workqueue: Allow modifying low level unbound workqueue cpumask

Allow to modify the low-level unbound workqueues cpumask through
sysfs. This is performed by traversing the entire workqueue list
and calling apply_wqattrs_prepare() on the unbound workqueues
with the new low level mask. Only after all the preparation are done,
we commit them all together.

Ordered workqueues are ignored from the low level unbound workqueue
cpumask, it will be handled in near future.

All the (default & per-node) pwqs are mandatorily controlled by
the low level cpumask. If the user configured cpumask doesn't overlap
with the low level cpumask, the low level cpumask will be used for the
wq instead.

The comment of wq_calc_node_cpumask() is updated and explicitly
requires that its first argument should be the attrs of the default
pwq.

The default wq_unbound_cpumask is cpu_possible_mask.  The workqueue
subsystem doesn't know its best default value, let the system manager
or the other subsystem set it when needed.

Changed from V8:
  merge the calculating code for the attrs of the default pwq together.
  minor change the code&comments for saving the user configured attrs.
  remove unnecessary list_del().
  minor update the comment of wq_calc_node_cpumask().
  update the comment of workqueue_set_unbound_cpumask();

Cc: Christoph Lameter <cl@linux.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Mike Galbraith <bitbucket@online.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Original-patch-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -424,6 +424,7 @@ struct workqueue_attrs *alloc_workqueue_attrs(gfp_t gfp_mask);
 void free_workqueue_attrs(struct workqueue_attrs *attrs);
 int apply_workqueue_attrs(struct workqueue_struct *wq,
 			  const struct workqueue_attrs *attrs);
+int workqueue_set_unbound_cpumask(cpumask_var_t cpumask);
 
 extern bool queue_work_on(int cpu, struct workqueue_struct *wq,
 			struct work_struct *work);

kernel/workqueue.c

@@ -299,7 +299,7 @@ static DEFINE_SPINLOCK(wq_mayday_lock);	/* protects wq->maydays list */
 static LIST_HEAD(workqueues);		/* PR: list of all workqueues */
 static bool workqueue_freezing;		/* PL: have wqs started freezing? */
 
-static cpumask_var_t wq_unbound_cpumask;
+static cpumask_var_t wq_unbound_cpumask; /* PL: low level cpumask for all unbound wqs */
 
 /* the per-cpu worker pools */
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct worker_pool [NR_STD_WORKER_POOLS],
@@ -3429,7 +3429,7 @@ static struct pool_workqueue *alloc_unbound_pwq(struct workqueue_struct *wq,
 
 /**
  * wq_calc_node_mask - calculate a wq_attrs' cpumask for the specified node
- * @attrs: the wq_attrs of interest
+ * @attrs: the wq_attrs of the default pwq of the target workqueue
  * @node: the target NUMA node
  * @cpu_going_down: if >= 0, the CPU to consider as offline
  * @cpumask: outarg, the resulting cpumask
@@ -3493,6 +3493,7 @@ static struct pool_workqueue *numa_pwq_tbl_install(struct workqueue_struct *wq,
 struct apply_wqattrs_ctx {
 	struct workqueue_struct	*wq;		/* target workqueue */
 	struct workqueue_attrs	*attrs;		/* attrs to apply */
+	struct list_head	list;		/* queued for batching commit */
 	struct pool_workqueue	*dfl_pwq;
 	struct pool_workqueue	*pwq_tbl[];
 };
@@ -3532,9 +3533,15 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 	if (!ctx || !new_attrs || !tmp_attrs)
 		goto out_free;
 
-	/* make a copy of @attrs and sanitize it */
+	/*
+	 * Calculate the attrs of the default pwq.
+	 * If the user configured cpumask doesn't overlap with the
+	 * wq_unbound_cpumask, we fallback to the wq_unbound_cpumask.
+	 */
 	copy_workqueue_attrs(new_attrs, attrs);
 	cpumask_and(new_attrs->cpumask, new_attrs->cpumask, wq_unbound_cpumask);
+	if (unlikely(cpumask_empty(new_attrs->cpumask)))
+		cpumask_copy(new_attrs->cpumask, wq_unbound_cpumask);
 
 	/*
 	 * We may create multiple pwqs with differing cpumasks.  Make a
@@ -3553,7 +3560,7 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 		goto out_free;
 
 	for_each_node(node) {
-		if (wq_calc_node_cpumask(attrs, node, -1, tmp_attrs->cpumask)) {
+		if (wq_calc_node_cpumask(new_attrs, node, -1, tmp_attrs->cpumask)) {
 			ctx->pwq_tbl[node] = alloc_unbound_pwq(wq, tmp_attrs);
 			if (!ctx->pwq_tbl[node])
 				goto out_free;
@@ -3563,7 +3570,11 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 		}
 	}
 
+	/* save the user configured attrs and sanitize it. */
+	copy_workqueue_attrs(new_attrs, attrs);
+	cpumask_and(new_attrs->cpumask, new_attrs->cpumask, cpu_possible_mask);
 	ctx->attrs = new_attrs;
+
 	ctx->wq = wq;
 	free_workqueue_attrs(tmp_attrs);
 	return ctx;
@@ -3704,11 +3715,11 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu,
 
