Re: [PATCH 1/1] x86: fix text_poke

[Jeremy Fitzhardinge <jeremy@goop.org>]

Mathieu Desnoyers wrote:
> * Linus Torvalds (torvalds@linux-foundation.org) wrote:
>   
>> On Fri, 25 Apr 2008, H. Peter Anvin wrote:
>>     
>>> Yes, that should work.  It's still ugly, and I have to say I find the
>>> complexity rather distasteful.  I am willing to be convinced it's worth it,
>>> but I would really like to see hard numbers.
>>>       
>> I really cannot imagine that this kind of pain is *ever* worth it. 
>>
>> Please give an example of something so important that we'd want to do 
>> complex code rewriting on the fly. What _is_ the point of imv_cond()?
>>
>> 		Linus
>>     
>
> The point is to provide a way to dynamically enable code at runtime
> without noticeable performance impact on the system. It's principally
> useful to control the markers in the kernel, which can be placed in very
> frequently executed code paths. The original markers add a memory read,
> test and conditional branch at each marker site. By using the immediate
> values patchset, it goes down to a load immediate value, test and branch.
>
> However, Ingo was still unhappy with the conditional branch, so I cooked
> this jump patching optimization on top of the immediate values. 

I think all this demonstrates that the conditional branch is a bearable 
cost compared to the alternative.  A conditional branch which almost 
always branches the same way is very predictable, and really shouldn't 
cost very much.

    J