Re: [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, May 13, 2020 at 06:47:12PM +0200, Frederic Weisbecker wrote:
> Allow a CPU's rdp to quit the callback offlined mode.

nit: s/offlined/offloaded/ ?

> The switch happens on the target with IRQs disabled and rdp->nocb_lock
> held to avoid races between local callbacks handling and kthread
> offloaded callbacks handling.
> nocb_cb kthread is first parked to avoid any future race with
> concurrent rcu_do_batch() executions. Then the cblist is set to offloaded
> so that the nocb_gp kthread ignores this rdp.

nit: you mean cblist is set to non-offloaded mode right?

Also, could you clarify better the rcu_barrier bits in the changelog. I know
there's some issue if the cblist has both offloaded and non-offloaded
callbacks, but it would be good to clarify this here better IMHO.

[...]
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index d8ea2bef5574..4bed48da7702 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -171,9 +171,9 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
>   * Mark the specified rcu_segcblist structure as offloaded.  This
>   * structure must be empty.
>   */
> -void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
> +void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload)
>  {
> -	rsclp->offloaded = 1;
> +	rsclp->offloaded = offload;
>  }
>  #endif
>  
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index 4c1503a82492..8f7c6c34cb1b 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -83,7 +83,7 @@ void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
>  void rcu_segcblist_init(struct rcu_segcblist *rsclp);
>  void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
>  #ifdef CONFIG_RCU_NOCB_CPU
> -void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
> +void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload);
>  #endif
>  bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp);
>  bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp);
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index f19e81e0c691..c74a4df8d5f2 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1943,6 +1943,10 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
>  		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
>  		raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> +		if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
> +			raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> +			continue;
> +		}
>  		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
>  		if (bypass_ncbs &&
>  		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
> @@ -2176,6 +2180,50 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
>  		do_nocb_deferred_wakeup_common(rdp);
>  }
>  
> +static void __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
> +{
> +	unsigned long flags;
> +	struct rcu_node *rnp = rdp->mynode;
> +
> +	printk("De-offloading %d\n", rdp->cpu);

nit: s/printk/pr_debug/ ?

thanks,

 - Joel

> +	kthread_park(rdp->nocb_cb_kthread);
> +
> +	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
> +	rcu_nocb_flush_bypass(rdp, NULL, jiffies);
> +	raw_spin_lock_rcu_node(rnp);
> +	rcu_segcblist_offload(&rdp->cblist, false);
> +	raw_spin_unlock_rcu_node(rnp);
> +	raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
> +}
> +
> +static long rcu_nocb_rdp_deoffload(void *arg)
> +{
> +	struct rcu_data *rdp = arg;
> +
> +	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
> +	__rcu_nocb_rdp_deoffload(rdp);
> +
> +	return 0;
> +}
> +
> +void rcu_nocb_cpu_deoffload(int cpu)
> +{
> +	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> +
> +	mutex_lock(&rcu_state.barrier_mutex);
> +	cpus_read_lock();
> +	if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
> +		if (cpu_online(cpu)) {
> +			work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
> +		} else {
> +			__rcu_nocb_rdp_deoffload(rdp);
> +		}
> +		cpumask_clear_cpu(cpu, rcu_nocb_mask);
> +	}
> +	cpus_read_unlock();
> +	mutex_unlock(&rcu_state.barrier_mutex);
> +}
> +
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> @@ -2218,7 +2266,7 @@ void __init rcu_init_nohz(void)
>  		rdp = per_cpu_ptr(&rcu_data, cpu);
>  		if (rcu_segcblist_empty(&rdp->cblist))
>  			rcu_segcblist_init(&rdp->cblist);
> -		rcu_segcblist_offload(&rdp->cblist);
> +		rcu_segcblist_offload(&rdp->cblist, true);
>  	}
>  	rcu_organize_nocb_kthreads();
>  }
> -- 
> 2.25.0
>