question about lazy FPU restore

question about lazy FPU restore

[Rik van Riel]

Hi Suresh,

I have a question about the lazy fpu restore code in
switch_fpu_prepare.  Specifically, about the case where
the old task did not use the FPU, and the new task's
FPU state is still in the cpu.

         } else {
                 old->fpu_counter = 0;
                 old->thread.fpu.last_cpu = ~0;
                 if (fpu.preload) {
                         new->fpu_counter++;
                         if (!use_eager_fpu() && fpu_lazy_restore(new, cpu))
                                 fpu.preload = 0;
                         else
                                 prefetch(new->thread.fpu.state);
                         __thread_fpu_begin(new);
                 }
         }

In this branch, we call fpu_lazy_restore, which
confirms that the CPU still has the new task's state
in it.

However, if we are in eager fpu mode, we still end up
calling restore_fpu_checking from switch_fpu_finish,
even if the new task's FPU state is still resident in
the CPU.

Is there a particular reason we do this?

Would it be possible to always set fpu.preload = 0,
call clts, and __thread_set_has_fpu if fpu_lazy_restore
returns true?

That would allow us to skip the loading of FPU state
when re-entering a process that went briefly idle, before
getting something else to do, a common occurrance in
message passing workloads.