Re: [PATCH 1/2] x86/apic: shutdown local APIC before I/O APIC during crash

[Wei, Jiangang]

Hi, Ingo

Thanks for your comments firstly.

On Fri, 2016-07-01 at 12:36 +0200, Ingo Molnar wrote:
> * Wei Jiangang <weijg.fnst@xxxxxxxxxxxxxx> wrote:
> 
> > commit <522e66464467> disables I/O APIC before shutdown of
> > the local APIC for both reboot and crash path.
> > and commit <2885432aaf15> declares that 'it still makes sense to
> > quiet IO APIC before disabling Local APIC'.
> 
> That's not how we refer to commits in changelogs.
> 
OK, I will fix it and pay attention to it in the following patch.

> > However, the former introduced a bug for crashdown.
> 
> What is 'crashdown'? It's not referred to in the kernel source even once.

well, I mean ...
 
If we trigger kernel panic with the following commands, the capture
kernel should boot normally and captures the dump image. 

#echo 1 > /proc/sys/kernel/sysrq
#echo c > /proc/sysrq-trigger

But due to commit 522e66464467 changes the APIC shutdown sequence in
native_machine_crash_shutdown(), the capture kernel doesn't boot
normally and
hang in calibrate_delay_converge(), waiting for the jiffies to be
updated.

BTW, without commit 522e66464467, the capture kernel works well.

> 
> > If specify 'notsc' for capture-kernel, and then trigger crashdown.
> > The capture-kernel will be blocked at calibrate_delay_converge().
> 
> This is a more readable way of saying the same:
> 
>   If we specify the 'notsc' boot parameter for the dump-capture kernel,
>   and then trigger a crash-down, then the dump-capture kernel will hang
>   in calibrate_delay_converge():
> 
> (Assuming the changelog first explains what a 'crash-down' is.)
> 
> > /* wait for "start of" clock tick */
> > ticks = jiffies;
> > while (ticks == jiffies)
> >     ; /* nothing */
> 
> Plase align quoted code to the right with at least a single tab.
> 
OK
> > serial console log as following,
> 
>   serial log of the hang is as follows:
> 
> > ............
> > [    0.000000] Linux version 4.7.0-rc2+ (root@xxxxxxxxxxxxxxxxxxxxx)
> > (gcc version 4.8.2 20140120 (Red Hat 4.8.2-16) (GCC) ) #2 SMP Wed Jun
> > 156
> > [    0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-4.7.0-rc2+
> > root=/dev/mapper/centos-root ro rd.lvm.lv=centos/swap
> > vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos/root crashkernel=256M
> > vconsole.keymap=us console=tty0 console=ttyS0,115200n8 LANG=en_US.UTF-8
> > irqpoll nr_cpus=1 reset_devices cgroup_disable=memory mce=off numa=off
> > panic=10 rootflags=nofail acpi_no_memhotplug notsc
> > ............
> > [    0.000000] tsc: Kernel compiled with CONFIG_X86_TSC, cannot disable
> > TSC completely
> > ............
> > [    0.000000] clocksource: hpet: mask: 0xffffffff max_cycles:
> > 0xffffffff, max_idle_ns: 133484882848 ns
> > [    0.000000] tsc: Fast TSC calibration using PIT
> > [    0.000000] tsc: Detected 3192.714 MHz processor
> > [    0.000000] Calibrating delay loop...
> 
> Just quote the last few lines and skip the useless timestamp column. Also, please 
> right-align this too.
OK
> 
> > The bug remains and unsolved for a long time, since 2013.
> > I find the arch-criminal by bisect.
> 
> What is an arch-criminal? Did you want to say:
> 
>   The bug has been introduced in 2013. I found the buggy commit via bisection.
> 
> ?
Yes, That's what i want to say.

> 
> > The commit <522e66464467> used to fix erratum AVR31 for "Intel Atom
> > Processor C2000 Product Family Specification Update".
> > You can find the doc at http://www.intel.com/content/dam/www/public/us
> > /en/documents/specification-updates/atom-c2000-family-spec-update.pdf.
> >
> > IMO,
> > It doesn't make sense that change the order of disabling between
> > I/O APIC and local APIC just for a certain model C2000.
> > And I couldn't find any related descriptions for Intel 64 and IA-32 Arch.
> > 
> > so, I want to revert the crash part of commit <522e66464467>.
> 
> So why does the crashdump kernel hang in calibrate_delay_converge()?

The jiffies value doesn't increase, which causes the capture kernel hang
in calibrate_delay_converge().

It seems that there's a relationship with the shutdown(disable) order
between IO APIC and local APIC.   I'm not sure of this point ....

One thing for sure by debugging is that do_timer() is not called while
capture kernel boots up. I suspect the timer interrupts (irq0) is not
passed to cpu by APIC.

> 
> To me it appears this is a weakness in the crashdump kernel: it is unable to boot 
> if we crash the original host system in a particular hardware state, right?

Maybe you're right ...
I specify 'notsc' only for capture-kernel, not the original host
system(first kernel).

And I suspect the APIC shutdown sequence in first kernel maybe bring
some bad influence on capture kernel.
I need to do more investigation.
Do you have any advice?
Thanks in advance.

Wei
> By reverting this change we'll just paper over the bug and re-introduce the bug 
> that can result in certain CPUs hanging if the IO-APIC sends an APIC message if 
> the lapic is disabled prematurely.
> Thanks,
> 
> 	Ingo
> 
>