Re: [PATCH 1/2] smpboot: Allow restricting per-CPU threads to a CPU mask

[Daniel Lezcano <daniel.lezcano@oss.qualcomm.com>]

Hi Thomas,

thanks for reviewing the patch

On 11/12/26 18:40, Thomas Gleixner wrote:
> On Tue, Nov 03 2026 at 00:09, Daniel Lezcano wrote:
>> smpboot_register_percpu_thread() creates a thread for every online CPU
>> and registers the descriptor for subsequent CPU hotplug operations.
>> There is currently no way to restrict these threads to a subset of
>> CPUs, even when the user only needs work performed on that subset.
>>
>> For example, an idle injection device targets a specific CPU mask.
>> Creating threads outside that mask consumes resources unnecessarily.
>> Supporting multiple such devices also requires each descriptor to
>> manage only the threads associated with its own CPUs.
>>
>> Add an optional CPU mask to struct smp_hotplug_thread and apply it
>> throughout the thread lifecycle. Registration creates threads only for
>> online CPUs in the mask. CPU bring-up creates threads for selected
>> CPUs that were offline at registration, while parking and unparking
>> are restricted to the same mask.
>>
>> Apply the restriction to thread destruction as well, both during
>> unregistration and registration failure cleanup. This is necessary
>> when descriptors with disjoint masks share the same per-CPU task
>> storage: tearing down one descriptor must not stop or clear threads
>> owned by another descriptor.
>>
>> The caller must keep the mask valid and unchanged until the descriptor
>> is unregistered. Callers sharing per-CPU task storage must ensure their
>> masks do not overlap and serialize ownership changes themselves.
> 
> This really wants a built-in check which prevents overlapping
> registrations for the same thread storage. It's easy enough to do:
> 
> smpboot_register_percpu_thread()
> 
> 	if (!pt->cpumask)
> 		pt->cpu_mask = cpu_possible_mask;
> 
> 	guard(cpus_read_lock)();
>          guard(mutex)(&smpboot_threads_lock);
>          list_for_each_entry(ht, &hotplug_threads, list) {
> 		if (ht->store != pt->store)
>                  	continue;
>                  if (WARN_ON_ONCE(cpumask_intersects(ht->cpumask, pt->cpumask)))
>                  	return -EBUSY;
>          }
>                  
> Hmm?
Yes, I agree it will prevent strange things happening

Thanks for the suggestion

   -- Daniel