Messages in this thread |  | | | Date | Fri, 25 Apr 2008 23:50:05 -0700 | | From | Jeremy Fitzhardinge <> | | Subject | Re: [PATCH 1/1] x86: fix text_poke | |
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
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