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SubjectRe: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups + docs
* Steven Rostedt (rostedt@goodmis.org) wrote:
[...]
> Actually, I was partly under the impression that we didn't care if it
> was in the fast path. Maybe we need three versions. Let me explain.
>
> We have cases where we want it default disabled and the code called when
> enabled should be as out of line as possible. Tracing definitely falls
> in this pattern. But we could push a "unlikely(static_branch())" for
> such a case (or keep it as very_unlikely()).
>
> Then we have cases where it is default enabled, where we can insert the
> code in the fast path. Or do we even care how the compiler places it?
> Because this leads us to the third use...
>
> The third use is the case were we don't know the branch should be taken
> or not until boot. We don't want the compiler to optimize the paths at
> all. This example is for things like CPU features or types (as HPA
> explained the "if (very_unlikely(cpu_vendor_amd))". This is the category
> that we want to have an efficient system for the running hardware. We
> can't bias one way or the other at compile time because frankly, we
> don't know the answer until run time. This could also be used by modules
> that are drivers for several types of hardware, and it can dynamically
> change itself to suit the hardware it is driving.
>
[...]
One possible naming that might be a good fit:

- read_always_likely()
- read_always_unlikely()
- read_always_branch()

I think it is important to convey both that it is expected to be always
read, pretty much never updated, and the bias, or absence of bias.

I also _like_ to have a relatively long name here, because the update
cost is so high that someone should really think before using this
facility. In my opinion, it's not "just" a stronger likely/unlikely.

Thoughts ?

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com


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