Re: [PATCH printk v5 29/30] rcu: Mark emergency sections in rcu stalls

[Petr Mladek]

On Thu 2024-05-02 23:44:38, John Ogness wrote:
> Mark emergency sections wherever multiple lines of
> rcu stall information are generated. In an emergency
> section the CPU will not perform console output for the
> printk() calls. Instead, a flushing of the console
> output is triggered when exiting the emergency section.
> This allows the full message block to be stored as
> quickly as possible in the ringbuffer.
>
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -612,6 +616,7 @@ static void synchronize_rcu_expedited_wait(void)
>  			}
>  			pr_cont("\n");
>  		}
> +		nbcon_cpu_emergency_flush();

It would make more sense to do the flush inside the cycle after each
dump_cpu_task(). Something like:

		rcu_for_each_leaf_node(rnp) {
			for_each_leaf_node_possible_cpu(rnp, cpu) {
				mask = leaf_node_cpu_bit(rnp, cpu);
				if (!(READ_ONCE(rnp->expmask) & mask))
					continue;
				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
				dump_cpu_task(cpu);
				preempt_enable();
+				nbcon_cpu_emergency_flush();
			}
			rcu_exp_print_detail_task_stall_rnp(rnp);
		}


Or maybe, it is limited onto to few CPUs by rcu_for_each_leaf_node(rnp)?


>  		rcu_for_each_leaf_node(rnp) {
>  			for_each_leaf_node_possible_cpu(rnp, cpu) {
>  				mask = leaf_node_cpu_bit(rnp, cpu);

Otherwise, it looks good to me.

Best Regards,
Petr