[RFC PATCH 1/2] cpu/hotplug: Skip disabled CPUs in cpuhp_smt_enable

[salil . mehta]

From: Salil Mehta <salil.mehta@xxxxxxxxxx>

The CPU enabled mask describes whether a present CPU may currently be
brought online. cpuhp_smt_enable() is an online operation, but currently
walks all present CPUs and only filters CPUs that are already online or
belong to offline NUMA nodes.

This can make it attempt _cpu_up() for a present CPU which firmware has
not enabled and which has not yet been registered as a CPU device.

Use the enabled mask for the policy decision it was introduced to
represent. This keeps present-but-disabled CPUs out of the SMT bring-up
path while still allowing registered offline SMT threads to be brought
back online.

Present and enabled are separate generic CPU states, so callers which
intend to bring CPUs online should not assume that every present CPU is
enabled.

Signed-off-by: Salil Mehta <salil.mehta@xxxxxxxxxx>
---
 kernel/cpu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index b3c8553d7bd6..988b7a1e8298 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2706,8 +2706,9 @@ int cpuhp_smt_enable(void)
 	cpu_maps_update_begin();
 	cpu_smt_control = CPU_SMT_ENABLED;
 	for_each_present_cpu(cpu) {
-		/* Skip online CPUs and CPUs on offline nodes */
-		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
+		/* Skip online/disabled CPUs and CPUs on offline nodes */
+		if (cpu_online(cpu) || !cpu_enabled(cpu) ||
+		    !node_online(cpu_to_node(cpu)))
 			continue;
 		if (!cpu_smt_thread_allowed(cpu) || !topology_is_core_online(cpu))
 			continue;
-- 
2.34.1