linux-kernel - Re: [PATCH] modules: Take a shortcut for checking if an address is in a module

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Date:	Wed, 18 Jun 2008 12:24:31 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] modules: Take a shortcut for checking if an address is
	in a module

On Wed, 2008-06-18 at 11:57 +0200, Ingo Molnar wrote:
> * Vegard Nossum <vegard.nossum@...il.com> wrote:
> 
> > >  > > Various pieces of the kernel (lockdep, latencytop, etc) tend to 
> > >  > > store backtraces, sometimes at a relatively high frequency. In 
> > >  > > itself this isn't a big performance deal (after all you're 
> > >  > > using diagnostics features), but there have been some 
> > >  > > complaints from people who have over 100 modules loaded that 
> > >  > > this is a tad too slow.
> > 
> > Would it be overkill to simply drop the module addresses in an rbtree 
> > and use that instead of a linear search over all the modules?
> > 
> > It would probably take a fair number of lines in C, and with a little 
> > memory overhead, but the speed-up should be great. Should I give it a 
> > try? (It would be arch-independent too.)
> 
> that's a tempting idea. rbtrees seem to be equally robust to plain lists 
> in my experience, so i'd not find the extra complexity a showstopper, as 
> long as the changes are well-tested. (radix trees on the other hand ... 
> ;-)

Radix trees are unsuited for this application, esp in their current
implementation.

> Rusty, Peter, Linus, any fundamental objections to Vegard's idea? Being 
> able to take a transparent stack-trace signature for debugging or 
> instrumentation purposes is important and performance does matter there 
> IMO.

A tree makes sense, although if more archs can do the same Arjan did for
x86 that'd be even better.

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