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

Thomas Gleixner
2024-03-21T11:14:32Z

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?

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

I found two other issues while trying to find a way to reproduce, but
those are completely unrelated to the problem you are observing.

Thanks,

tglx
---
arch/x86/kernel/cpu/topology.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -176,6 +176,8 @@ static __init void topo_register_apic(u3
{
int cpu, dom;

+ pr_info("APIC: %x %d\n", apic_id, present);
+
if (present) {
set_bit(apic_id, phys_cpu_present_map);

@@ -277,10 +279,23 @@ int topology_get_logical_id(u32 apicid,
/* Remove the bits below @at_level to get the proper level ID of @apicid */
unsigned int lvlid = topo_apicid(apicid, at_level);

- if (lvlid >= MAX_LOCAL_APIC)
+ pr_info("APIC logical ID: %x %x %d\n", apicid, lvlid, at_level);
+
+ if (WARN_ON_ONCE(lvlid >= MAX_LOCAL_APIC))
return -ERANGE;
- if (!test_bit(lvlid, apic_maps[at_level].map))
+
+ /*
+ * 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)
+ return 0;
+
+ if (WARN_ON_ONCE(!test_bit(lvlid, apic_maps[at_level].map)))
return -ENODEV;
+
/* Get the number of set bits before @lvlid. */
return bitmap_weight(apic_maps[at_level].map, lvlid);
}