Mathieu Desnoyers wrote:
* Linus Torvalds (torvalds@xxxxxxxxxxxxxxxxxxxx) 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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