tracing: 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.

Cc: Lai Jiangshan <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Link: https://lore.kernel.org/[email protected]
Suggested-by: Tejun Heo <[email protected]>
Signed-off-by: Marco Crivellari <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: DispatchCode

kernel/trace/trace_events_user.c

@@ -835,7 +835,7 @@ void user_event_mm_remove(struct task_struct *t)
 	 * so we use a work queue after call_rcu() to run within.
 	 */
 	INIT_RCU_WORK(&mm->put_rwork, delayed_user_event_mm_put);
-	queue_rcu_work(system_wq, &mm->put_rwork);
+	queue_rcu_work(system_percpu_wq, &mm->put_rwork);
 }
 
 void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)