linux-kernel - Re: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too

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Date:	Wed, 14 Jan 2015 21:49:04 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...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 06/11] x86,fpu: lazily skip fpu restore with eager
 fpu mode, too

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On 01/14/2015 01:36 PM, Oleg Nesterov wrote:
> On 01/11, riel@...hat.com wrote:
>> 
>> If the next task still has its FPU state present in the FPU
>> registers, there is no need to restore it from memory.
> 
> Another patch I can't understand...
> 
>> --- a/arch/x86/include/asm/fpu-internal.h +++
>> b/arch/x86/include/asm/fpu-internal.h @@ -435,13 +435,9 @@ static
>> inline void switch_fpu_prepare(struct task_struct *old, struct
>> task_struc old->thread.fpu.last_cpu = ~0; if (preload) { 
>> new->thread.fpu_counter++; -			if (!use_eager_fpu() &&
>> fpu_lazy_restore(new, cpu)) -				/* XXX: is this safe against
>> ptrace??? */ -				__thread_fpu_begin(new); -			else { +
>> set_thread_flag(TIF_LOAD_FPU); +			if (!fpu_lazy_restore(new,
>> cpu)) prefetch(new->thread.fpu.state); -
>> set_thread_flag(TIF_LOAD_FPU); -			}
> 
> It is not clear to me why do we set TIF_LOAD_FPU if
> fpu_lazy_restore() succeeds. __thread_fpu_begin() is cheap.
> 
> At the same time, if switch_fpu_finish() does fpu_lazy_restore()
> anyway, why this patch doesn't remove it from switch_fpu_prepare()
> ?

I have it removed now, because the prefetch does not make
much sense (as was pointed out by either you or Andy).

>> @@ -466,6 +462,10 @@ static inline void switch_fpu_finish(void)
>> 
>> __thread_fpu_begin(tsk);
>> 
>> +	/* The FPU registers already have this task's FPU state. */ +
>> if (fpu_lazy_restore(tsk, raw_smp_processor_id())) +		return; +
> 
> Now that this is called before return to user-mode, I am not sure
> this is correct. Note that __kernel_fpu_begin() doesn't clear
> fpu_owner_task if use_eager_fpu().

However, __kernel_fpu_begin() does call __thread_clear_has_fpu(),
which clears the per-cpu fpu_owner variable, which is also
evaluated by fpu_lazy_restore(), so I think this is actually
correct.

- -- 
All rights reversed
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