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

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Date:	Tue, 13 Jan 2015 19:22:39 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <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 01/13, Rik van Riel wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 01/13/2015 12:11 PM, Oleg Nesterov wrote:
> > 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.
>
> I suppose we could make kernel_fpu_begin() explicitly point
> the cpu's fpu pointer at a special value to indicate that
> we are in the middle of a kernel_fpu_begin() / kernel_fpu_end()
> session, and should not use the FPU from interrupt context
> right now.

Not sure I understand...

But yes, I think we need the per-cpu "in_kernel_fpu" and irq_fpu_usable()
must die. Please look at http://marc.info/?l=linux-kernel&m=14096628660929

Until then imo this series should try to ensure that kernel_fpu_begin/end
will work if interrupted thread is idle task.

Oleg.

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