Re: unwind_init() takes 100 ms

From: Josh Poimboeuf
Date: Mon Oct 08 2018 - 13:34:23 EST

On Mon, Oct 08, 2018 at 11:52:46AM +0200, Paul Menzel wrote:
> Dear Josh, dear Linux folks,
> Trying to decrease the boot time of the 64-bit Linux kernel (Linux 4.19-rc7
> (0238df64)) on a Asus F2A85-M PRO with an AMD processor, I noticed
> `unwind_init()` called from `setup_arch()` `arch/x86/kernel/setup.c` takes
> over 100 ms to initialize according to Linux time stamps (which are present
> for early boot now).
> ```
> [â]
> [ 0.047650] ACPI: HPET id: 0x10228210 base: 0xfed00000
> [ 0.047653] smpboot: Allowing 4 CPUs, 2 hotplug CPUs
> [ 0.047670] [mem 0x8f000000-0xfebfffff] available for PCI devices
> [ 0.047672] clocksource: refined-jiffies: mask: 0xffffffff max_cycles:
> 0xffffffff, max_idle_ns: 1910969940391419 ns
> [ 0.151426] setup_percpu: NR_CPUS:64 nr_cpumask_bits:64 nr_cpu_ids:4
> nr_node_ids:1
> [ 0.151892] percpu: Embedded 44 pages/cpu @(____ptrval____) s142552 r8192
> d29480 u524288
> [â]
> ```
> Thatâs a third of the time to get to unpacking the initramfs.
> Therefore, I have several questions.
> 1. Can you reproduce that on your systems?

I haven't tried to reproduce, but it wouldn't surprise me because it
sorts the big ORC table during boot.

> 2. Can this be decreased or moved out of the âhot pathâ?

It would be better to sort it at build time, but I haven't had a chance
to implement that yet. I'll try to get to it one of these days...

> 3. If not, how can this be disabled? I was unable to do so in the
> configuration menu. I could always select an unwinder under *Kernel
> hacking*, and was not able to disable it.

If you select CONFIG_UNWINDER_FRAME_POINTER, the boot time penalty
should go away.

Or do CONFIG_EXPERT and CONFIG_UNWINDER_GUESS if you don't want the
general slowdown caused by frame pointers.

> 4. Would a command line parameter be reasonable `disable_unwind`, so people
> could decrease their boot time with distribution kernels, and easily turn it
> back on, when they need a stacktrace without having to rebuild the Linux
> kernel?

I think a boot cmdline option to disable ORC would be ok. However,
disabling ORC would need to fall back to the "guess" unwinder.
Otherwise it would make debugging impossible. That shouldn't be too
hard, but it would require restructuring the code a bit.

I'm not sure about a runtime toggle though. It might be racy and I'd
rather avoid that completely, unless there's a solid justification for