linux-kernel - Re: [patch 2/3] RCU move trace defines to rcupdate

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Date:	Fri, 17 Apr 2009 09:14:27 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: [patch 2/3] RCU move trace defines to rcupdate_types.h

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