Re: [patch 2/3] RCU move trace defines to rcupdate_types.h

Jeremy Fitzhardinge
2009-04-17T16:14:39Z

Mathieu Desnoyers wrote:
> * Jeremy Fitzhardinge (jeremy@goop.org) wrote:
>
>> Mathieu Desnoyers wrote:
>>
>>> Given the simplicity of the preempt_disable/enable_notrace found in
>>> preempt.h, we could move them to
>>>
>>> include/preempt_types.h too, and that would solve all problems, wouldn't
>>> it ?
>>>
>>>
>> No, it still needs linux/thread_info.h -> asm/thread_info.h, which in
>> turn gets quite a lot of things on x86 (and would need to be audited in
>> each architecture).
>>
>> J
>>
>
> Well, I think it's a good time to do some cleanup then. Why on earth
> would thread_info.h be anything else than a "_types"-like header ?
>

Why indeed? Because it includes a number of other headers to get the
definitions it needs, and defines various functions needed to operate on
the thread_info structure (including the all-important
current_thread_info()).

Yes, it can be refactored into thread_info.h and thread_info_types.h,
and all the headers it includes can be similarly refactored, and
linux/thread_info.h can also be split, and all the asm/*/thread_info.hs
can be split too, and it can be made to work for all arches under all
configs...

But that's going to take a long time, and if its a pre-requisite for
getting tracing going, then we're not going to see it merged this year.

> If headers has become in such a state in the kernel, then IMHO the
> solution is not to shove more out-of-line functions under the carpet,
> but rather to do the cleanup.
>

Besides, I'm still not convinced that putting the code inline is a good
idea. Direct call/return are not inherently expensive, and they're
something that CPU vendors have a lot of motivation to optimise for. In
particular, the call itself is no more expensive than a jmp other than
the return-address push, and the ret is also cheap because it will use
the return address cache rather than having to be a full indirect jmp.

And it would be much easier to justify leaving tracing compile-time
enabled all the time if each tracepoint really does have a minimal
icache profile when not enabled.

J