Re: [patch 5/9] x86: Cure per CPU madness on UP

Guenter Roeck
2024-03-21T14:06:56Z

On 3/21/24 04:14, Thomas Gleixner wrote:
> On Wed, Mar 20 2024 at 08:46, Guenter Roeck wrote:
>> On 3/20/24 01:58, Thomas Gleixner wrote:
>>> On Fri, Mar 15 2024 at 09:17, Guenter Roeck wrote:
>>>> I don't know the code well enough to determine what is wrong.
>>>> Please let me know what I can do to help debugging the problem.
>>>
>>> Could you provide me the config and the qemu command line?
>>>
>>
>> defconfig-CONFIG_SMP and
>>
>> qemu-system-x86_64 -kernel arch/x86/boot/bzImage -cpu Haswell \
>> --append "console=ttyS0" -nographic -monitor none
>>
>> The cpu doesn't really matter as long as it is an Intel CPU.
>> A root file system isn't needed since the boot doesn't get that far.
>
> Now it get's interesting because I can't reproduce it with that setup at
> all.
>
> What's weird is that I saw it exactly once on 64-bit in a VM with a UP
> config two days ago, but when I started to add instrumentation it never
> came back even after backing the instrumentation changes out. I have
> seriously no idea what's going on there.
>
> Is it fully reproducible on your side?
>

Yes, always.

> If so can you please provide a full dmesg and then apply the patch below
> and provide the resulting full dmesg too?
>

You'll find everything at http://server.roeck-us.net/qemu/x86-nosmp/

The crash is gone after applying your patch. The difference is:

+ /*
+ * If there was no APIC registered, then the map check below would
+ * fail. With no APIC this is guaranteed to be an UP system and
+ * therefore all topology levels have only one entry and their
+ * logical ID is obviously 0.
+ */
+ if (topo_info.boot_cpu_apic_id == BAD_APICID) {
+ pr_info("#### topo_info.boot_cpu_apic_id == BAD_APICID\n");
^^^^ I added this
+ return 0;
+ }
+

I see the "#### topo_info.boot_cpu_apic_id == BAD_APICID" message
twice in the log. See patched.log at the page pointed to above.

Hope the helps,
Guenter