Re: [for-linus][PATCH 2/4] tracing: relax trace_event_eval_update() execution with cond_resched()

[Steven Rostedt <rostedt@goodmis.org>]

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On Sat, 30 Sep 2023 16:32:15 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: =?UTF-8?q?Cl=C3=A9ment=20L=C3=A9ger?= <cleger@rivosinc.com>
> 
> 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/20230929191637.416931-1-cleger@rivosinc.com
> 
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> Tested-by: Atish Patra <atishp@rivosinc.com>
> Reviewed-by: Atish Patra <atishp@rivosinc.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  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);
>  }