linux-ext4 - Re: [PATCH 1/1] x86: fix text

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Date:	Fri, 25 Apr 2008 23:50:05 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>, Jiri Slaby <jirislaby@...il.com>,
	David Miller <davem@...emloft.net>, zdenek.kabelac@...il.com,
	rjw@...k.pl, paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org, herbert@...dor.apana.org.au,
	penberg@...helsinki.fi, clameter@....com,
	linux-kernel@...r.kernel.org, pageexec@...email.hu
Subject: Re: [PATCH 1/1] x86: fix text_poke

Mathieu Desnoyers wrote:
> * Linus Torvalds (torvalds@...ux-foundation.org) 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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