[PATCH 3/5] workqueue: make the default pwq optional

[Breno Leitao]

apply_wqattrs_prepare() always creates a dfl_pwq, the fallback a CPU
uses when it has no pwq of its own. This is a CPU unbound field, so, we
don't need it for the per cpu affinity version of workqueue.

A percpu workqueue has no use for one. Skip the allocation for
a WQ_PERCPU workqueue and skip installing what was not allocated.

Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 kernel/workqueue.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 7feefe0a4a5ff..8fac93574becb 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5513,15 +5513,18 @@ apply_wqattrs_prepare(struct workqueue_struct *wq,
 
 	/*
 	 * If something goes wrong during CPU up/down, we'll fall back to
-	 * the default pwq covering whole @attrs->cpumask.  Always create
-	 * it even if we don't use it immediately.
+	 * the default pwq covering whole @attrs->cpumask.  Create it even
+	 * if we don't use it immediately.  A percpu workqueue has a pwq on
+	 * every possible CPU and never falls back, so it has no default.
 	 */
 	copy_workqueue_attrs(new_attrs, attrs);
 	wqattrs_actualize_cpumask(new_attrs, unbound_cpumask);
 	cpumask_copy(new_attrs->__pod_cpumask, new_attrs->cpumask);
-	ctx->dfl_pwq = alloc_unbound_pwq(wq, new_attrs);
-	if (!ctx->dfl_pwq)
-		goto out_free;
+	if (!(wq->flags & WQ_PERCPU)) {
+		ctx->dfl_pwq = alloc_unbound_pwq(wq, new_attrs);
+		if (!ctx->dfl_pwq)
+			goto out_free;
+	}
 
 	for_each_possible_cpu(cpu) {
 		if (new_attrs->ordered) {
@@ -5573,7 +5576,8 @@ static void apply_wqattrs_commit(struct apply_wqattrs_ctx *ctx)
 	for_each_possible_cpu(cpu)
 		ctx->pwq_tbl[cpu] = install_unbound_pwq(ctx->wq, cpu,
 							ctx->pwq_tbl[cpu]);
-	ctx->dfl_pwq = install_unbound_pwq(ctx->wq, -1, ctx->dfl_pwq);
+	if (ctx->dfl_pwq)
+		ctx->dfl_pwq = install_unbound_pwq(ctx->wq, -1, ctx->dfl_pwq);
 
 	/* update node_nr_active->max, which only unbound workqueues have */
 	if (ctx->wq->flags & WQ_UNBOUND)

-- 
2.53.0-Meta