linux-kernel - [PATCH v2] tracing: relax trace_event_eval_update() execution with cond

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Date:   Fri, 29 Sep 2023 21:16:37 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Clément Léger <cleger@...osinc.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Atish Patra <atishp@...osinc.com>
Subject: [PATCH v2] 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.

Signed-off-by: Clément Léger <cleger@...osinc.com>
---

Changes in v2:
 - Use cond_resched() instead of schedule()

 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..f49d6ddb6342 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]);
 			}
 		}
+		cond_resched();
 	}
 	up_write(&trace_event_sem);
 }
-- 
2.40.1