bpf: replace use of system_wq with system_percpu_wq

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.

system_wq is a per-CPU worqueue, yet nothing in its name tells about that
CPU affinity constraint, which is very often not required by users. Make
it clear by adding a system_percpu_wq.

queue_work() / queue_delayed_work() mod_delayed_work() will now use the
new per-cpu wq: whether the user still stick on the old name a warn will
be printed along a wq redirect to the new one.

This patch add the new system_percpu_wq except for mm, fs and net
subsystem, whom are handled in separated patches.

The old wq will be kept for a few release cylces.

Suggested-by: Tejun Heo <[email protected]>
Signed-off-by: Marco Crivellari <[email protected]>

Author: DispatchCode

kernel/bpf/cgroup.c

@@ -27,7 +27,7 @@ EXPORT_SYMBOL(cgroup_bpf_enabled_key);
 /*
  * cgroup bpf destruction makes heavy use of work items and there can be a lot
  * of concurrent destructions.  Use a separate workqueue so that cgroup bpf
- * destruction work items don't end up filling up max_active of system_wq
+ * destruction work items don't end up filling up max_active of system_percpu_wq
  * which may lead to deadlock.
  */
 static struct workqueue_struct *cgroup_bpf_destroy_wq;

kernel/bpf/cpumap.c

@@ -550,7 +550,7 @@ static void __cpu_map_entry_replace(struct bpf_cpu_map *cmap,
 	old_rcpu = unrcu_pointer(xchg(&cmap->cpu_map[key_cpu], RCU_INITIALIZER(rcpu)));
 	if (old_rcpu) {
 		INIT_RCU_WORK(&old_rcpu->free_work, __cpu_map_entry_free);
-		queue_rcu_work(system_wq, &old_rcpu->free_work);
+		queue_rcu_work(system_percpu_wq, &old_rcpu->free_work);
 	}
 }