linux-kernel - Re: CONFIG_FRAME_POINTER [was [PATCH] x86: BUILD

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Date:	Mon, 28 Jul 2008 16:41:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Mike Travis <travis@....com>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org,
	the arch/x86 maintainers <x86@...nel.org>,
	Linuxppc-dev@...abs.org, Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: CONFIG_FRAME_POINTER [was [PATCH] x86: BUILD_IRQ say .text]


* Hugh Dickins <hugh@...itas.com> wrote:

> [PATCH] sched: move sched_clock before first use
> 
> Move sched_clock() up to stop warning: weak declaration of 
> `sched_clock' after first use results in unspecified behavior (if 
> -fno-unit-at-a-time).
> 
> Signed-off-by: Hugh Dickins <hugh@...itas.com>

applied to tip/sched/urgent - thanks Hugh.

> I rather think CONFIG_FRAME_POINTER shouldn't exist at all (or be a 
> private, config-user-invisible, specific-to-a-few-arches thing): what 
> one wants to configure is how far to sacrifice cpu performance and 
> kernel smallness to getting a good stacktrace.  Frame pointer is just 
> an implementation detail on that, appropriate to some arches. Perhaps 
> three settings: no stacktrace, fair stacktrace, best stacktrace.

actually, we consciously use and rely on frame pointers on x86. The 
runtime cost on 64-bit is miniscule and the improved backtrace output in 
recent kernels makes backtraces _much_ easier to interpret:

 Call Trace:
  <NMI>  [<ffffffff80480779>] _raw_spin_trylock+0x19/0x50
  [<ffffffff808fb2e9>] _spin_lock_irqsave+0x59/0x90
  [<ffffffff80261ab4>] atomic_notifier_chain_register+0x24/0x60
  [<ffffffff80262f38>] ? __profile_tick+0x58/0x90
  [<ffffffff808fd1a9>] nmi_watchdog_tick+0x59/0x1e0
  [<ffffffff808fc79a>] default_do_nmi+0x6a/0x220
  [<ffffffff808fc9b4>] do_nmi+0x64/0xb0
  [<ffffffff808fc032>] nmi+0xa2/0xc2
  [<ffffffff80285bd1>] ? stopmachine+0x61/0xd0
  <<EOE>>  [<ffffffff8020dca9>] child_rip+0xa/0x11
  [<ffffffff8020cf3e>] ? restore_args+0x0/0x30
  [<ffffffff80285b70>] ? stopmachine+0x0/0xd0
  [<ffffffff8020dc9f>] ? child_rip+0x0/0x11

we experimented with using dwarf2 data in the past but it proved to be 
very fragile in practice - we depended too much on the whims of 
gcc/binutils being absolutely correct, etc.

Something as fundamental to the kernel's general health as backtraces 
must not be fragile. So the EBP based backtracing code was ported to 
64-bit as well and it was improved further upon.

kudos to Arjan for that.

	Ingo
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