Re: [PATCH v1] cpu: Add missing check to cpuhp_smt_enable()

[Dave Hansen <dave.hansen@intel.com>]

On 9/2/25 08:05, Rafael J. Wysocki wrote:
> On Fri, Aug 29, 2025 at 10:01 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>> Christian has reported that commit a430c11f4015 ("intel_idle: Rescan
>> "dead" SMT siblings during initialization") broke the use case in

Does "dead" here mean present but offline?

>> which both nosmt and maxcpus were added to the kernel command line
>> because it caused CPUs that were not SMT siblings to be brought
>> online during the intel_idle driver initialization in violation of the
>> maxcpus limit.

How does intel_idle fit in here? I don't immediately see it calling
cpuhp_smt_enable().

>> The underlying reason for this is a missing topology_is_primary_thread()
>> check in cpuhp_smt_enable() which causes that function to put online
>> more CPUs than just the SMT siblings that it is supposed to handle.
>>
>> Add the missing check to address the issue.

We should probably add a bit more checking in cpuhp_smt_enable() to make
sure that it's being called under expected conditions like a:

	WARN_ON_ONCE(cpu_smt_control != CPU_SMT_DISABLED);

and probably also some comments about how it is expected to work.

cpuhp_smt_enable() doesn't _really_ enable SMT. It specifically takes it
from DISABLED=>ENABLED. Thinking about it in that context, enabling
_just_ the secondary (disabled) threads makes a lot of sense.

But that can come later, after the bug fix.

>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -2710,6 +2710,12 @@

No 'diff -p', eh?

>>         cpu_maps_update_begin();
>>         cpu_smt_control = CPU_SMT_ENABLED;
>>         for_each_present_cpu(cpu) {
>> +               /*
>> +                * Avoid accidentally onlining primary thread CPUs that have
>> +                * been taken offline.
>> +                */
>> +               if (topology_is_primary_thread(cpu))
>> +                       continue;
>>                 /* Skip online CPUs and CPUs on offline nodes */
>>                 if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
>>                         continue;
Is there a more generic problem with this not respecting 'maxcpus'? If
maxcpus had forced a primary thread offline, this would still online the
secondary thread, even with the fix. Taking _that_ online might still
bring you over the maxcpus limit.