linux-kernel - Re: [PATCH 0/3] x86, fpu: more eagerfpu cleanups

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Date:	Fri, 20 Feb 2015 14:30:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.com>,
	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: [PATCH 0/3] x86, fpu: more eagerfpu cleanups

On 02/20, Borislav Petkov wrote:
>
> On Mon, Jan 19, 2015 at 07:51:09PM +0100, Oleg Nesterov wrote:
> > math_state_restore() and its usage doesn't look nice. init_fpu() too,
>
> Yeah, about that:
>
> I see:
>
> math_state_restore
> ...
> 	 if (!tsk_used_math(tsk))
> 		init_fpu()
>
> and init_fpu() then does:
>
> 	if (tsk_used_math(tsk))
>

Yes, and more. math_state_restore() assumes that it is called with irqs
disabled. At least if !tsk_used_math. That is why 3/3 calls init_fpu()
first. Not only this doesn't look clean, this is simply not true in
general.

The comment above init_fpu() is simply wrong. And unlazy_fpu() from
there doesn't look nice. This mixes 2 completely differents things.

> Could use a cleanup and so on... Perhaps it is in the works already :)

Yes, I'll try to make the cleanups on top of these changes. And let me
repeat that there is another reason for 1/3 and 2/3 at least (3/3 makes
sense too), if we add TIF_LOAD_FPU we need to avoid the performance
regression (irq_fpu_usable() should not fail if !__thread_has_fpu()).

Oleg.

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