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

[Steven Rostedt]

[ added Arjan ]

On Fri, 17 Apr 2009, Jeremy Fitzhardinge wrote:

> Mathieu Desnoyers wrote:
> > * Jeremy Fitzhardinge (jeremy@xxxxxxxx) 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.

I was talking with Arjan about this in San Francisco. The expense of doing 
function calls. He told me (and he can correct me if I'm wrong here) that 
function calls are like branch predictions. The branch part is the fact 
that a retq is a jmp that can go to different locations. There's logic in 
the CPU to match calls with retqs to speed this up.

He also told me that the "mcount" retq that I do is actually more 
expensive. The logic does not expect a function to return immediately. 
(for stubs, I'm not sure that was a good design).

Hence,

	call mcount

[...]

mcount:
	retq


is expensive, compared to a call to a function that actually does 
something.

Again, Arjan can correct me here, since I'm just trying to paraphrase what 
he told me.

-- Steve

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