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

Rik van Riel
2015-01-13T17:55:08Z

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On 01/13/2015 12:11 PM, Oleg Nesterov wrote:
> On 01/11, riel@redhat.com wrote:
>>
>> Defer restoring the FPU state, if so desired, until the task
>> returns to userspace.
>
> And I have another concern.
>
> Afaocs with this patch the idle threads will run with TIF_LOAD_FPU
> set but without fpu.has_fpu.
>
> This is fine by itself, but this (performance-wise) breaks
> kernel_fpu_begin() if use_eager_fpu() == T. Please see the
> changelog in 5187b28ff08249ab8a162e8 and note that this patch cc's
> @stable.
>
> Yes, yes, even if I am right we should blame kernel_fpu_begin() and
> other eager_fpu oddities. I tried to start the cleanups/fixes in
> this area some time ago, but they were ignored.

I suppose we could make kernel_fpu_begin() explicitly point
the cpu's fpu pointer at a special value to indicate that
we are in the middle of a kernel_fpu_begin() / kernel_fpu_end()
session, and should not use the FPU from interrupt context
right now.

Does that seem a reasonable fix?

- --
All rights reversed
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