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

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Date:	Fri, 1 Jun 2007 14:03:56 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/9] Conditional Calls - Hash Table

* Matt Mackall (mpm@...enic.com) wrote:
> On Fri, Jun 01, 2007 at 12:46:23PM -0400, Mathieu Desnoyers wrote:
> > * Matt Mackall (mpm@...enic.com) wrote:
> > > On Thu, May 31, 2007 at 03:42:50PM +0200, Andi Kleen wrote:
> > > > Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> writes:
> > > > 
> > > > > Reimplementation of the cond calls which uses a hash table to hold the active
> > > > > cond_calls. It permits to first arm a cond_call and then load supplementary
> > > > > modules that contain this cond_call.
> > > > 
> > > > Hash table is probably overkill. This is a very very slow path operation.
> > > > Can you simplify the code? Just a linked list of all the condcall segments
> > > > should be enough  and then walk it.
> > > 
> > > I think it could be greatly simplified by using symbols instead of
> > > strings.
> > > 
> > > That is, doing cond_call(foo, func()) rather than cond_call("foo",
> > > func()). Here foo is a structure or type holding the relevant info to
> > > deal with the cond_call infrastructure. For unoptimized architectures,
> > > it can simply be a bool, which will be faster.
> > > 
> > > This has the added advantage that the compiler will automatically pick
> > > up any misspellings of these things. And it saves the space we'd use
> > > on the hash table too.
> > > 
> > 
> > The idea is interesting, but does not fit the problem: AFAIK, it will
> > not be possible to do multiple declarations of the same symbol, which is
> > needed whenever we want to declare a cond_call() more than once or to
> > embed it in an inline function.
> 
> It's not clear to me why either of those things are necessary. An
> example please?
> 

Case where we want to declare the same cond_call multiple times :

function_a(int var)
{
  ...
  cond_call(profile_on, profile_hit(...));
  ...
}


function_b(int var, int var2)
{
  ...
  cond_call(profile_on, profile_hit(...));
  ...
}


Case in inline function :

static inline myinlinefct()
{
  ...
  cond_call(profile_on, profile_hit(...));
  ...
}


somefct()
{
  ...
  myinlinefct();
  ...
  myinlinefct();
  ...
}

Those will result in multiple declarations of the cond_call.


-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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