linux-kernel - Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end() if use_eager

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Message-ID: <54CAABF0.3090705@redhat.com>
Date:	Thu, 29 Jan 2015 16:53:52 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86, fpu: unlazy_fpu: don't do __thread_fpu_end()
 if use_eager_fpu()

On 01/29/2015 04:49 PM, Oleg Nesterov wrote:

> Yes. But in this case tsk != current, and we ensure that __switch_to()
> was finished. wait_task_inactive().
> 
>> When we switch to the traced task, the __thread_fpu_end()
>> that was called from init_fpu() ensures that
>> switch_fpu_begin() will actually load the new FPU state
>> from memory into the registers, and we will not take
>> the fpu_lazy_restore() branch.
> 
> No. in this case we rely on "tsk->thread.fpu.last_cpu = ~0" which disables
> fpu_lazy_restore().

I should have known that.

Especially considering I have a patch here that
converts "tsk->thread.fpu.last_cpu = ~0" into
a disable_lazy_restore() call...

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