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SubjectRe: [RFC patch] cmpxchg_double: remove local variables to get better performance
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On Fri, 2012-03-02 at 08:54 +0000, Jan Beulich wrote:
> >>> On 02.03.12 at 09:31, Alex Shi <alex.shi@intel.com> wrote:
> > There are some local variables in cmpxchg_double macro, seems these are
> > used to for force casting on input variables to transfer them into '*p1'
> > type. May there are some reason I don't know. But I just saw 2 problems
> > here:
> >
> > 1, user may mis-use the macro, like give a 'long' type o1, but just use
> > a 'int*' or 'char*' p1.
>
> No - see the BUILD_BUG_ON()s right after the lines you suggest to
> remove.
>
> Further, it seems to be intentional to allow _compatible_ types for
> o1 and o2 - you could pass in a literal number without L suffix here,
> which I don't think you can anymore with the intermediate variable
> removed.

Yes, we can use cast for intermediate data. And actually, current kernel
has live mis-used case on cmpxchg(), that I plan to point out too.

-- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -203,12 +203,12 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)

void kvm_flush_remote_tlbs(struct kvm *kvm)
{
- int dirty_count = kvm->tlbs_dirty;
+ long dirty_count = kvm->tlbs_dirty;

smp_mb();
if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
++kvm->stat.remote_tlb_flush;
- cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
+ cmpxchg(&kvm->tlbs_dirty, dirty_count, 0L);
}

>
> > If we remove the force cast here, gcc will check the mis-using in
> > compiling. and user can get the error report in compiling for such
> > issues.
> >
> > 2, local variable increased the data section, and bring extra memory bus
>
> These aren't static, so the data section can't possibly increase.

sorry, it is text section increasing.
> > accesses, that hurt performance in this critical macro.
>
> With optimization enabled, the compiler should eliminate all unnecessary
> intermediate variables.

oh, I don't know now. I will recheck this point.
>
> > I did a little experiment on my nhm i7 desktop, to run the macro with a
> > fixed times, here is the data:
> > using local vars no local variable
> > with lock prefix, 267700578ns 232079696ns
> > without lock prefix, 34715666ns 34687566ns
> >
> > So, we may need rethink about the local variable usage here.
> >
> > Signed-off-by: Alex Shi <alex.shi@intel.com>
>
> Sorry, but if this counts, this is a nack from me.
>
> Jan
>
> > diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
> > index b3b7332..8bf9127 100644
> > --- a/arch/x86/include/asm/cmpxchg.h
> > +++ b/arch/x86/include/asm/cmpxchg.h
> > @@ -210,17 +210,15 @@ extern void __add_wrong_size(void)
> > #define __cmpxchg_double(pfx, p1, p2, o1, o2, n1, n2) \
> > ({ \
> > bool __ret; \
> > - __typeof__(*(p1)) __old1 = (o1), __new1 = (n1); \
> > - __typeof__(*(p2)) __old2 = (o2), __new2 = (n2); \
> > BUILD_BUG_ON(sizeof(*(p1)) != sizeof(long)); \
> > BUILD_BUG_ON(sizeof(*(p2)) != sizeof(long)); \
> > VM_BUG_ON((unsigned long)(p1) % (2 * sizeof(long))); \
> > VM_BUG_ON((unsigned long)((p1) + 1) != (unsigned long)(p2)); \
> > asm volatile(pfx "cmpxchg%c4b %2; sete %0" \
> > - : "=a" (__ret), "+d" (__old2), \
> > + : "=a" (__ret), "+d" (o2), \
> > "+m" (*(p1)), "+m" (*(p2)) \
> > - : "i" (2 * sizeof(long)), "a" (__old1), \
> > - "b" (__new1), "c" (__new2)); \
> > + : "i" (2 * sizeof(long)), "a" (o1), \
> > + "b" (n1), "c" (n2)); \
> > __ret; \
> > })
> >
>
>
>




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