Messages in this thread | | | Date | Mon, 28 Jul 2008 16:41:33 +0200 | From | Ingo Molnar <> | Subject | Re: CONFIG_FRAME_POINTER [was [PATCH] x86: BUILD_IRQ say .text] |
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* Hugh Dickins <hugh@veritas.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@veritas.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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