Re: [PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the RCU lock

Ingo Molnar
2015-04-10T09:13:00Z

* Ingo Molnar <mingo@kernel.org> wrote:

> 0000000000000030 <mutex_spin_on_owner.isra.4>:
> 30: 48 3b 37 cmp (%rdi),%rsi
> 33: 48 8d 4e 28 lea 0x28(%rsi),%rcx
> 37: 75 4e jne 87 <mutex_spin_on_owner.isra.4+0x57>
> 39: 55 push %rbp
> 3a: 45 31 c0 xor %r8d,%r8d
> 3d: 65 4c 8b 0c 25 00 00 mov %gs:0x0,%r9
> 44: 00 00
> 46: 48 89 e5 mov %rsp,%rbp
> 49: 48 83 ec 10 sub $0x10,%rsp
> 4d: eb 08 jmp 57 <mutex_spin_on_owner.isra.4+0x27>
> 4f: 90 nop
> 50: f3 90 pause
> 52: 48 3b 37 cmp (%rdi),%rsi
> 55: 75 29 jne 80 <mutex_spin_on_owner.isra.4+0x50>
> 57: 44 89 c0 mov %r8d,%eax
> 5a: 90 nop
> 5b: 90 nop
> 5c: 90 nop
> 5d: 8b 11 mov (%rcx),%edx
> 5f: 90 nop
> 60: 90 nop
> 61: 90 nop

Yeah, so what I missed here are those nops: placeholders for the
STAC/CLAC instructions on x86... and this is what Linus mentioned
about the clac() overhead.

But this could be solved I think: by adding a
copy_from_kernel_inatomic() primitive which simply leaves out the
STAC/CLAC sequence: as these are always guaranteed to be kernel
addresses, the SMAP fault should not be generated.

Thanks,

Ingo