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 <tj@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -7014,10 +7014,10 @@
 			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

@@ -358,7 +358,11 @@ on one of the CPUs which share the last level cache with the issuing CPU.
 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
@@ -392,6 +396,9 @@ directory.
 ``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
   item starts execution, workqueue makes a best-effort attempt to ensure

include/linux/workqueue.h

@@ -126,6 +126,7 @@ struct rcu_work {
 };
 
 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 */
@@ -133,8 +134,6 @@ enum wq_affn_scope {
 	WQ_AFFN_SYSTEM,			/* one pod across the whole system */
 
 	WQ_AFFN_NR_TYPES,
-
-	WQ_AFFN_DFL = WQ_AFFN_CACHE,
 };
 
 /**

kernel/workqueue.c

@@ -339,9 +339,10 @@ struct wq_pod_type {
 };
 
 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",
@@ -3734,7 +3735,7 @@ struct workqueue_attrs *alloc_workqueue_attrs(void)
 		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);
@@ -3815,7 +3816,18 @@ static void wqattrs_actualize_cpumask(struct 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))
@@ -5847,13 +5859,29 @@ static int parse_affn_scope(const char *val)
 
 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;
 }
 
@@ -6033,8 +6061,13 @@ static ssize_t wq_affn_scope_show(struct device *dev,
 	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;