linux-kernel - Re: [PATCH 3/3] x86, fpu: don't abuse FPU in kernel threads if use_eager

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Message-ID: <54BE6BFD.80603@redhat.com>
Date:	Tue, 20 Jan 2015 09:53:49 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.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: [PATCH 3/3] x86, fpu: don't abuse FPU in kernel threads if use_eager_fpu()

On 01/19/2015 01:52 PM, Oleg Nesterov wrote:
> Afaics there is no reason why kernel threads should have FPU context
> even if use_eager_fpu() == T. Now that interrupted_kernel_fpu_idle()
> does not check __thread_has_fpu() we can remove the init_fpu() code
> from eager_fpu_init() and change flush_thread() called by do_execve()
> to initialize FPU.
>
> Note: of course, the change in flush_thread() is horrible and must be
> cleanuped. We need the new helper, and flush_thread() should return the
> error if init_fpu() fails.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Nice. This should help prevent us from loading and saving
the totally unused FPU state for things like idle threads
and other kernel threads.

Reviewed-by: Rik van Riel <riel@...hat.com>

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