linux-kernel - Re: x86/kconfig: Consolidate unwinders into multiple choice selection

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Date:   Wed, 6 Sep 2017 16:49:45 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Dave Jones <davej@...emonkey.org.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86/kconfig: Consolidate unwinders into multiple choice selection

On Tue, Sep 05, 2017 at 01:09:53PM -0400, Dave Jones wrote:
> On Mon, Sep 04, 2017 at 08:05:13PM +0000, Linux Kernel wrote:
>  > Web:        https://git.kernel.org/torvalds/c/81d387190039c14edac8de2b3ec789beb899afd9
>  > Commit:     81d387190039c14edac8de2b3ec789beb899afd9
>  > Parent:     a34a766ff96d9e88572e35a45066279e40a85d84
>  > Refname:    refs/heads/master
>  > Author:     Josh Poimboeuf <jpoimboe@...hat.com>
>  > AuthorDate: Tue Jul 25 08:54:24 2017 -0500
>  > Committer:  Ingo Molnar <mingo@...nel.org>
>  > CommitDate: Wed Jul 26 14:05:36 2017 +0200
>  > 
>  >     x86/kconfig: Consolidate unwinders into multiple choice selection
>  >     
>  >     There are three mutually exclusive unwinders.  Make that more obvious by
>  >     combining them into a multiple-choice selection:
>  >     
>  >       CONFIG_FRAME_POINTER_UNWINDER
>  >       CONFIG_ORC_UNWINDER
>  >       CONFIG_GUESS_UNWINDER (if CONFIG_EXPERT=y)
> 
> The help texts for the various unwinders are now attached to the wrong
> kconfig item.
> 
>  > +choice
>  > +	prompt "Choose kernel unwinder"
>  > +	default FRAME_POINTER_UNWINDER
>  > +	---help---
>  > +	  This determines which method will be used for unwinding kernel stack
>  > +	  traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
>  > +	  livepatch, lockdep, and more.
> 
> This is what gets displayed, but tells me nothing about what the
> benefits/downsides are of each (or even what they are; I had to read the
> Kconfig file to figure out what 'GUESS' meant)
> 
> 
> an oldconfig run ..
> 
> 
> Choose kernel unwinder
> > 1. Frame pointer unwinder (FRAME_POINTER_UNWINDER) (NEW)
>   2. ORC unwinder (ORC_UNWINDER) (NEW)
>   3. Guess unwinder (GUESS_UNWINDER) (NEW)
> choice[1-3?]: ?
> 
> This determines which method will be used for unwinding kernel stack
> traces for panics, oopses, bugs, warnings, perf, /proc/<pid>/stack,
> livepatch, lockdep, and more.
> 
> Prompt: Choose kernel unwinder
>   Location:
>     -> Kernel hacking
>   Defined at arch/x86/Kconfig.debug:359
>   Selected by: m
> 
> 
> 
> Choose kernel unwinder
> > 1. Frame pointer unwinder (FRAME_POINTER_UNWINDER) (NEW)
>   2. ORC unwinder (ORC_UNWINDER) (NEW)
>   3. Guess unwinder (GUESS_UNWINDER) (NEW)
> choice[1-3?]: 

This is a quirk of the config tool.  It's not very intuitive, but to see
the help for a given option you have to type the number appended with a
'?', like:

  > 1. Frame pointer unwinder (FRAME_POINTER_UNWINDER) (NEW)
    2. ORC unwinder (ORC_UNWINDER) (NEW)
  choice[1-2?]: 1?
  
  CONFIG_FRAME_POINTER_UNWINDER:
  
  This option enables the frame pointer unwinder for unwinding kernel
  stack traces.
  
  The unwinder itself is fast and it uses less RAM than the ORC
  unwinder, but the kernel text size will grow by ~3% and the kernel's
  overall performance will degrade by roughly 5-10%.
  
  This option is recommended if you want to use the livepatch
  consistency model, as this is currently the only way to get a
  reliable stack trace (CONFIG_HAVE_RELIABLE_STACKTRACE).
  
  Symbol: FRAME_POINTER_UNWINDER [=y]
  Type  : boolean
  Prompt: Frame pointer unwinder
    Location:
      -> Kernel hacking
        -> Choose kernel unwinder (<choice> [=y])
    Defined at arch/x86/Kconfig.debug:367
    Depends on: <choice>
    Selects: FRAME_POINTER [=y]

-- 
Josh