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

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Date:	Fri, 18 Jul 2008 12:44:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	systemtap@...rceware.org, jbeulich@...ell.com,
	arjan <arjan@...radead.org>
Subject: Re: [RFC] systemtap: begin the process of using proper kernel APIs
	(part1: use kprobe symbol_name/offset instead of address)

On Fri, 2008-07-18 at 12:31 +0200, Andi Kleen wrote:

> > 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.

I hardly ever compile a kernel without frame pointers, as debugability
is top priority for me, so I'm afraid I'm not sure how bad it gets
without.

But it used to be that x86_64 was crap even with frame pointers, and
without it it was just random gibberish - Arjan fixed that up somewhere
around .25 iirc.

> - 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.

No argument on i386 vs frame pointers, fortunately I hardly ever build a
32bit kernel these days, 32bit hardware is disappearing fast here :-) 

As to merging the dwarf unwinder, I have no particular objection to
getting that merged as I'm rather ignorant on these matters - but from
reading emails around the last merge attempt, Linus had some strong
opinions on the matter.

Have those been resolved since?

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