Kthreads attached to a preferred NUMA node for their task structure
allocation can also be assumed to run preferrably within that same node.
A more precise affinity is usually notified by calling
kthread_create_on_cpu() or kthread_bind[_mask]() before the first wakeup.
For the others, a default affinity to the node is desired and sometimes
implemented with more or less success when it comes to deal with hotplug
events and nohz_full / CPU Isolation interactions:
- kcompactd is affine to its node and handles hotplug but not CPU Isolation
- kswapd is affine to its node and ignores hotplug and CPU Isolation
- A bunch of drivers create their kthreads on a specific node and
don't take care about affining further.
Handle that default node affinity preference at the generic level
instead, provided a kthread is created on an actual node and doesn't
apply any specific affinity such as a given CPU or a custom cpumask to
bind to before its first wake-up.
This generic handling is aware of CPU hotplug events and CPU isolation
such that:
* When a housekeeping CPU goes up that is part of the node of a given
kthread, the related task is re-affined to that own node if it was
previously running on the default last resort online housekeeping set
from other nodes.
* When a housekeeping CPU goes down while it was part of the node of a
kthread, the running task is migrated (or the sleeping task is woken
up) automatically by the scheduler to other housekeepers within the
same node or, as a last resort, to all housekeepers from other nodes.
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
include/linux/cpuhotplug.h | 1 +
kernel/kthread.c | 106 ++++++++++++++++++++++++++++++++++++-
2 files changed, 106 insertions(+), 1 deletion(-)
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 2361ed4d2b15..228f27150a93 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -239,6 +239,7 @@ enum cpuhp_state {
CPUHP_AP_WORKQUEUE_ONLINE,
CPUHP_AP_RANDOM_ONLINE,
CPUHP_AP_RCUTREE_ONLINE,
+ CPUHP_AP_KTHREADS_ONLINE,
CPUHP_AP_BASE_CACHEINFO_ONLINE,
CPUHP_AP_ONLINE_DYN,
CPUHP_AP_ONLINE_DYN_END = CPUHP_AP_ONLINE_DYN + 40,
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1527a522cdd3..736276d313c2 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -35,6 +35,9 @@ static DEFINE_SPINLOCK(kthread_create_lock);
static LIST_HEAD(kthread_create_list);
struct task_struct *kthreadd_task;
+static LIST_HEAD(kthreads_hotplug);
+static DEFINE_MUTEX(kthreads_hotplug_lock);
+
struct kthread_create_info
{
/* Information passed to kthread() from kthreadd. */
@@ -53,6 +56,7 @@ struct kthread_create_info
struct kthread {
unsigned long flags;
unsigned int cpu;
+ unsigned int node;
int started;
int result;
int (*threadfn)(void *);
@@ -64,6 +68,8 @@ struct kthread {
#endif
/* To store the full name if task comm is truncated. */
char *full_name;
+ struct task_struct *task;
+ struct list_head hotplug_node;
};
enum KTHREAD_BITS {
@@ -122,8 +128,11 @@ bool set_kthread_struct(struct task_struct *p)
init_completion(&kthread->exited);
init_completion(&kthread->parked);
+ INIT_LIST_HEAD(&kthread->hotplug_node);
p->vfork_done = &kthread->exited;
+ kthread->task = p;
+ kthread->node = tsk_fork_get_node(current);
p->worker_private = kthread;
return true;
}
@@ -314,6 +323,11 @@ void __noreturn kthread_exit(long result)
{
struct kthread *kthread = to_kthread(current);
kthread->result = result;
+ if (!list_empty(&kthread->hotplug_node)) {
+ mutex_lock(&kthreads_hotplug_lock);
+ list_del(&kthread->hotplug_node);
+ mutex_unlock(&kthreads_hotplug_lock);
+ }
do_exit(0);
}
EXPORT_SYMBOL(kthread_exit);
@@ -339,6 +353,48 @@ void __noreturn kthread_complete_and_exit(struct completion *comp, long code)
}
EXPORT_SYMBOL(kthread_complete_and_exit);
+static void kthread_fetch_affinity(struct kthread *kthread, struct cpumask *cpumask)
+{
+ cpumask_and(cpumask, cpumask_of_node(kthread->node),
+ housekeeping_cpumask(HK_TYPE_KTHREAD));
+
+ if (cpumask_empty(cpumask))
+ cpumask_copy(cpumask, housekeeping_cpumask(HK_TYPE_KTHREAD));
+}
+
+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 (kthread->node == NUMA_NO_NODE) {
+ housekeeping_affine(current, HK_TYPE_RCU);
+ } else {
+ if (!zalloc_cpumask_var(&affinity, GFP_KERNEL)) {
+ WARN_ON_ONCE(1);
+ return;
+ }
+
+ mutex_lock(&kthreads_hotplug_lock);
+ WARN_ON_ONCE(!list_empty(&kthread->hotplug_node));
+ list_add_tail(&kthread->hotplug_node, &kthreads_hotplug);
+ /*
+ * 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(&kthreads_hotplug_lock);
+
+ free_cpumask_var(affinity);
+ }
+}
+
static int kthread(void *_create)
{
static const struct sched_param param = { .sched_priority = 0 };
@@ -369,7 +425,6 @@ static int kthread(void *_create)
* back to default in case they have been changed.
*/
sched_setscheduler_nocheck(current, SCHED_NORMAL, ¶m);
- set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_TYPE_KTHREAD));
/* OK, tell user we're spawned, wait for stop or wakeup */
__set_current_state(TASK_UNINTERRUPTIBLE);
@@ -385,6 +440,9 @@ static int kthread(void *_create)
self->started = 1;
+ if (!(current->flags & PF_NO_SETAFFINITY))
+ kthread_affine_node();
+
ret = -EINTR;
if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
cgroup_kthread_ready();
@@ -779,6 +837,52 @@ int kthreadd(void *unused)
return 0;
}
+/*
+ * Re-affine kthreads according to their preferences
+ * and the newly online CPU. The CPU down part is handled
+ * by select_fallback_rq() which default re-affines to
+ * housekeepers in case the preferred affinity doesn't
+ * apply anymore.
+ */
+static int kthreads_online_cpu(unsigned int cpu)
+{
+ cpumask_var_t affinity;
+ struct kthread *k;
+ int ret;
+
+ guard(mutex)(&kthreads_hotplug_lock);
+
+ if (list_empty(&kthreads_hotplug))
+ return 0;
+
+ if (!zalloc_cpumask_var(&affinity, GFP_KERNEL))
+ return -ENOMEM;
+
+ ret = 0;
+
+ list_for_each_entry(k, &kthreads_hotplug, hotplug_node) {
+ if (WARN_ON_ONCE((k->task->flags & PF_NO_SETAFFINITY) ||
+ kthread_is_per_cpu(k->task) ||
+ k->node == NUMA_NO_NODE)) {
+ ret = -EINVAL;
+ continue;
+ }
+ kthread_fetch_affinity(k, affinity);
+ set_cpus_allowed_ptr(k->task, affinity);
+ }
+
+ free_cpumask_var(affinity);
+
+ return ret;
+}
+
+static int kthreads_init(void)
+{
+ return cpuhp_setup_state(CPUHP_AP_KTHREADS_ONLINE, "kthreads:online",
+ kthreads_online_cpu, NULL);
+}
+early_initcall(kthreads_init);
+
void __kthread_init_worker(struct kthread_worker *worker,
const char *name,
struct lock_class_key *key)
--
2.46.0
