workqueue: Make default affinity_scope dynamically updatable

While workqueue.default_affinity_scope is writable, it only affects
workqueues which are created afterwards and isn't very useful. Instead,
let's introduce explicit "default" scope and update the effective scope
dynamically when workqueue.default_affinity_scope is changed.

Signed-off-by: Tejun Heo <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			information, see the Affinity Scopes section in
 			Documentation/core-api/workqueue.rst.
 
-			This can be updated after boot through the matching
-			file under /sys/module/workqueue/parameters.
-			However, the changed default will only apply to
-			unbound workqueues created afterwards.
+			This can be changed after boot by writing to the
+			matching /sys/module/workqueue/parameters file. All
+			workqueues with the "default" affinity scope will be
+			updated accordignly.
 
 	workqueue.debug_force_rr_cpu
 			Workqueue used to implicitly guarantee that work

Documentation/core-api/workqueue.rst

···
 Once started, the worker may or may not be allowed to move outside the scope
 depending on the ``affinity_strict`` setting of the scope.
 
-Workqueue currently supports the following five affinity scopes.
+Workqueue currently supports the following affinity scopes.
+
+``default``
+  Use the scope in module parameter ``workqueue.default_affinity_scope``
+  which is always set to one of the scopes below.
 
 ``cpu``
   CPUs are not grouped. A work item issued on one CPU is processed by a
···
 
 ``affinity_scope``
   Read to see the current affinity scope. Write to change.
+
+  When default is the current scope, reading this file will also show the
+  current effective scope in parentheses, for example, ``default (cache)``.
 
 ``affinity_strict``
   0 by default indicating that affinity scopes are not strict. When a work

include/linux/workqueue.h

···
 };
 
 enum wq_affn_scope {
+	WQ_AFFN_DFL,			/* use system default */
 	WQ_AFFN_CPU,			/* one pod per CPU */
 	WQ_AFFN_SMT,			/* one pod poer SMT */
 	WQ_AFFN_CACHE,			/* one pod per LLC */
···
 	WQ_AFFN_SYSTEM,			/* one pod across the whole system */
 
 	WQ_AFFN_NR_TYPES,
-
-	WQ_AFFN_DFL = WQ_AFFN_CACHE,
 };
 
 /**

kernel/workqueue.c

···
 };
 
 static struct wq_pod_type wq_pod_types[WQ_AFFN_NR_TYPES];
-static enum wq_affn_scope wq_affn_dfl = WQ_AFFN_DFL;
+static enum wq_affn_scope wq_affn_dfl = WQ_AFFN_CACHE;
 
 static const char *wq_affn_names[WQ_AFFN_NR_TYPES] = {
+	[WQ_AFFN_DFL]			= "default",
 	[WQ_AFFN_CPU]			= "cpu",
 	[WQ_AFFN_SMT]			= "smt",
 	[WQ_AFFN_CACHE]			= "cache",
···
 		goto fail;
 
 	cpumask_copy(attrs->cpumask, cpu_possible_mask);
-	attrs->affn_scope = wq_affn_dfl;
+	attrs->affn_scope = WQ_AFFN_DFL;
 	return attrs;
 fail:
 	free_workqueue_attrs(attrs);
···
 static const struct wq_pod_type *
 wqattrs_pod_type(const struct workqueue_attrs *attrs)
 {
-	struct wq_pod_type *pt = &wq_pod_types[attrs->affn_scope];
+	enum wq_affn_scope scope;
+	struct wq_pod_type *pt;
+
+	/* to synchronize access to wq_affn_dfl */
+	lockdep_assert_held(&wq_pool_mutex);
+
+	if (attrs->affn_scope == WQ_AFFN_DFL)
+		scope = wq_affn_dfl;
+	else
+		scope = attrs->affn_scope;
+
+	pt = &wq_pod_types[scope];
 
 	if (!WARN_ON_ONCE(attrs->affn_scope == WQ_AFFN_NR_TYPES) &&
 	    likely(pt->nr_pods))
···
 
 static int wq_affn_dfl_set(const char *val, const struct kernel_param *kp)
 {
-	int affn;
+	struct workqueue_struct *wq;
+	int affn, cpu;
 
 	affn = parse_affn_scope(val);
 	if (affn < 0)
 		return affn;
+	if (affn == WQ_AFFN_DFL)
+		return -EINVAL;
+
+	cpus_read_lock();
+	mutex_lock(&wq_pool_mutex);
 
 	wq_affn_dfl = affn;
+
+	list_for_each_entry(wq, &workqueues, list) {
+		for_each_online_cpu(cpu) {
+			wq_update_pod(wq, cpu, cpu, true);
+		}
+	}
+
+	mutex_unlock(&wq_pool_mutex);
+	cpus_read_unlock();
+
 	return 0;
 }
 
···
 	int written;
 
 	mutex_lock(&wq->mutex);
-	written = scnprintf(buf, PAGE_SIZE, "%s\n",
-			    wq_affn_names[wq->unbound_attrs->affn_scope]);
+	if (wq->unbound_attrs->affn_scope == WQ_AFFN_DFL)
+		written = scnprintf(buf, PAGE_SIZE, "%s (%s)\n",
+				    wq_affn_names[WQ_AFFN_DFL],
+				    wq_affn_names[wq_affn_dfl]);
+	else
+		written = scnprintf(buf, PAGE_SIZE, "%s\n",
+				    wq_affn_names[wq->unbound_attrs->affn_scope]);
 	mutex_unlock(&wq->mutex);
 
 	return written;