workqueue: Generalize unbound CPU pods

While renamed to pod, the code still assumes that the pods are defined by
NUMA boundaries. Let's generalize it:

* workqueue_attrs->affn_scope is added. Each enum represents the type of
  boundaries that define the pods. There are currently two scopes -
  WQ_AFFN_NUMA and WQ_AFFN_SYSTEM. The former is the same behavior as before
  - one pod per NUMA node. The latter defines one global pod across the
  whole system.

* struct wq_pod_type is added which describes how pods are configured for
  each affnity scope. For each pod, it lists the member CPUs and the
  preferred NUMA node for memory allocations. The reverse mapping from CPU
  to pod is also available.

* wq_pod_enabled is dropped. Pod is now always enabled. The previously
  disabled behavior is now implemented through WQ_AFFN_SYSTEM.

* get_unbound_pool() wants to determine the NUMA node to allocate memory
  from for the new pool. The variables are renamed from node to pod but the
  logic still assumes they're one and the same. Clearly distinguish them -
  walk the WQ_AFFN_NUMA pods to find the matching pod and then use the pod's
  NUMA node.

* wq_calc_pod_cpumask() was taking @pod but assumed that it was the NUMA
  node. Take @cpu instead and determine the cpumask to use from the pod_type
  matching @attrs.

* apply_wqattrs_prepare() is update to return ERR_PTR() on error instead of
  NULL so that it can indicate -EINVAL on invalid affinity scopes.

This patch allows CPUs to be grouped into pods however desired per type.
While this patch causes some internal behavior changes, nothing material
should change for workqueue users.

v2: Trigger WARN_ON_ONCE() in wqattrs_pod_type() if affn_scope is
    WQ_AFFN_NR_TYPES which indicates that the function is called with a
    worker_pool's attrs instead of a workqueue's.
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -125,6 +125,15 @@ struct rcu_work {
 	struct workqueue_struct *wq;
 };
 
+enum wq_affn_scope {
+	WQ_AFFN_NUMA,			/* one pod per NUMA node */
+	WQ_AFFN_SYSTEM,			/* one pod across the whole system */
+
+	WQ_AFFN_NR_TYPES,
+
+	WQ_AFFN_DFL = WQ_AFFN_NUMA,
+};
+
 /**
  * struct workqueue_attrs - A struct for workqueue attributes.
  *
@@ -141,12 +150,26 @@ struct workqueue_attrs {
 	 */
 	cpumask_var_t cpumask;
 
+	/*
+	 * Below fields aren't properties of a worker_pool. They only modify how
+	 * :c:func:`apply_workqueue_attrs` select pools and thus don't
+	 * participate in pool hash calculations or equality comparisons.
+	 */
+
 	/**
-	 * @ordered: work items must be executed one by one in queueing order
+	 * @affn_scope: unbound CPU affinity scope
 	 *
-	 * Unlike other fields, ``ordered`` isn't a property of a worker_pool. It
-	 * only modifies how :c:func:`apply_workqueue_attrs` select pools and thus
-	 * doesn't participate in pool hash calculations or equality comparisons.
+	 * CPU pods are used to improve execution locality of unbound work
+	 * items. There are multiple pod types, one for each wq_affn_scope, and
+	 * every CPU in the system belongs to one pod in every pod type. CPUs
+	 * that belong to the same pod share the worker pool. For example,
+	 * selecting %WQ_AFFN_NUMA makes the workqueue use a separate worker
+	 * pool for each NUMA node.
+	 */
+	enum wq_affn_scope affn_scope;
+
+	/**
+	 * @ordered: work items must be executed one by one in queueing order
 	 */
 	bool ordered;
 };