Re: [PATCH 22/33] kthread: Include unbound kthreads in the managed affinity list

Waiman Long
2025-10-21T22:43:11Z

On 10/13/25 4:31 PM, Frederic Weisbecker wrote:
> The managed affinity list currently contains only unbound kthreads that
> have affinity preferences. Unbound kthreads globally affine by default
> are outside of the list because their affinity is automatically managed
> by the scheduler (through the fallback housekeeping mask) and by cpuset.
>
> However in order to preserve the preferred affinity of kthreads, cpuset
> will delegate the isolated partition update propagation to the
> housekeeping and kthread code.
>
> Prepare for that with including all unbound kthreads in the managed
> affinity list.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
> kernel/kthread.c | 59 ++++++++++++++++++++++++------------------------
> 1 file changed, 30 insertions(+), 29 deletions(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index c4dd967e9e9c..cba3d297f267 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -365,9 +365,10 @@ static void kthread_fetch_affinity(struct kthread *kthread, struct cpumask *cpum
> if (kthread->preferred_affinity) {
> pref = kthread->preferred_affinity;
> } else {
> - if (WARN_ON_ONCE(kthread->node == NUMA_NO_NODE))
> - return;
> - pref = cpumask_of_node(kthread->node);
> + if (kthread->node == NUMA_NO_NODE)
> + pref = housekeeping_cpumask(HK_TYPE_KTHREAD);
> + else
> + pref = cpumask_of_node(kthread->node);
> }
>
> cpumask_and(cpumask, pref, housekeeping_cpumask(HK_TYPE_KTHREAD));
> @@ -380,32 +381,29 @@ static void kthread_affine_node(void)
> struct kthread *kthread = to_kthread(current);
> cpumask_var_t affinity;
>
> - WARN_ON_ONCE(kthread_is_per_cpu(current));
> + if (WARN_ON_ONCE(kthread_is_per_cpu(current)))
> + return;
>
> - if (kthread->node == NUMA_NO_NODE) {
> - housekeeping_affine(current, HK_TYPE_KTHREAD);
> - } else {
> - if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
> - WARN_ON_ONCE(1);
> - return;
> - }
> -
> - mutex_lock(&kthread_affinity_lock);
> - WARN_ON_ONCE(!list_empty(&kthread->affinity_node));
> - list_add_tail(&kthread->affinity_node, &kthread_affinity_list);
> - /*
> - * The node cpumask is racy when read from kthread() but:
> - * - a racing CPU going down will either fail on the subsequent
> - * call to set_cpus_allowed_ptr() or be migrated to housekeepers
> - * afterwards by the scheduler.
> - * - a racing CPU going up will be handled by kthreads_online_cpu()
> - */
> - kthread_fetch_affinity(kthread, affinity);
> - set_cpus_allowed_ptr(current, affinity);
> - mutex_unlock(&kthread_affinity_lock);
> -
> - free_cpumask_var(affinity);
> + if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
> + WARN_ON_ONCE(1);
> + return;
> }
> +
> + mutex_lock(&kthread_affinity_lock);
> + WARN_ON_ONCE(!list_empty(&kthread->affinity_node));
> + list_add_tail(&kthread->affinity_node, &kthread_affinity_list);
> + /*
> + * The node cpumask is racy when read from kthread() but:
> + * - a racing CPU going down will either fail on the subsequent
> + * call to set_cpus_allowed_ptr() or be migrated to housekeepers
> + * afterwards by the scheduler.
> + * - a racing CPU going up will be handled by kthreads_online_cpu()
> + */
> + kthread_fetch_affinity(kthread, affinity);
> + set_cpus_allowed_ptr(current, affinity);
> + mutex_unlock(&kthread_affinity_lock);
> +
> + free_cpumask_var(affinity);
> }
>
> static int kthread(void *_create)
> @@ -924,8 +922,11 @@ static int kthreads_online_cpu(unsigned int cpu)
> ret = -EINVAL;
> continue;
> }
> - kthread_fetch_affinity(k, affinity);
> - set_cpus_allowed_ptr(k->task, affinity);
> +
> + if (k->preferred_affinity || k->node != NUMA_NO_NODE) {
> + kthread_fetch_affinity(k, affinity);
> + set_cpus_allowed_ptr(k->task, affinity);
> + }
> }

My understanding of kthreads_online_cpu() is that hotplug won't affect
the affinity returned from kthread_fetch_affinity(). However,
set_cpus_allowed_ptr() will mask out all the offline CPUs. So if the
given "cpu" to be brought online is in the returned affinity, we should
call set_cpus_allowed_ptr() to add this cpu into its affinity mask
though the current code will call it even it is not strictly necessary.
This change will not do this update to NUMA_NO_NODE kthread with no
preferred_affinity, is this a problem?

Cheers,
Longman