[PATCH v2 5/9] cpu/SMT: Create topology_smt_thread_allowed()

[Laurent Dufour]

From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

Some architectures allows partial SMT states, ie. when not all SMT
threads are brought online.

To support that, add an architecture helper which checks whether a given
CPU is allowed to be brought online depending on how many SMT threads are
currently enabled. Since this is only applicable to architecture supporting
partial SMT, only these architectures should select the new configuration
variable CONFIG_SMT_NUM_THREADS_DYNAMIC. For the other architectures, not
supporting the partial SMT states, there is no need to define
topology_cpu_smt_allowed(), the generic code assumed that all the threads
are allowed or only the primary ones.

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@xxxxxxxxxxxxxx>
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
[ldufour: slightly reword the commit's description]
[ldufour: introduce CONFIG_SMT_NUM_THREADS_DYNAMIC]
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
---
 arch/Kconfig |  3 +++
 kernel/cpu.c | 24 +++++++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 205fd23e0cad..c69e9c662a87 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -34,6 +34,9 @@ config ARCH_HAS_SUBPAGE_FAULTS
 config HOTPLUG_SMT
 	bool
 
+config SMT_NUM_THREADS_DYNAMIC
+	bool
+
 config GENERIC_ENTRY
 	bool
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index e354af92b2b8..29bf310651c6 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -466,9 +466,23 @@ static int __init smt_cmdline_disable(char *str)
 }
 early_param("nosmt", smt_cmdline_disable);
 
+/*
+ * For Archicture supporting partial SMT states check if the thread is allowed.
+ * Otherwise this has already been checked through cpu_smt_max_threads when
+ * setting the SMT level.
+ */
+static inline bool cpu_smt_thread_allowed(unsigned int cpu)
+{
+#ifdef CONFIG_SMT_NUM_THREADS_DYNAMIC
+	return topology_smt_thread_allowed(cpu);
+#else
+	return true;
+#endif
+}
+
 static inline bool cpu_smt_allowed(unsigned int cpu)
 {
-	if (cpu_smt_control == CPU_SMT_ENABLED)
+	if (cpu_smt_control == CPU_SMT_ENABLED && cpu_smt_thread_allowed(cpu))
 		return true;
 
 	if (topology_is_primary_thread(cpu))
@@ -2283,6 +2297,12 @@ 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 && cpu_smt_thread_allowed(cpu))
+			continue;
 		ret = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
 		if (ret)
 			break;
@@ -2317,6 +2337,8 @@ 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 (!cpu_smt_thread_allowed(cpu))
+			continue;
 		ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
 		if (ret)
 			break;
-- 
2.41.0