linux-kernel - Re: [RFC 0/5] kernel: backtrace unwind support

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Date:	Sun, 12 Feb 2012 00:36:58 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, paulus@...ba.org,
	cjashfor@...ux.vnet.ibm.com, fweisbec@...il.com,
	linux-kernel@...r.kernel.org,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Jan Blunck <jblunck@...e.de>
Subject: Re: [RFC 0/5] kernel: backtrace unwind support

On Sat, Feb 11, 2012 at 03:38:09PM +0100, Ingo Molnar wrote:
> 
> * Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > > I had a quick peek and I don't think it's constructed in a 
> > > resilent enough form right now. For example there's no clear 
> > > separation and checking of what comes from GCC and what not.
> > 
> > yes, there's nothing like this in now, I'll see what can be 
> > done about that..
> 
> Another resilience feature of lockdep is the 'one strike and you 
> are out!' aspect: the first error or unexpected condition we 
> detect results in the very quick shutting down of all things 
> lockdep. It prints exactly one error message, then it 
> deactivates and never ever runs again.
> 
> The equivalent of this in the scope of your dwarf unwind kernel 
> feature would be to fall back to the regular guess and 
> framepointer based stack backtrace method the moment any error 
> is detected.
> 
> Maybe print a single line that indicates that the fallback has 
> been activated, and after that the dwarf code should never run 
> again. Make sure nobody comes away a "oh, no, the dwarf unwind 
> messed up things!' impression, even if it *does* run into some 
> trouble (such as unexpected debuginfo generated by GCC - or 
> debuginfo *corrupted* by a kernel bug [a very real 
> possibility]).

right, such fallback seems necessary

> 
> What is totally unacceptable is for the dwarf code to *cause* 
> crashes, or to destroy stack trace information.
> 
> > yep, looks interesting.. not sure about the mathematical proof 
> > though ;)
> 
> In the physical sense even mathematics is always and unavoidably 
> probability based (or brain and all our senses are 
> probabilistic), so you can probably replace 'mathematical proof' 
> with 'very robust design and a very, very good track record', 
> before bothering Linus with it next time around ;-)

I wasn't aware of such kernel unwind history ;) was just curious,
if anyone is interested, before spending more time on that..

> 
> And we might as well conclude "it's simply not worth it", at 
> some point down he road. I *do* think that it's worth it though, 
> and I do think it can be designed and implemented robustly, so 
> I'd be willing to try out these patches in -tip for a kernel 
> release or two, without pushing it to Linus.

thanks a lot for your ideas, I'll start working on that

jirka
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