Re: [PATCH v4 0/3] x86, apic, kexec: Add disable_cpu_apic kernelparameter

From: HATAYAMA Daisuke
Date: Tue Oct 29 2013 - 20:47:29 EST

(2013/10/29 23:21), Baoquan He wrote:

I am reviewing this patchset, and found there's a cpu0 hotplug feature
posted by intel which we can borrow an idea from. In that implementation,
CPU0 is waken up by nmi not INIT to avoid the realmode bootstrap code
execution. I tried it by below patch which includes one line of change.

By console printing, I got the boot cpu is always 0(namely cpu=0),
however the apicid related to each processor keeps the same as in 1st
kernel. In my HP Z420 machine, the apicid for BSP is 0, so I just make a
test patch which depends on the fact that apicid for BSP is 0. Maybe
generally the apicid for BSP can't be guaranteed, then passing it from
1st kernel to 2nd kernel in cmdline is very helpful, just as you have
done for disable_cpu_apic.

On my HP z420, I add nr_cpus=4 in /etc/sysconfig/kdump, and then execute
below command, then 3 APs (1 boot cpu and 2 AP) can be waken up
correctly, but BSP failed because NMI received for unknown reason 21 on
CPU0. I think I need further check why BSP failed to wake up by nmi. But
3 processors are brought up successfully and kdump is successful too.

sudo taskset -c 1 sh -c "echo c >/proc/sysrq-trigger"

[ 0.296831] smpboot: Booting Node 0, Processors # 1
[ 0.302095]
*****************************************************cpu=1, apicid=0, wakeup_cpu_via_init_nmi
[ 0.311942] cpu=1, apicid=0, register_nmi_handlercpu=1, apicid=0, wakeup_secondary_cpu_via_nmi
[ 0.320826] Uhhuh. NMI received for unknown reason 21 on CPU 0.
[ 0.327129] Do you have a strange power saving mode enabled?
[ 0.333858] Dazed and confused, but trying to continue
[ 0.339290] cpu=1, apicid=0, wakeup_cpu_via_init_nmi
[ 2.409099] Uhhuh. NMI received for unknown reason 21 on CPU 0.
[ 2.415393] Do you have a strange power saving mode enabled?
[ 2.421142] Dazed and confused, but trying to continue
[ 5.379519] smpboot: CPU1: Not responding
[ 5.383692] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.

We've already discussed this approach and concluded this is not applicable
to our issue.

The reason is:

- The cpu0-hotplugging approach assumes BSP to be halting before initiating
NMI to it while in our case, BSP is halting in the 1st kernel or is
running in arbitrary position of the 1st kernel in catastrophic state.

- In general, NMI modifies stack, which means if throwing NMI to the BSP
in the 1st kernel, stack on the 1st kernel is modified. It's unpermissible
from kdump's perspective.

- On x86_64, there are two cases where stack is changed to another one
when receiving interrupts. One is when receiving interrupt in user mode.
The other is when using Interrupt Stack Table (IST), which is already
used in the current x86_64 implementation.

By using either, it would be possible to wake up BSP in the 1st kernel
by modifying the contexts on the 2nd kernel's NMI stack pushed on when NMI
to the 1st kernel is initiated.

However, this approach depends on the logic in the 1st kernel, there's
no guarantee that it works well. Consider severely buggy situation again.

- To do this approach rigorously, we need to check if states of BSP and APs
are kept in just what we assume in the place where logic is guaranteed to be
sane, i.e., at least after purgatory. However, adding new logic in the
purgatory means we are forced to introduce additional dependency between
kernel and kexec. The process performed in purgatory itself is not so
simple.I don't like this complication.

To sum up, I think the current idea is simple enough approach.


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