linux-kernel - Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to userspace

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Message-ID: <20150113171134.GA27519@redhat.com>
Date:	Tue, 13 Jan 2015 18:11:34 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to
	userspace

On 01/11, riel@...hat.com wrote:
>
> Defer restoring the FPU state, if so desired, until the task returns to
> userspace.

And I have another concern.

Afaocs with this patch the idle threads will run with TIF_LOAD_FPU set but
without fpu.has_fpu.

This is fine by itself, but this (performance-wise) breaks kernel_fpu_begin()
if use_eager_fpu() == T. Please see the changelog in 5187b28ff08249ab8a162e8
and note that this patch cc's @stable.

Yes, yes, even if I am right we should blame kernel_fpu_begin() and other
eager_fpu oddities. I tried to start the cleanups/fixes in this area some
time ago, but they were ignored.

Oleg.

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