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Date: Tue, 1 Sep 2015 18:03:27 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Rusty Russell <rusty@...tcorp.com.au>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Uros Bizjak <ubizjak@...il.com>
Subject: Re: [PATCH 1/2] x86/bitops: implement __test_bit
On Tue, Sep 01, 2015 at 01:39:42PM +0200, Ingo Molnar wrote:
>
> * Michael S. Tsirkin <mst@...hat.com> wrote:
>
> > On Tue, Sep 01, 2015 at 11:24:22AM +0200, Ingo Molnar wrote:
> > >
> > > * Michael S. Tsirkin <mst@...hat.com> 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 have no strong feelings either way, it simply strikes me as misguided to
> explicitly optimize for something that is listed as a high overhead instruction.
>
> Assuming it really is high overhead:
>
> > 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.
Ouch. I meant the reverse:
[mst@...in test]$ diff a.c b.c
31c31
< if (__variable_test_bit(1, &addr))
---
> if (__constant_test_bit(1, &addr))
[mst@...in test]$ gcc -Wall -O2 a.c; time ./a.out
real 0m0.532s
user 0m0.531s
sys 0m0.000s
[mst@...in test]$ gcc -Wall -O2 b.c; time ./a.out
real 0m0.517s
user 0m0.517s
sys 0m0.000s
So __constant_test_bit is faster even though it's using more
instructions
$ gcc -Wall -O2 a.c; -objdump -ld ./a.out
08048414 <main>:
main():
8048414: 8d 4c 24 04 lea 0x4(%esp),%ecx
8048418: 83 e4 f8 and $0xfffffff8,%esp
804841b: ff 71 fc pushl -0x4(%ecx)
804841e: 55 push %ebp
804841f: 89 e5 mov %esp,%ebp
8048421: 51 push %ecx
8048422: 83 ec 14 sub $0x14,%esp
8048425: c7 45 ec 00 00 00 00 movl $0x0,-0x14(%ebp)
804842c: c7 45 f0 00 00 00 00 movl $0x0,-0x10(%ebp)
8048433: c7 45 f4 00 00 00 00 movl $0x0,-0xc(%ebp)
804843a: eb 1a jmp 8048456 <main+0x42>
804843c: 8d 45 ec lea -0x14(%ebp),%eax
804843f: 50 push %eax
8048440: 6a 01 push $0x1
8048442: e8 b4 ff ff ff call 80483fb <__variable_test_bit>
8048447: 83 c4 08 add $0x8,%esp
804844a: 85 c0 test %eax,%eax
804844c: 74 00 je 804844e <main+0x3a>
804844e: 83 45 f0 01 addl $0x1,-0x10(%ebp)
8048452: 83 55 f4 00 adcl $0x0,-0xc(%ebp)
8048456: 8b 45 f0 mov -0x10(%ebp),%eax
8048459: 8b 55 f4 mov -0xc(%ebp),%edx
804845c: 83 fa 00 cmp $0x0,%edx
804845f: 72 db jb 804843c <main+0x28>
8048461: 83 fa 00 cmp $0x0,%edx
8048464: 77 07 ja 804846d <main+0x59>
8048466: 3d ff c9 9a 3b cmp $0x3b9ac9ff,%eax
804846b: 76 cf jbe 804843c <main+0x28>
804846d: b8 00 00 00 00 mov $0x0,%eax
8048472: 8b 4d fc mov -0x4(%ebp),%ecx
8048475: c9 leave
8048476: 8d 61 fc lea -0x4(%ecx),%esp
8048479: c3 ret
804847a: 66 90 xchg %ax,%ax
804847c: 66 90 xchg %ax,%ax
804847e: 66 90 xchg %ax,%ax
$ gcc -Wall -O2 b.c; -objdump -ld ./a.out
080483fb <main>:
main():
80483fb: 8d 4c 24 04 lea 0x4(%esp),%ecx
80483ff: 83 e4 f8 and $0xfffffff8,%esp
8048402: ff 71 fc pushl -0x4(%ecx)
8048405: 55 push %ebp
8048406: 89 e5 mov %esp,%ebp
8048408: 51 push %ecx
8048409: 83 ec 24 sub $0x24,%esp
804840c: c7 45 e4 00 00 00 00 movl $0x0,-0x1c(%ebp)
8048413: c7 45 f0 00 00 00 00 movl $0x0,-0x10(%ebp)
804841a: c7 45 f4 00 00 00 00 movl $0x0,-0xc(%ebp)
8048421: eb 44 jmp 8048467 <main+0x6c>
8048423: c7 45 ec 01 00 00 00 movl $0x1,-0x14(%ebp)
804842a: 8d 45 e4 lea -0x1c(%ebp),%eax
804842d: 89 45 e8 mov %eax,-0x18(%ebp)
8048430: 8b 45 ec mov -0x14(%ebp),%eax
8048433: c1 f8 05 sar $0x5,%eax
8048436: 8d 14 85 00 00 00 00 lea 0x0(,%eax,4),%edx
804843d: 8b 45 e8 mov -0x18(%ebp),%eax
8048440: 01 d0 add %edx,%eax
8048442: 8b 10 mov (%eax),%edx
8048444: 8b 45 ec mov -0x14(%ebp),%eax
8048447: 83 e0 1f and $0x1f,%eax
804844a: 89 c1 mov %eax,%ecx
804844c: d3 ea shr %cl,%edx
804844e: 89 d0 mov %edx,%eax
8048450: 83 e0 01 and $0x1,%eax
8048453: 85 c0 test %eax,%eax
8048455: 0f 95 c0 setne %al
8048458: 0f b6 c0 movzbl %al,%eax
804845b: 85 c0 test %eax,%eax
804845d: 74 00 je 804845f <main+0x64>
804845f: 83 45 f0 01 addl $0x1,-0x10(%ebp)
8048463: 83 55 f4 00 adcl $0x0,-0xc(%ebp)
8048467: 8b 45 f0 mov -0x10(%ebp),%eax
804846a: 8b 55 f4 mov -0xc(%ebp),%edx
804846d: 83 fa 00 cmp $0x0,%edx
8048470: 72 b1 jb 8048423 <main+0x28>
8048472: 83 fa 00 cmp $0x0,%edx
8048475: 77 07 ja 804847e <main+0x83>
8048477: 3d ff c9 9a 3b cmp $0x3b9ac9ff,%eax
804847c: 76 a5 jbe 8048423 <main+0x28>
804847e: b8 00 00 00 00 mov $0x0,%eax
8048483: 83 c4 24 add $0x24,%esp
8048486: 59 pop %ecx
8048487: 5d pop %ebp
8048488: 8d 61 fc lea -0x4(%ecx),%esp
804848b: c3 ret
804848c: 66 90 xchg %ax,%ax
804848e: 66 90 xchg %ax,%ax
--
MST
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