linux-kernel - Re: [PATCH] improve_stack: make stack dump output useful again

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Date:	Sun, 16 Mar 2014 11:55:03 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] improve_stack: make stack dump output useful again

On Sun, Mar 16, 2014 at 11:33 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> The only useful thing above is the function name. Due to the amount of
> different kernel code versions and various configurations being used, the
> kernel address and the offset into the function are not really helpful in
> determining where the problem actually occured.

Actually, the offset into the function is very useful both for a local
kernel (when it tells you exactly where it is) and for external
bug-reports (where it's more of a hint about where things are).

So I think the "not really helpful" description about the offset in
particular is misleading and actively incorrect.

The size part is not generally so useful, although for external
bug-reports it's an important part of making sense of the offset
(because, as you say, config options can have such huge impact on code
generation).

HOWEVER. I agree that *if* you have debug info, and can look up file
and line number, then both offset and size end up being less than
interesting. So I wonder if your script should remove offset/size iff
the debug info can be found. IOW, for your example:

  [  324.019502]  dump_stack+0x52/0x7f (lib/dump_stack.c:52)
  [  324.020206]  warn_slowpath_common+0x8c/0xc0 (kernel/panic.c:418)
  [  324.020289]  ? noop_count+0x10/0x10 (kernel/locking/lockdep.c:1315)
  [  324.020289]  warn_slowpath_null+0x1a/0x20 (kernel/panic.c:453)
  [  324.020289]  __bfs+0x113/0x240 (kernel/locking/lockdep.c:962
kernel/locking/lockdep.c:1027)
  [  324.020289]  find_usage_backwards+0x80/0x90 (kernel/locking/lockdep.c:1365)
  [  324.020289]  check_usage_backwards+0xb7/0x100
(kernel/locking/lockdep.c:2379)

maybe you could simplify this to just

  [  324.019502]  dump_stack (lib/dump_stack.c:52)
  [  324.020206]  warn_slowpath_common (kernel/panic.c:418)
  [  324.020289]  ? noop_count (kernel/locking/lockdep.c:1315)
  [  324.020289]  warn_slowpath_null (kernel/panic.c:453)
  [  324.020289]  __bfs (kernel/locking/lockdep.c:962
kernel/locking/lockdep.c:1027)
  [  324.020289]  find_usage_backwards (kernel/locking/lockdep.c:1365)
  [  324.020289]  check_usage_backwards (kernel/locking/lockdep.c:2379)

but only do that when addr2line actually works. Right now you don't
seem to handle the "addr2line fails" case (it seems to return 0
regardless, and just output "??:?" when it can't find line number
information).

Other than that nit, the concept certainly looks fine to me.

              Linus
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