Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

H. Peter Anvin
2011-10-06T18:30:20Z

On 10/06/2011 11:26 AM, Steven Rostedt wrote:
> On Thu, 2011-10-06 at 14:10 -0400, Jason Baron wrote:
>
>>> Looks like jmp2 is about 5% faster than jmp5 on Sandybridge with this
>>> benchmark.
>>>
>>> But insignificant difference on Nehalem.
>>>
>>> J
>>
>> It would be cool if we could make the total width 2-bytes, when
>> possible. It might be possible by making the initial 'JUMP_LABEL_INITIAL_NOP'
>> as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
>> time or link time - letting the compiler pick the width. In that way we could
>> get the optimal width...
>
> Why not just do it?
>
> jump_label is encapsulated in arch_static_branch() which on x86 looks
> like:
>
> static __always_inline bool arch_static_branch(struct jump_label_key *key)
> {
> asm goto("1:"
> JUMP_LABEL_INITIAL_NOP
> ".pushsection __jump_table, \"aw\" \n\t"
> _ASM_ALIGN "\n\t"
> _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
> ".popsection \n\t"
> : : "i" (key) : : l_yes);
> return false;
> l_yes:
> return true;
> }
>
>
> That jmp to l_yes should easily be a two byte jump.
>
> If not I'm sure it would be easy to catch it before modifying the code.
> And then complain real loudly about it.
>

The important thing is that it requires the build-time elimination of
jumps. It's just work.

-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel. I don't speak on their behalf.