On 7/26/24 11:56 PM, Frederic Weisbecker wrote:
> Affining kthreads follow either of three existing different patterns:
>
> 1) Per-CPU kthreads must stay affine to a single CPU and never execute
> relevant code on any other CPU. This is currently handled by smpboot
> code which takes care of CPU-hotplug operations.
>
> 2) Kthreads that _have_ to be affine to a specific set of CPUs and can't
> run anywhere else. The affinity is set through kthread_bind_mask()
> and the subsystem takes care by itself to handle CPU-hotplug operations.
>
> 3) Kthreads that have a _preferred_ affinity but that can run anywhere
> without breaking correctness. Userspace can overwrite the affinity.
> It is set manually like any other task and CPU-hotplug is supposed
> to be handled by the relevant subsystem so that the task is properly
> reaffined whenever a given CPU from the preferred affinity comes up
> or down. Also care must be taken so that the preferred affinity
> doesn't cross housekeeping cpumask boundaries.
>
> Currently the preferred affinity pattern has at least 4 identified
> users, with more or less success when it comes to handle CPU-hotplug
> operations and housekeeping cpumask.
>
> Provide an infrastructure to handle this usecase patter. A new
> kthread_affine_preferred() API is introduced, to be used just like
> kthread_bind_mask(), right after kthread creation and before the first
> wake up. The kthread is then affine right away to the cpumask passed
> through the API if it has online housekeeping CPUs. Otherwise it will
> be affine to all online housekeeping CPUs as a last resort.
>
> It is aware of CPU hotplug events such that:
>
> * When a housekeeping CPU goes up and is part of the preferred affinity
> of a given kthread, it is added to its applied affinity set (and
> possibly the default last resort online housekeeping set is removed
> from the set).
>
> * When a housekeeping CPU goes down while it was part of the preferred
> affinity of a kthread, it is removed from the kthread's applied
> affinity. The last resort is to affine the kthread to all online
> housekeeping CPUs.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Nit:
> +int kthread_affine_preferred(struct task_struct *p, const struct cpumask *mask)
> +{
> + struct kthread *kthread = to_kthread(p);
> + cpumask_var_t affinity;
> + unsigned long flags;
> + int ret;
> +
> + if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE) || kthread->started) {
> + WARN_ON(1);
> + return -EINVAL;
> + }
> +
Should we also fail if kthread->preferred_affinity already exist? In
case somebody calls this twice.
Also for some of the use cases (kswapd, kcompactd) it would make sense
to be able to add cpus of a node as they are onlined. Which seems we
didn't do, except some corner case handling in kcompactd, but maybe we
should? I wonder if the current implementation of onlining a completely
new node with cpus does the right thing as a result of the individual
onlining operations, or we end up with being affined to a single cpu (or
none).
But that would need some kind of kthread_affine_preferred_update()
implementation?
> + if (!zalloc_cpumask_var(&affinity, GFP_KERNEL))
> + return -ENOMEM;
> +
> + kthread->preferred_affinity = kzalloc(sizeof(struct cpumask), GFP_KERNEL);
> + if (!kthread->preferred_affinity) {
> + ret = -ENOMEM;
> + goto out;
> + }
> +
> + mutex_lock(&kthreads_hotplug_lock);
> + cpumask_copy(kthread->preferred_affinity, mask);
> + list_add_tail(&kthread->hotplug_node, &kthreads_hotplug);
> + kthread_fetch_affinity(kthread, affinity);
> +
> + /* It's safe because the task is inactive. */
> + raw_spin_lock_irqsave(&p->pi_lock, flags);
> + do_set_cpus_allowed(p, mask);
> + raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +
> + mutex_unlock(&kthreads_hotplug_lock);
> +out:
> + free_cpumask_var(affinity);
> +
> + return 0;
> +}
> +
> +static int kthreads_hotplug_update(void)
> +{
> + cpumask_var_t affinity;
> + struct kthread *k;
> + int err = 0;
> +
> + if (list_empty(&kthreads_hotplug))
> + return 0;
> +
> + if (!zalloc_cpumask_var(&affinity, GFP_KERNEL))
> + return -ENOMEM;
> +
> + list_for_each_entry(k, &kthreads_hotplug, hotplug_node) {
> + if (WARN_ON_ONCE(!k->preferred_affinity)) {
> + err = -EINVAL;
> + break;
> + }
> + kthread_fetch_affinity(k, affinity);
> + set_cpus_allowed_ptr(k->task, affinity);
> + }
> +
> + free_cpumask_var(affinity);
> +
> + return err;
> +}
> +
> +static int kthreads_offline_cpu(unsigned int cpu)
> +{
> + int ret = 0;
> +
> + mutex_lock(&kthreads_hotplug_lock);
> + cpumask_clear_cpu(cpu, &kthread_online_mask);
> + ret = kthreads_hotplug_update();
> + mutex_unlock(&kthreads_hotplug_lock);
> +
> + return ret;
> +}
> +
> +static int kthreads_online_cpu(unsigned int cpu)
> +{
> + int ret = 0;
> +
> + mutex_lock(&kthreads_hotplug_lock);
> + cpumask_set_cpu(cpu, &kthread_online_mask);
> + ret = kthreads_hotplug_update();
> + mutex_unlock(&kthreads_hotplug_lock);
> +
> + return ret;
> +}
> +
> +static int kthreads_init(void)
> +{
> + return cpuhp_setup_state(CPUHP_AP_KTHREADS_ONLINE, "kthreads:online",
> + kthreads_online_cpu, kthreads_offline_cpu);
> +}
> +early_initcall(kthreads_init);
> +
> void __kthread_init_worker(struct kthread_worker *worker,
> const char *name,
> struct lock_class_key *key)
