linux-kernel - Re: [PATCH 2/2] i387: support lazy restore of FPU state

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Message-ID: <20120220075101.GB30810@elte.hu>
Date:	Mon, 20 Feb 2012 08:51:01 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] i387: support lazy restore of FPU state


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> +DECLARE_PER_CPU(struct task_struct *, fpu_owner_task);

> +static inline int fpu_lazy_restore(struct task_struct *new, unsigned int cpu)
> +{
> +	return new == percpu_read_stable(fpu_owner_task) &&
> +		cpu == new->thread.fpu.last_cpu;
> +}

I guess the CPU hotplug case deserves a comment in the code: CPU 
hotplug + replug of the same (but meanwhile reset) CPU is safe 
because fpu_owner_task[cpu] gets reset to NULL.

Likewise, if we hot-unplug two CPUs and then insert the second 
one, it's the same CPU index but not the same FPU state anymore. 
There too the CPU hotplug code resetting fpu_owner_task to NULL 
makes this optimization safe.

Thanks,

	Ingo
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