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

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Message-ID: <54EC8DFC.3030005@redhat.com>
Date:	Tue, 24 Feb 2015 09:43:08 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>
CC:	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.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

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On 02/23/2015 09:31 PM, Andy Lutomirski wrote:
> On Mon, Feb 23, 2015 at 6:14 PM, Maciej W. Rozycki
> <macro@...ux-mips.org> wrote:

>> That's an interesting case too, although not necessarily related.
>> If you say that we always save the FP context eagerly for the
>> purpose of process migration, then sure, that invalidates any
>> benefit we'd have from letting the x87 proceed.
>> 
>> However I can see different ways to address this case avoiding
>> the need of eager FP context saving or an IPI:
>> 
>> 1. We could bind any currently suspended process with an unsaved
>> FP context to the CPU it last executed on.
> 
> This would be insane.

The task would only be bound to the CPU as long as nothing else
ran that saved the FPU state to memory.  This means the task
would be bound to the CPU while the CPU is idle, or running a
kernel thread. When another user space thread is about to run,
the FPU state would be saved.

This sounds slightly less insane already.

Of course, once you figure in CPU power saving and CPU hot
plug, the level of insanity is far beyond what you seem to
imply above...

In other words, almost certainly not worth doing :)

- -- 
All rights reversed
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