EXP rcu: Revert expedited GP parallelization cleverness

[Paul E. McKenney]

> (Commit 258ba8e089db23f760139266c232f01bad73f85c from linux-rcu)
> 
> This commit reverts a series of commits starting with fcc635436501 ("rcu:
> Make expedited GPs handle CPU 0 being offline") and its successors, thus
> queueing each rcu_node structure's expedited grace-period initialization
> work on the first CPU of that rcu_node structure.
> 
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 0b2c2ad69629..a0486414edb4 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -472,7 +472,6 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
>  static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
>  				     smp_call_func_t func)
>  {
> -	int cpu;
>  	struct rcu_node *rnp;
>  
>  	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset"));
> @@ -494,13 +493,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
>  			continue;
>  		}
>  		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
> -		preempt_disable();
> -		cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
> -		/* If all offline, queue the work on an unbound CPU. */
> -		if (unlikely(cpu > rnp->grphi))
> -			cpu = WORK_CPU_UNBOUND;
> -		queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> -		preempt_enable();
> +		queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work);
>  		rnp->exp_need_flush = true;
>  	}

How about instead changing the earlier "if" statement to read as follows?

		if (!READ_ONCE(rcu_par_gp_wq) ||
		    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
		    rcu_is_last_leaf_node(rnp) ||
		    IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) {
			/* No workqueues yet or last leaf, do direct call. */
			sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
			continue;
		}

This just adds the "|| IS_ENABLED(CONFIG_PREEMPT_RT_FULL)" to the "if"
condition.

The advantage of this approach is that it leaves the parallelization
alone for mainline, and avoids the overhead of the workqueues for -rt.

							Thanx, Paul