[PATCH RFC v2 5/8] workqueue: add workqueue_set_affn_scope()

[Chuck Lever <cel@kernel.org>]

An unbound workqueue's affinity scope can be changed only through
sysfs. A module whose workqueue contends on the pool lock at the
default scope has no in-kernel way to select a finer one, because
alloc_workqueue_attrs() and apply_workqueue_attrs() are not
exported. Exporting them would also invite a caller to apply freshly
allocated attributes, which resets the nice level, cpumask, and
strict affinity the workqueue already carries.

Add workqueue_set_affn_scope(), which copies the workqueue's current
attributes, replaces only the scope, and applies the result under
wq_pool_mutex, as the sysfs affinity_scope store does. Export it so
that SUNRPC and NFS can set the scope of their workqueues when they
create them.

Signed-off-by: Chuck Lever <cel@kernel.org>
---
 include/linux/workqueue.h |  2 ++
 kernel/workqueue.c        | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index c8a36423cb34..585c32d8dc98 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -618,6 +618,8 @@ struct workqueue_attrs *alloc_workqueue_attrs_noprof(void);
 void free_workqueue_attrs(struct workqueue_attrs *attrs);
 int apply_workqueue_attrs(struct workqueue_struct *wq,
 			  const struct workqueue_attrs *attrs);
+int workqueue_set_affn_scope(struct workqueue_struct *wq,
+			     enum wq_affn_scope affn_scope);
 extern int workqueue_unbound_housekeeping_update(const struct cpumask *hk);
 
 extern bool queue_work_on(int cpu, struct workqueue_struct *wq,
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 3c034cbc5bb3..0c2b8e87cd59 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5610,6 +5610,43 @@ int apply_workqueue_attrs(struct workqueue_struct *wq,
 	return ret;
 }
 
+/**
+ * workqueue_set_affn_scope - change the affinity scope of an unbound workqueue
+ * @wq: the target unbound workqueue
+ * @affn_scope: the new scope, or %WQ_AFFN_DFL for the system default
+ *
+ * Reapply @wq's current attributes with only the affinity scope
+ * replaced, so the nice level, cpumask, and strict affinity the
+ * workqueue already carries survive. Pool-workqueue replacement
+ * proceeds as for apply_workqueue_attrs().
+ *
+ * Context: Process context. Takes wq_pool_mutex and performs
+ * GFP_KERNEL allocations.
+ *
+ * Return: 0 on success and -errno on failure.
+ */
+int workqueue_set_affn_scope(struct workqueue_struct *wq,
+			     enum wq_affn_scope affn_scope)
+{
+	struct workqueue_attrs *attrs;
+	int ret = -ENOMEM;
+
+	if ((unsigned int)affn_scope >= WQ_AFFN_NR_TYPES)
+		return -EINVAL;
+
+	mutex_lock(&wq_pool_mutex);
+	attrs = alloc_workqueue_attrs();
+	if (attrs) {
+		copy_workqueue_attrs(attrs, wq->attrs);
+		attrs->affn_scope = affn_scope;
+		ret = apply_workqueue_attrs_locked(wq, attrs);
+	}
+	mutex_unlock(&wq_pool_mutex);
+	free_workqueue_attrs(attrs);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(workqueue_set_affn_scope);
+
 /**
  * unbound_wq_update_pwq - update a pwq slot for CPU hot[un]plug
  * @wq: the target workqueue

-- 
2.55.0