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

Oleg Nesterov
2015-01-13T17:12:37Z

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.

Oleg.