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

[Bradley Morgan]

On 29 September 2026 20:41:29 BST, salil.mehta@xxxxxxxxxx wrote:
>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.

Ok.

>
>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)))

Ehh, have you had a issue with this code? Like something e.g: a splat.


> 			continue;
> 		if (!cpu_smt_thread_allowed(cpu) || !topology_is_core_online(cpu))
> 			continue;
>

--- Thanks!
"I'm not a very positive person" - Linus torvalds