x86/topology: Implement topology_is_core_online() to address SMT regression

JIRA: https://issues.redhat.com/browse/RHEL-114092

commit 2066f00
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Sun Sep 21 10:56:40 2025 +0200

Christian reported that commit a430c11 ("intel_idle: Rescan "dead" SMT
siblings during initialization") broke the use case in which both 'nosmt'
and 'maxcpus' are on the kernel command line because it onlines primary
threads, which were offline due to the maxcpus limit.

The initially proposed fix to skip primary threads in the loop is
inconsistent. While it prevents the primary thread to be onlined, it then
onlines the corresponding hyperthread(s), which does not really make sense.

The CPU iterator in cpuhp_smt_enable() contains a check which excludes all
threads of a core, when the primary thread is offline. The default
implementation is a NOOP and therefore not effective on x86.

Implement topology_is_core_online() on x86 to address this issue. This
makes the behaviour consistent between x86 and PowerPC.

Fixes: a430c11 ("intel_idle: Rescan "dead" SMT siblings during initialization")
Fixes: f694481 ("ACPI: processor: Rescan "dead" SMT siblings during initialization")
Closes: https://lore.kernel.org/linux-pm/724616a2-6374-4ba3-8ce3-ea9c45e2ae3b@arm.com/
Reported-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/12740505.O9o76ZdvQC@rafael.j.wysocki
Signed-off-by: Mark Langsdorf <mlangsdo@redhat.com>

Author: mlangsdorf

arch/x86/include/asm/topology.h

@@ -230,6 +230,16 @@ static inline bool topology_is_primary_thread(unsigned int cpu)
 	return cpumask_test_cpu(cpu, cpu_primary_thread_mask);
 }
 
+int topology_get_primary_thread(unsigned int cpu);
+
+static inline bool topology_is_core_online(unsigned int cpu)
+{
+	int pcpu = topology_get_primary_thread(cpu);
+
+	return pcpu >= 0 ? cpu_online(pcpu) : false;
+}
+#define topology_is_core_online topology_is_core_online
+
 #else /* CONFIG_SMP */
 static inline int topology_phys_to_logical_pkg(unsigned int pkg) { return 0; }
 static inline int topology_max_smt_threads(void) { return 1; }

arch/x86/kernel/cpu/topology.c

@@ -371,6 +371,19 @@ unsigned int topology_unit_count(u32 apicid, enum x86_topology_domains which_uni
 	return topo_unit_count(lvlid, at_level, apic_maps[which_units].map);
 }
 
+#ifdef CONFIG_SMP
+int topology_get_primary_thread(unsigned int cpu)
+{
+	u32 apic_id = cpuid_to_apicid[cpu];
+
+	/*
+	 * Get the core domain level APIC id, which is the primary thread
+	 * and return the CPU number assigned to it.
+	 */
+	return topo_lookup_cpuid(topo_apicid(apic_id, TOPO_CORE_DOMAIN));
+}
+#endif
+
 #ifdef CONFIG_ACPI_HOTPLUG_CPU
 /**
  * topology_hotplug_apic - Handle a physical hotplugged APIC after boot