Messages in this thread | | | Date | Wed, 14 Jan 2015 19:36:06 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too |
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On 01/11, riel@redhat.com wrote: > > If the next task still has its FPU state present in the FPU registers, > there is no need to restore it from memory.
Another patch I can't understand...
> --- 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); > - set_thread_flag(TIF_LOAD_FPU); > - }
It is not clear to me why do we set TIF_LOAD_FPU if fpu_lazy_restore() succeeds. __thread_fpu_begin() is cheap.
At the same time, if switch_fpu_finish() does fpu_lazy_restore() anyway, why this patch doesn't remove it from switch_fpu_prepare() ?
However,
> @@ -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; > +
Now that this is called before return to user-mode, I am not sure this is correct. Note that __kernel_fpu_begin() doesn't clear fpu_owner_task if use_eager_fpu().
OK, interrupted_kernel_fpu_idle() should fail in this case... but as we already discussed this means the perfomance regression, so this should be changed.
Oleg.
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