[PATCH 5/9] cpu/SMT: Create topology_smt_thread_allowed()

[Michael Ellerman]

A subsequent patch will enable partial SMT states, ie. when not all SMT
threads are brought online.

To support that, add an arch helper which checks whether a given CPU is
allowed to be brought online depending on how many SMT threads are
currently enabled.

Call the helper from cpu_smt_enable(), and cpu_smt_allowed() when SMT is
enabled, to check if the particular thread should be onlined. Notably,
also call it from cpu_smt_disable() if CPU_SMT_ENABLED, to allow
offlining some threads to move from a higher to lower number of threads
online.

Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
---
 arch/x86/include/asm/topology.h |  2 ++
 arch/x86/kernel/smpboot.c       | 15 +++++++++++++++
 kernel/cpu.c                    | 10 +++++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index 197ec2589f5d..c6245ea6e589 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -145,6 +145,7 @@ int topology_phys_to_logical_die(unsigned int die, unsigned int cpu);
 bool topology_is_primary_thread(unsigned int cpu);
 bool topology_smt_supported(void);
 bool topology_smt_threads_supported(unsigned int threads);
+bool topology_smt_thread_allowed(unsigned int cpu);
 #else
 #define topology_max_packages()			(1)
 static inline int
@@ -159,6 +160,7 @@ static inline int topology_max_smt_threads(void) { return 1; }
 static inline bool topology_is_primary_thread(unsigned int cpu) { return true; }
 static inline bool topology_smt_supported(void) { return false; }
 static inline bool topology_smt_threads_supported(unsigned int threads) { return false; }
+static inline bool topology_smt_thread_allowed(unsigned int cpu) { return false; }
 #endif
 
 static inline void arch_fix_phys_package_id(int num, u32 slot)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index c7ba62beae3e..244b4df40600 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -298,6 +298,21 @@ bool topology_smt_threads_supported(unsigned int threads)
 	return threads == 1 || threads == smp_num_siblings;
 }
 
+/**
+ * topology_smt_thread_allowed - When enabling SMT check whether this particular
+ *				 CPU thread is allowed to be brought online.
+ * @cpu:	CPU to check
+ */
+bool topology_smt_thread_allowed(unsigned int cpu)
+{
+	/*
+	 * No extra logic s required here to support different thread values
+	 * because threads will always == 1 or smp_num_siblings because of
+	 * topology_smt_threads_supported().
+	 */
+	return true;
+}
+
 /**
  * topology_phys_to_logical_pkg - Map a physical package id to a logical
  *
diff --git a/kernel/cpu.c b/kernel/cpu.c
index a08dd8f93675..72b9a03a4fef 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -458,7 +458,7 @@ early_param("nosmt", smt_cmdline_disable);
 
 static inline bool cpu_smt_allowed(unsigned int cpu)
 {
-	if (cpu_smt_control == CPU_SMT_ENABLED)
+	if (cpu_smt_control == CPU_SMT_ENABLED && topology_smt_thread_allowed(cpu))
 		return true;
 
 	if (topology_is_primary_thread(cpu))
@@ -2273,6 +2273,12 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
 	for_each_online_cpu(cpu) {
 		if (topology_is_primary_thread(cpu))
 			continue;
+		/*
+		 * Disable can be called with CPU_SMT_ENABLED when changing
+		 * from a higher to lower number of SMT threads per core.
+		 */
+		if (ctrlval == CPU_SMT_ENABLED && topology_smt_thread_allowed(cpu))
+			continue;
 		ret = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
 		if (ret)
 			break;
@@ -2307,6 +2313,8 @@ int cpuhp_smt_enable(void)
 		/* Skip online CPUs and CPUs on offline nodes */
 		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
 			continue;
+		if (!topology_smt_thread_allowed(cpu))
+			continue;
 		ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
 		if (ret)
 			break;
-- 
2.40.1