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

Rik van Riel
2015-01-15T02:49:30Z

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On 01/14/2015 01:36 PM, Oleg Nesterov wrote:
> On 01/11, riel@redhat.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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