linux-kernel - Re: Random panic in load

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Message-ID: <20140728172737.GA17598@pg-vmw-gw1>
Date:	Mon, 28 Jul 2014 10:27:39 -0700
From:	Alexei Starovoitov <ast@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Michel Dänzer <michel@...nzer.net>,
	Jakub Jelinek <jakub@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Debian GCC Maintainers <debian-gcc@...ts.debian.org>,
	Debian Kernel Team <debian-kernel@...ts.debian.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Mon, Jul 28, 2014 at 09:45:45AM -0700, Linus Torvalds wrote:
> On Mon, Jul 28, 2014 at 5:26 AM, Frank Ch. Eigler <fche@...hat.com> wrote:
> >
> > Please note that the data produced by "-g -fvar-tracking" is consumed
> > by tools like systemtap, perf, crash, and makes a significant
> > difference to the observability of debug AND non-debug kernels.
> 
> Yeah, and compared to having a buggy kernel, I care exactly this much: "".
> 
> Besides, "significant difference" is very debatable indeed. It may be
> noticeable, it's likely not actually significant.

as far as I can see in gcc code, -fno-var-tracking-assignments is the main
switch for 'vartrack' pass. So -fno-var-tracking-assignments is pretty
much equivalent to -fno-var-tracking
Both kill majority of debug info for local variables and function arguments.
The end effect for perf/systemtap will be quite noticeable.

It's not pretty, but adding it unconditionally was the right thing to do.
Black listing compiler versions is too fragile.
Look at the flip side: now size of build dir will be much smaller :)

For gcc it's obviously a big blow. Now 10k lines pass developed
over 10+ years will not be used :(

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