linux-kernel - Re: [patch 2/9] Conditional Calls

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Message-ID: <20070601204434.GN7217@one.firstfloor.org>
Date:	Fri, 1 Jun 2007 22:44:34 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Andi Kleen <andi@...stfloor.org>, Matt Mackall <mpm@...enic.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/9] Conditional Calls - Hash Table

On Fri, Jun 01, 2007 at 04:33:06PM -0400, Mathieu Desnoyers wrote:
> * Andi Kleen (andi@...stfloor.org) wrote:
> > > Yes, but as you have probably understood, I want to have everything
> > > embedded at the cond_call() site rather than polluting the rest of the
> > > code with declarations.
> > 
> > A cond call is essentially a fancy variable. And the Linux kernel
> > is written in C and in C you declare variables before you use them.
> > Also it would allow compile time checking against typos and 
> > allow removing some nasty hash table code. The proposal sounds like a 
> > clear winner to me. 
> > 
> 
> You could not declare in advance a structure that would contain pointers
> to every load immediate instruction of the optimized cond_calls. Unless

To find them you just walk the sections.  Changing cond call is a slow
path operation. That is similar to how the smp lock switching
works today.

> I understand that if we limit ourselves to applications like the two
> toy examples I proposed (enabling profiling and bug fixups), it could
> make sense to try to declare a variable somewhere and later use it in
> the body of functions (except the fact that it cannot work, due to
> incapacity to declare pointers to each load immediate instruction, as
> stated above). Even if it would work, the main purpose here is to
> support the Linux Kernel Markers, where the goal is to provide the
> ability to declare a marker within the body of a function without
> requiring more hassle than a printk, but with less performance impact
> than the latter. Also, we would not want the whole kernel to recompile
> simply because someone chose to add one or two marker in his own driver
> to extract some more information and had to add them to some globally
> included header file.

Sounds similar to config.h then when Kconfig keeps track of those
dependencies for the CONFIG_*s and only recompiles what is needed. Perhaps 
this infrastructure could be reused.

-Andi  
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