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

Andy Lutomirski
2015-01-13T17:11:40Z

On Sun, Jan 11, 2015 at 1:46 PM, <riel@redhat.com> wrote:
> From: Rik van Riel <riel@redhat.com>
>
> If the next task still has its FPU state present in the FPU registers,
> there is no need to restore it from memory.
>
> This is no big deal on bare metal, where XSAVEOPT / XRSTOR are heavily
> optimized, but those optimizations do not carry across VMENTER / VMEXIT.
>
> Skipping the call to fpu_restore_checking when the FPU state is already
> loaded in the CPU could save a little bit of overhead on bare metal too,
> so this is not just a KVM optimization.
>
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> arch/x86/include/asm/fpu-internal.h | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h
> index 4db8781..a5a40c7 100644
> --- 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);

Is this prefetch still worth keeping? Wouldn't prefetching the fpu
effectively require grabbing more than one cacheline anyway?

--Andy

> - set_thread_flag(TIF_LOAD_FPU);
> - }
> } else
> /*
> * The new task does not want an FPU state restore,
> @@ -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;
> +
> if (unlikely(restore_fpu_checking(tsk)))
> drop_init_fpu(tsk);
> }
> --
> 1.9.3
>

--
Andy Lutomirski
AMA Capital Management, LLC