu_buf();
>  
> -	/* Auto enable eagerfpu for xsaveopt */
> -	if (cpu_has_xsaveopt && eagerfpu != DISABLE)
> +	/* Auto enable eagerfpu for everyone */
> +	if (eagerfpu != DISABLE)
>  		eagerfpu = ENABLE;

Well, but if we want this change then perhaps we should simply change
the default value? This way "AUTO" still can work.

Oleg.

--- x/arch/x86/kernel/xsave.c
+++ x/arch/x86/kernel/xsave.c
@@ -563,7 +563,7 @@ static void __init setup_init_fpu_buf(vo
 	xsave_state_booting(init_xstate_buf, -1);
 }
 
-static enum { AUTO, ENABLE, DISABLE } eagerfpu = AUTO;
+static enum { AUTO, ENABLE, DISABLE } eagerfpu = ENABLE;
 static int __init eager_fpu_setup(char *s)
 {
 	if (!strcmp(s, "on"))

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