Re: [PATCH] tracing: relax trace_event_eval_update() execution with schedule()

[Steven Rostedt]

On Fri, 29 Sep 2023 16:13:48 +0200
Clément Léger <cleger@xxxxxxxxxxxx> wrote:

> 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 <cleger@xxxxxxxxxxxx>
> ---
>  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.

-- Steve

>  	}
>  	up_write(&trace_event_sem);
>  }

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