Messages in this thread Patch in this message | | | Date | Wed, 14 Jan 2015 21:49:04 -0500 | From | Rik van Riel <> | Subject | Re: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too |
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On 01/14/2015 01:36 PM, Oleg Nesterov wrote: > 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() > ?
I have it removed now, because the prefetch does not make much sense (as was pointed out by either you or Andy).
>> @@ -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().
However, __kernel_fpu_begin() does call __thread_clear_has_fpu(), which clears the per-cpu fpu_owner variable, which is also evaluated by fpu_lazy_restore(), so I think this is actually correct.
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