linux-kernel - Re: [PATCH tip/core/rcu 12/23] rcu: Prevent force_quiescent

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Date:	Sun, 2 Sep 2012 03:47:07 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 12/23] rcu: Prevent force_quiescent_state()
 memory contention

On Thu, Aug 30, 2012 at 11:18:27AM -0700, Paul E. McKenney wrote:
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
[...]
> @@ -1824,16 +1825,35 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
>  static void force_quiescent_state(struct rcu_state *rsp)
>  {
>  	unsigned long flags;
> -	struct rcu_node *rnp = rcu_get_root(rsp);
> +	bool ret;
> +	struct rcu_node *rnp;
> +	struct rcu_node *rnp_old = NULL;
> +
> +	/* Funnel through hierarchy to reduce memory contention. */
> +	rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;

What makes this use of raw_smp_processor_id() safe?  (And, could you
document the answer here?)

> +	for (; rnp != NULL; rnp = rnp->parent) {
> +		ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
> +		      !raw_spin_trylock(&rnp->fqslock);

So, the root lock will still get trylocked by one CPU per second-level
tree node, just not by every CPU?

> @@ -2721,10 +2741,14 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
>  static void __init rcu_init_one(struct rcu_state *rsp,
>  		struct rcu_data __percpu *rda)
>  {
> -	static char *buf[] = { "rcu_node_level_0",
> -			       "rcu_node_level_1",
> -			       "rcu_node_level_2",
> -			       "rcu_node_level_3" };  /* Match MAX_RCU_LVLS */
> +	static char *buf[] = { "rcu_node_0",
> +			       "rcu_node_1",
> +			       "rcu_node_2",
> +			       "rcu_node_3" };  /* Match MAX_RCU_LVLS */

Why rename these?

- Josh Triplett
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