On 29/09/2023 17:06, Steven Rostedt wrote:

On Fri, 29 Sep 2023 16:13:48 +0200
Clément Léger [off-list ref] wrote:

quoted

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

Signed-off-by: Clément Léger <redacted>
---
 kernel/trace/trace_events.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 91951d038ba4..dbdf57a081c0 100644
--- a/kernel/trace/trace_events.c
+++ b/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]);
 			}
 		}
+		schedule();

The proper answer to this is "cond_resched()" but still, there's going
to be work to get rid of all that soon [1]. But I'll take a cond_resched()
now until that is implemented.

Hi Steven,

Thanks for the information, I'll update the patch and send a V2.

Clément

-- Steve

quoted

 	}
 	up_write(&trace_event_sem);
 }

[1] https://lore.kernel.org/all/87cyyfxd4k.ffs@tglx/ (local)