linux-kernel - Re: [PATCH] rcu: Rework tick dependency setting into rcu_exp

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Message-ID: <Y7WX8Uy7t+33FyUO@lothringen>
Date:   Wed, 4 Jan 2023 16:14:57 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     paulmck@...nel.org, quic_neeraju@...cinc.com,
        joelfernandes.org@...hringen, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Rework tick dependency setting into
 rcu_exp_handler()

On Wed, Jan 04, 2023 at 07:47:22PM +0800, Zqiang wrote:
> @@ -586,39 +586,19 @@ static bool synchronize_rcu_expedited_wait_once(long tlimit)
>  static void synchronize_rcu_expedited_wait(void)
>  {
>  	int cpu;
> -	unsigned long j;
>  	unsigned long jiffies_stall;
>  	unsigned long jiffies_start;
>  	unsigned long mask;
>  	int ndetected;
> -	struct rcu_data *rdp;
>  	struct rcu_node *rnp;
>  	struct rcu_node *rnp_root = rcu_get_root();
> -	unsigned long flags;
>  
>  	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
>  	jiffies_stall = rcu_exp_jiffies_till_stall_check();
>  	jiffies_start = jiffies;
> -	if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
> -		if (synchronize_rcu_expedited_wait_once(1))
> -			return;
> -		rcu_for_each_leaf_node(rnp) {
> -			raw_spin_lock_irqsave_rcu_node(rnp, flags);
> -			mask = READ_ONCE(rnp->expmask);
> -			for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
> -				rdp = per_cpu_ptr(&rcu_data, cpu);
> -				if (rdp->rcu_forced_tick_exp)
> -					continue;
> -				rdp->rcu_forced_tick_exp = true;
> -				if (cpu_online(cpu))
> -					tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
> -			}
> -			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> -		}
> -		j = READ_ONCE(jiffies_till_first_fqs);
> -		if (synchronize_rcu_expedited_wait_once(j + HZ))
> -			return;
> -	}
> +
> +	if (synchronize_rcu_expedited_wait_once(1))
> +		return;

Do we still need this? This is immediately followed by the same call with
a higher time limit.

>  
>  	for (;;) {
>  		if (synchronize_rcu_expedited_wait_once(jiffies_stall))
> @@ -653,8 +633,6 @@ static void synchronize_rcu_expedited_wait(void)
>  		if (ndetected) {
>  			pr_err("blocking rcu_node structures (internal RCU debug):");
>  			rcu_for_each_node_breadth_first(rnp) {
> -				if (rnp == rnp_root)
> -					continue; /* printed unconditionally */

Is that chunk from another patch?

>  				if (sync_rcu_exp_done_unlocked(rnp))
>  					continue;
>  				pr_cont(" l=%u:%d-%d:%#lx/%c",
> @@ -729,6 +707,17 @@ static void rcu_exp_sel_wait_wake(unsigned long s)
>  
>  #ifdef CONFIG_PREEMPT_RCU
>  
> +static void rcu_exp_set_tickdep(int cpu)
> +{
> +	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);

This is only used locally so you can use this_cpu_ptr() or even
better you can pass the rdp from rcu_exp_handler()

> +
> +	if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended() &&

You can directly test tick_nohz_full_cpu(smp_processor_id()), this has the
advantage to only evaluate smp_processor_id() if nohz_full is current running
(using static key). It's a micro optimization, but still...


> +						cpu_online(cpu)) {

Right good point, this could happen if rcu_exp_handler() is called from
smpcfd_dying_cpu(). In which case we can't requeue an IPI or an IRQ work.
Which is not a problem since rcu_report_dead() -> rcu_preempt_deferred_qs()
then does the QS report. Can you add a comment about that?


> +		rdp->rcu_forced_tick_exp = true;
> +		tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
> +	}
> +}
> +
>  /*
>   * Remote handler for smp_call_function_single().  If there is an
>   * RCU read-side critical section in effect, request that the
> @@ -743,6 +732,7 @@ static void rcu_exp_handler(void *unused)
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  	struct rcu_node *rnp = rdp->mynode;
>  	struct task_struct *t = current;
> +	int cpu = raw_smp_processor_id();

And then no need to fetch smp_processor_id() here.

Thanks.