tracing: relax trace_event_eval_update() execution with cond_resched()

When kernel is compiled without preemption, the eval_map_work_func()
(which calls trace_event_eval_update()) will not be preempted up to its
complete execution. This can actually cause a problem since if another
CPU call stop_machine(), the call will have to wait for the
eval_map_work_func() function to finish executing in the workqueue
before being able to be scheduled. This problem was observe on a SMP
system at boot time, when the CPU calling the initcalls executed
clocksource_done_booting() which in the end calls stop_machine(). We
observed a 1 second delay because one CPU was executing
eval_map_work_func() and was not preempted by the stop_machine() task.

Adding a call to cond_resched() in trace_event_eval_update() allows
other tasks to be executed and thus continue working asynchronously
like before without blocking any pending task at boot time.

Link: https://lore.kernel.org/linux-trace-kernel/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Signed-off-by: Clément Léger <[email protected]>
Tested-by: Atish Patra <[email protected]>
Reviewed-by: Atish Patra <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

kernel/trace/trace_events.c

@@ -2770,6 +2770,7 @@ void trace_event_eval_update(struct trace_eval_map **map, int len)
 				update_event_fields(call, map[i]);
 			}
 		}
+		cond_resched();
 	}
 	up_write(&trace_event_sem);
 }