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Message-ID: <20070320174159.GA4286@bingen.suse.de>
Date: Tue, 20 Mar 2007 18:42:00 +0100
From: Andi Kleen <ak@...e.de>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Andi Kleen <ak@...e.de>, David Miller <davem@...emloft.net>,
torvalds@...ux-foundation.org,
virtualization@...ts.linux-foundation.org, jbeulich@...ell.com,
jeremy@...p.org, xen-devel@...ts.xensource.com,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
chrisw@...s-sol.org, virtualization@...ts.osdl.org,
anthony@...emonkey.ws, akpm@...ux-foundation.org, mingo@...e.hu
Subject: Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable
On Tue, Mar 20, 2007 at 10:25:20AM -0600, Eric W. Biederman wrote:
> What I recall observing is call traces that made no sense. Not just
> extra noise in the stack trace but things like seeing a function that
> has exactly one path to it, and not seeing all of the functions on
> that path in the call trace.
That's tail call/sibling call optimization. No unwinder can untangle that
because the return address is lost. But it's also an quite important optimization.
> >
> > In 2.4 it was often very reasonable to just sort out the false positives,
> > but with sometimes 20-30+ level deep call chains in 2.6 with many callbacks that
> > just
> > gets far too tenuous.
>
> Hmm. I haven't seen those traces, but I wonder if the size of those
> stack traces indicates potential stack overflow problems.
Most functions have quite small frames, so 20-30 is still not a problem
> Do you also validate the unwind data?
There are many sanity checks in the unwind code and it will fall back
to the old unwinder when it gets stuck.
>
> > Although in future it would be good if people did some more analysis in root
> > causes for failures before let the paranoia take over and revert patches.
> >
> > We see a good example here of what I call the JFS/ACPI effect: code gets merged
> > too early with some visible problems. It gets a bad name and afterwards people
> > never look objectively at it again and just trust their prejudices.
>
> I don't know. The impression I got was the root cause analysis stopped
> when it was observed that the code was unsuitable for solving the problem.
No, me and Jan fixed all reported bugs as far as I know.
-Andi
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