[PATCH 1/2] cpu/SMT: Enable SMT only if a core is online

["Nysal Jan K.A." <nysal@linux.ibm.com>]

From: "Nysal Jan K.A" <nysal@linux.ibm.com>

If a core is offline then enabling SMT should not online CPUs of
this core. By enabling SMT, what is intended is either changing the SMT
value from "off" to "on" or setting the SMT level (threads per core) from a
lower to higher value.

On PowerPC the ppc64_cpu utility can be used, among other things, to
perform the following functions:

ppc64_cpu --cores-on                # Get the number of online cores
ppc64_cpu --cores-on=X              # Put exactly X cores online
ppc64_cpu --offline-cores=X[,Y,...] # Put specified cores offline
ppc64_cpu --smt={on|off|value}      # Enable, disable or change SMT level

If the user has decided to offline certain cores, enabling SMT should
not online CPUs in those cores. This patch fixes the issue and changes
the behaviour as described, by introducing an arch specific function
topology_is_core_online(). It is currently implemented only for PowerPC.

Fixes: 73c58e7e1412 ("powerpc: Add HOTPLUG_SMT support")
Reported-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Closes: https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ
Signed-off-by: Nysal Jan K.A <nysal@linux.ibm.com>
---
 kernel/cpu.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 563877d6c28b..73c4a6f98c4a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2676,6 +2676,16 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
 	return ret;
 }
 
+/**
+ * Check if the core a CPU belongs to is online
+ */
+#if !defined(topology_is_core_online)
+static inline bool topology_is_core_online(unsigned int cpu)
+{
+	return true;
+}
+#endif
+
 int cpuhp_smt_enable(void)
 {
 	int cpu, ret = 0;
@@ -2686,7 +2696,7 @@ 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))
+		if (!cpu_smt_thread_allowed(cpu) || !topology_is_core_online(cpu))
 			continue;
 		ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
 		if (ret)
-- 
2.35.3