linux-kernel - Re: [RFC][PATCH] module: Optimize __module

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Message-ID: <20150227143008.GQ5029@twins.programming.kicks-ass.net>
Date:	Fri, 27 Feb 2015 15:30:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andi Kleen <ak@...ux.intel.com>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	mathieu.desnoyers@...icios.com, oleg@...hat.com,
	paulmck@...ux.vnet.ibm.com, mingo@...nel.org
Subject: Re: [RFC][PATCH] module: Optimize __module_address() using a latched
 RB-tree

On Fri, Feb 27, 2015 at 10:32:39PM +1030, Rusty Russell wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> > One of the users of this is __kernel_text_address() which is employed in
> > many stack unwinders; which in turn are used by perf-callchain (possibly
> > from NMI context).
> 
> Um, so the stack unwinders use "does this look like a kernel address"
> because we omit the frame pointer?

Not only because of that I think; also as a general robustness check.
I'm not sure people want their stack unwinder to go off into the woods.

> To keep that optimization, we add 220 non-trivial lines to module.c?

Can I make you feel better by writing more comments? I'm not sure we can
convince the arch people to take this test out. Its all over the place.

> Don't get me wrong, it's cute code, but I do wonder if at some point a
> grown up is going to come along and tell us to stop :)

Now where's the fun in that ;-)
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