[PATCH v6 1/4] cpu/SMT: Provide a default topology_is_primary_thread()

[Yicong Yang]

From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>

Currently if architectures want to support HOTPLUG_SMT they need to
provide a topology_is_primary_thread() telling the framework which
thread in the SMT cannot offline. However arm64 doesn't have a
restriction on which thread in the SMT cannot offline, a simplest
choice is that just make 1st thread as the "primary" thread. So
just make this as the default implementation in the framework and
let architectures like x86 that have special primary thread to
override this function (which they've already done).

There's no need to provide a stub function if !CONFIG_SMP or
!CONFIG_HOTPLUG_SMP. In such case the testing CPU is already
the 1st CPU in the SMT so it's always the primary thread.

Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
---
 arch/powerpc/include/asm/topology.h |  1 +
 arch/x86/include/asm/topology.h     |  2 +-
 include/linux/topology.h            | 14 ++++++++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index 16bacfe8c7a2..da15b5efe807 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/arch/powerpc/include/asm/topology.h
@@ -152,6 +152,7 @@ static inline bool topology_is_primary_thread(unsigned int cpu)
 {
 	return cpu == cpu_first_thread_sibling(cpu);
 }
+#define topology_is_primary_thread topology_is_primary_thread
 
 static inline bool topology_smt_thread_allowed(unsigned int cpu)
 {
diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
index aef70336d624..3a7c18851016 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -219,11 +219,11 @@ static inline bool topology_is_primary_thread(unsigned int cpu)
 {
 	return cpumask_test_cpu(cpu, cpu_primary_thread_mask);
 }
+#define topology_is_primary_thread topology_is_primary_thread
 
 #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; }
-static inline bool topology_is_primary_thread(unsigned int cpu) { return true; }
 static inline unsigned int topology_amd_nodes_per_pkg(void) { return 1; }
 #endif /* !CONFIG_SMP */
 
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 52f5850730b3..9c9f6b9f3462 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -240,6 +240,20 @@ static inline const struct cpumask *cpu_smt_mask(int cpu)
 }
 #endif
 
+#ifndef topology_is_primary_thread
+#define topology_is_primary_thread topology_is_primary_thread
+static inline bool topology_is_primary_thread(unsigned int cpu)
+{
+	/*
+	 * On SMT hotplug the primary thread of the SMT won't be disabled.
+	 * Architectures do have a special primary thread (e.g. x86) need
+	 * to override this function. Otherwise just make the first thread
+	 * in the SMT as the primary thread.
+	 */
+	return cpu == cpumask_first(topology_sibling_cpumask(cpu));
+}
+#endif
+
 static inline const struct cpumask *cpu_cpu_mask(int cpu)
 {
 	return cpumask_of_node(cpu_to_node(cpu));
-- 
2.24.0