linux-kernel - Re: [RFC] systemtap: begin the process of using proper kernel APIs (part1: use kprobe symbol

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Message-ID: <488070F1.9030903@firstfloor.org>
Date:	Fri, 18 Jul 2008 12:31:13 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	James Bottomley <James.Bottomley@...senPartnership.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	systemtap@...rceware.org, jbeulich@...ell.com
Subject: Re: [RFC] systemtap: begin the process of using proper kernel APIs
 (part1: use kprobe symbol_name/offset instead of address)

>  1) stap ought to use the kernel's infrastructure and not re-implement
> its own.
> 
>  2) if the kernel's infrastructure doesn't meet requirements, improve
> it.

No argument on either of those. Right now the kernel infrastructure
is only comparable to what systemtap overs at very high overhead
costs (see below)

> But while the x86 might not be perfect, its fairly ok these days. Its
> not the utter piece of shite x86_64 had for a long time 

Not sure what you're referring to with this. AFAIK the x86-64 unwinder
for a normal frame pointer less kernel was not any worse (or better)
than a i386 kernel without frame pointers.

- today's traces
> mostly make sense.

If you enable frame pointers? Making your complete kernel slower?
Generating much worse code on i386 by wasting >20% of its available
registers?  Getting pipeline stalls on each function call/exit on many CPUs?

Right now unfortunately there are a few rogue CONFIGs who select that
so more and more kernels have, but I found that always distateful because
enabling frame pointers has such a large impact on all kernel code, especially
on the register starved i386.

I still think the right solution eventually is to have a dwarf2 unwinder
by default for i386/x86-64 and get rid of all these nasty "select
FRAME_POINTER"s which have unfortunately sneaked in.

-Andi

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