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    SubjectRe: [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.

    - --
    All rights reversed
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