cpu/SMT: Provide a default topology_is_primary_thread()

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_SMT. In such case the testing CPU is already
the 1st CPU in the SMT so it's always the primary thread.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20250311075143.61078-2-yangyicong@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index 16bacfe8c7a2ca1f97d403d2889e7135effbd978..da15b5efe8071a70676349c14d1f2ff68381e6f2 100644 (file)
--- 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 ec134b7191446d4bf349a31512c25e613295606c..6c79ee7c0957a751390c19102ca1300e357843a4 100644 (file)
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -229,11 +229,11 @@ 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 52f5850730b3e52c0e8f3bd95552adb69baafddc..f8bb02c5e8a477f83dcdf1916832dbe3b7c86780 100644 (file)
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -240,6 +240,29 @@ static inline const struct cpumask *cpu_smt_mask(int cpu)
 }
 #endif
 
+#ifndef topology_is_primary_thread
+
+static inline bool topology_is_primary_thread(unsigned int cpu)
+{
+       /*
+        * When disabling SMT, the primary thread of the SMT will remain
+        * enabled/active. Architectures that have a special primary thread
+        * (e.g. x86) need to override this function. Otherwise the first
+        * thread in the SMT can be made the primary thread.
+        *
+        * The sibling cpumask of an offline CPU always contains the CPU
+        * itself on architectures using the implementation of
+        * CONFIG_GENERIC_ARCH_TOPOLOGY for building their topology.
+        * Other architectures not using CONFIG_GENERIC_ARCH_TOPOLOGY for
+        * building their topology have to check whether to use this default
+        * implementation or to override it.
+        */
+       return cpu == cpumask_first(topology_sibling_cpumask(cpu));
+}
+#define topology_is_primary_thread topology_is_primary_thread
+
+#endif
+
 static inline const struct cpumask *cpu_cpu_mask(int cpu)
 {
        return cpumask_of_node(cpu_to_node(cpu));