[PATCH v10 1/4] cpu/SMT: Provide a default topology_is_primary_thread()

[Yicong Yang]

From: Yicong Yang <yangyicong at hisilicon.com>

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 at hisilicon.com>
---
As questioned in v9 [1] whether this works on architectures not using
CONFIG_GENERIC_ARCH_TOPOLOGY, hacked on LoongArch VM and this also works.
Architectures should use this on their own situation.
[1] https://lore.kernel.org/linux-arm-kernel/427bd639-33c3-47e4-9e83-68c428eb1a7d@arm.com/

[root at localhost smt]# uname -m
loongarch64
[root at localhost smt]# pwd
/sys/devices/system/cpu/smt
[root at localhost smt]# cat ../possible 
0-3
[root at localhost smt]# cat ../online 
0-3
[root at localhost smt]# cat control 
on
[root at localhost smt]# echo off > control 
[root at localhost smt]# cat control 
off
[root at localhost smt]# cat ../online 
0,2
[root at localhost smt]# echo on > control 
[root at localhost smt]# cat control 
on
[root at localhost smt]# cat ../online 
0-3

 arch/powerpc/include/asm/topology.h |  1 +
 arch/x86/include/asm/topology.h     |  2 +-
 include/linux/topology.h            | 22 ++++++++++++++++++++++
 3 files changed, 24 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 fd41103ad342..faa0d6334ea4 100644
--- a/arch/x86/include/asm/topology.h
+++ b/arch/x86/include/asm/topology.h
@@ -228,11 +228,11 @@ 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..b3aba443c4eb 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -240,6 +240,28 @@ 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.
+	 *
+	 * The sibling cpumask of an offline CPU contains always the CPU
+	 * itself for architectures using CONFIG_GENERIC_ARCH_TOPOLOGY.
+	 * Other architectures should use this depend on their own
+	 * situation.
+	 */
+	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