cgroup: WQ_PERCPU added to alloc_workqueue users

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.
This lack of consistentcy cannot be addressed without refactoring the API.

alloc_workqueue() treats all queues as per-CPU by default, while unbound
workqueues must opt-in via WQ_UNBOUND.

This default is suboptimal: most workloads benefit from unbound queues,
allowing the scheduler to place worker threads where they’re needed and
reducing noise when CPUs are isolated.

This patch adds a new WQ_PERCPU flag to explicitly request the use of
the per-CPU behavior. Both flags coexist for one release cycle to allow
callers to transition their calls.

Once migration is complete, WQ_UNBOUND can be removed and unbound will
become the implicit default.

With the introduction of the WQ_PERCPU flag (equivalent to !WQ_UNBOUND),
any alloc_workqueue() caller that doesn’t explicitly specify WQ_UNBOUND
must now use WQ_PERCPU.

All existing users have been updated accordingly.

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

kernel/cgroup/cgroup-v1.c

···
 	 * Cap @max_active to 1 too.
 	 */
 	cgroup_pidlist_destroy_wq = alloc_workqueue("cgroup_pidlist_destroy",
-						    0, 1);
+						    WQ_PERCPU, 1);
 	BUG_ON(!cgroup_pidlist_destroy_wq);
 	return 0;
 }

kernel/cgroup/cgroup.c

···
 	 * We would prefer to do this in cgroup_init() above, but that
 	 * is called before init_workqueues(): so leave this until after.
 	 */
-	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
+	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", WQ_PERCPU, 1);
 	BUG_ON(!cgroup_destroy_wq);
 	return 0;
 }