Re: [RFC PATCH 04/11] x86,fpu: defer FPU restore until return to userspace

[Rik van Riel]

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On 01/13/2015 12:57 PM, Andy Lutomirski wrote:
> On Tue, Jan 13, 2015 at 9:44 AM, Rik van Riel <riel@xxxxxxxxxx>
> wrote: On 01/13/2015 12:18 PM, Andy Lutomirski wrote:

>>>> - Task is not current and FPU is in memory. - Task is not
>>>> current and FPU is loaded into some cpu.
> 
>> Same for this one. When the task is not current, the FPU state 
>> will have been saved to memory. If we try running the task 
>> somewhere else, it devolves to "FPU is in memory".
> 
> 
> Isn't there a case where the FPU is in memory *and* in the cpu
> regs? Isn't that how you can skip reloading the FPU after going
> idle and returning?  Is this what fpu_lazy_restore is for?
> Confused.

Indeed, if we end up running the task on the same CPU again, and the
FPU still has the state loaded, we may skip restoring the FPU state.

>>>> Am I missing anything?  (In lazy mode, there are a few more 
>>>> involving CR0.TS.)
>>>> 
>>>> That's five states, plus an optional cpu number.  But we have
>>>> tons of state variable that can express all kinds of nonsense
>>>> things.
>>>> 
>>>> If we asserted that we were in a sensible state and fixed
>>>> the things that exited the sensible states, maybe this would
>>>> be easier to understand and debug.
> 
> Lets see what things we could test, at different points.
> 
> 1) At context switch time, we need to make sure that the previous
> task will no longer have __thread_has_fpu()
> 
> 2) When loading the FPU state, we need to make sure that the
> current task does not have __thread_has_fpu()
> 
>> Examples, any of which may be wrong:
> 
>> If !current, then !TIF_LOAD_FPU

We set TIF_LOAD_CPU on the next task at context switch time,
which is different from the current task. I suspect we can
deal with that exception, though :)

What we can test is that "new" does not already have TIF_LOAD_CPU
set...

>> If switching out a task with TIF_LOAD_FPU set, then !has_fpu

... and that old does not have both TIF_LOAD_FPU and has_fpu.

>> If last_cpu == smp_processor_id(), then fpu_owner == fpu.

Not necessarily, since the task may not have entered userspace in
a very long time, so it may not have loaded its FPU context.

>> If has_fpu, then the task must be current somewhere and last_cpu
>> must be the cpu on which it's current.

Indeed, if has_fpu, then last_cpu must match the current cpu.


- -- 
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