 	/*
 	 * Let's determine what needs to be done.  If the target cpumask is
-	 * different from wq's, we need to compare it to @pwq's and create
-	 * a new one if they don't match.  If the target cpumask equals
-	 * wq's, the default pwq should be used.
+	 * different from the default pwq's, we need to compare it to @pwq's
+	 * and create a new one if they don't match.  If the target cpumask
+	 * equals the default pwq's, the default pwq should be used.
 	 */
-	if (wq_calc_node_cpumask(wq->unbound_attrs, node, cpu_off, cpumask)) {
+	if (wq_calc_node_cpumask(wq->dfl_pwq->pool->attrs, node, cpu_off, cpumask)) {
 		if (cpumask_equal(cpumask, pwq->pool->attrs->cpumask))
 			goto out_unlock;
 	} else {
@@ -4731,6 +4742,84 @@ void thaw_workqueues(void)
 }
 #endif /* CONFIG_FREEZER */
 
+static int workqueue_apply_unbound_cpumask(void)
+{
+	LIST_HEAD(ctxs);
+	int ret = 0;
+	struct workqueue_struct *wq;
+	struct apply_wqattrs_ctx *ctx, *n;
+
+	lockdep_assert_held(&wq_pool_mutex);
+
+	list_for_each_entry(wq, &workqueues, list) {
+		if (!(wq->flags & WQ_UNBOUND))
+			continue;
+		/* creating multiple pwqs breaks ordering guarantee */
+		if (wq->flags & __WQ_ORDERED)
+			continue;
+
+		ctx = apply_wqattrs_prepare(wq, wq->unbound_attrs);
+		if (!ctx) {
+			ret = -ENOMEM;
+			break;
+		}
+
+		list_add_tail(&ctx->list, &ctxs);
+	}
+
+	list_for_each_entry_safe(ctx, n, &ctxs, list) {
+		if (!ret)
+			apply_wqattrs_commit(ctx);
+		apply_wqattrs_cleanup(ctx);
+	}
+
+	return ret;
+}
+
+/**
+ *  workqueue_set_unbound_cpumask - Set the low-level unbound cpumask
+ *  @cpumask: the cpumask to set
+ *
+ *  The low-level workqueues cpumask is a global cpumask that limits
+ *  the affinity of all unbound workqueues.  This function check the @cpumask
+ *  and apply it to all unbound workqueues and updates all pwqs of them.
+ *
+ *  Retun:	0	- Success
+ *  		-EINVAL	- Invalid @cpumask
+ *  		-ENOMEM	- Failed to allocate memory for attrs or pwqs.
+ */
+int workqueue_set_unbound_cpumask(cpumask_var_t cpumask)
+{
+	int ret = -EINVAL;
+	cpumask_var_t saved_cpumask;
+
+	if (!zalloc_cpumask_var(&saved_cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	get_online_cpus();
+	cpumask_and(cpumask, cpumask, cpu_possible_mask);
+	if (!cpumask_empty(cpumask)) {
+		mutex_lock(&wq_pool_mutex);
+
+		/* save the old wq_unbound_cpumask. */
+		cpumask_copy(saved_cpumask, wq_unbound_cpumask);
+
+		/* update wq_unbound_cpumask at first and apply it to wqs. */
+		cpumask_copy(wq_unbound_cpumask, cpumask);
+		ret = workqueue_apply_unbound_cpumask();
+
+		/* restore the wq_unbound_cpumask when failed. */
+		if (ret < 0)
+			cpumask_copy(wq_unbound_cpumask, saved_cpumask);
+
+		mutex_unlock(&wq_pool_mutex);
+	}
+	put_online_cpus();
+
+	free_cpumask_var(saved_cpumask);
+	return ret;
+}
+
 #ifdef CONFIG_SYSFS
 /*
  * Workqueues with WQ_SYSFS flag set is visible to userland via
@@ -4952,14 +5041,34 @@ static ssize_t wq_unbound_cpumask_show(struct device *dev,
 {
 	int written;
 
+	mutex_lock(&wq_pool_mutex);
 	written = scnprintf(buf, PAGE_SIZE, "%*pb\n",
 			    cpumask_pr_args(wq_unbound_cpumask));
+	mutex_unlock(&wq_pool_mutex);
 
 	return written;
 }
 
+static ssize_t wq_unbound_cpumask_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	cpumask_var_t cpumask;
+	int ret;
+
+	if (!zalloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+
+	ret = cpumask_parse(buf, cpumask);
+	if (!ret)
+		ret = workqueue_set_unbound_cpumask(cpumask);
+
+	free_cpumask_var(cpumask);
+	return ret ? ret : count;
+}
+
 static struct device_attribute wq_sysfs_cpumask_attr =
-	__ATTR(cpumask, 0444, wq_unbound_cpumask_show, NULL);
+	__ATTR(cpumask, 0644, wq_unbound_cpumask_show,
+	       wq_unbound_cpumask_store);
 
 static int __init wq_sysfs_init(void)
 {