Re: [PATCH 1/2] x86/bitops: implement __test_bit

[Michael S. Tsirkin]

On Tue, Sep 01, 2015 at 11:24:22AM +0200, Ingo Molnar wrote:
> 
> * Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> 
> > I applied this patch on top of mine:
> 
> Yeah, looks similar to the one I sent.
> 
> > -static inline int __variable_test_bit(long nr, const unsigned long *addr)
> > -{
> > -	int oldbit;
> > -
> > -	asm volatile("bt %2,%1\n\t"
> > -		     "sbb %0,%0"
> > -		     : "=r" (oldbit)
> > -		     : "m" (*addr), "Ir" (nr));
> > -
> > -	return oldbit;
> > -}
> 
> > And the code size went up:
> > 
> >    134836    2997    8372  146205   23b1d arch/x86/kvm/kvm-intel.ko  ->
> >    134846    2997    8372  146215   23b27 arch/x86/kvm/kvm-intel.ko     
> > 
> >    342690   47640     441  390771   5f673 arch/x86/kvm/kvm.ko ->
> >    342738   47640     441  390819   5f6a3 arch/x86/kvm/kvm.ko   
> > 
> > I tried removing  __always_inline, this had no effect.
> 
> But code size isn't the only factor.
> 
> Uros Bizjak pointed out that the reason GCC does not use the "BT reg,mem" 
> instruction is that it's highly suboptimal even on recent microarchitectures, 
> Sandy Bridge is listed as having a 10 cycles latency (!) for this instruction:
> 
>    http://www.agner.org/optimize/instruction_tables.pdf
> 
> this instruction had bad latency going back to Pentium 4 CPUs.
> 
> ... so unless something changed in this area with Skylake I think using the 
> __variable_test_bit() code of the kernel is a bad choice and looking at kernel 
> size only is misleading.
> 
> It makes sense for atomics, but not for unlocked access.
> 
> Thanks,
> 
> 	Ingo

Hmm - so do you take back the ack?

I compared this:
int main(int argc, char **argv)
{

        long long int i;
        const unsigned long addr = 0;
        for (i = 0; i < 1000000000ull; ++i) {
                asm volatile("");
                if (__variable_test_bit(1, &addr))
                asm volatile("");
        }
        return 0;
}

with the __constant_test_bit variant.

__constant_test_bit code does appear to be slower on an i7 processor.

test_bit isn't atomic either. Maybe drop variable_test_bit there too?

-- 
MST
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