linux-kernel - Re: [PATCH 3/3] rcu: Use rcu_num_nodes instead of NUM_RCU

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Date:	Wed, 16 Jul 2014 09:26:04 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] rcu: Use rcu_num_nodes instead of NUM_RCU_NODES

On 07/16/2014 08:45 AM, Paul E. McKenney wrote:
> On Tue, Jul 15, 2014 at 06:31:49PM -0400, Pranith Kumar wrote:
>> NUM_RCU_NODES is set at build time and is usually a huge number. We calculate the
>> actual number of rcu nodes necessary at boot time based on nr_cpu_ids in
>> rcu_init_geometry() and store it in rcu_num_nodes. We should use this variable
>> instead of NUM_RCU_NODES.
>>
>> This commit changes all such uses of NUM_RCU_NODES to rcu_num_nodes.
>>
>> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
>> ---
>>  kernel/rcu/tree_plugin.h | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index cedb020..17ccb62 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -46,6 +46,8 @@ static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
>>  static char __initdata nocb_buf[NR_CPUS * 5];
>>  #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
>>
>> +extern int rcu_num_nodes;
> 
> This should not be necessary given the existing declaration in
> kernel/rcu/tree.c way before the #include of kernel/rcu/tree_plugin.h.
> 
> Or did you get a build failure without this?  (And if you did get a build
> failure, I would be really curious how that happened!)
> 
> So please send an updated patch or tell me how your build managed to fail.
> 

Indeed. The extern was unnecessary. Please find an updated patch below.

--
Pranith

From: Pranith Kumar <bobby.prani@...il.com>
Date: Wed, 16 Jul 2014 09:21:49 -0400
Subject: [PATCH 1/1] rcu: Use rcu_num_nodes instead of NUM_RCU_NODES

NUM_RCU_NODES is set at build time and is usually a huge number. We calculate the
actual number of rcu nodes necessary at boot time based on nr_cpu_ids in
rcu_init_geometry() and store it in rcu_num_nodes. We should use this variable
instead of NUM_RCU_NODES.

This commit changes all such uses of NUM_RCU_NODES to rcu_num_nodes.

Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
---
 kernel/rcu/tree_plugin.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cedb020..b99055a 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -885,7 +885,7 @@ void synchronize_rcu_expedited(void)
 	/* Snapshot current state of ->blkd_tasks lists. */
 	rcu_for_each_leaf_node(rsp, rnp)
 		sync_rcu_preempt_exp_init(rsp, rnp);
-	if (NUM_RCU_NODES > 1)
+	if (rcu_num_nodes > 1)
 		sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));
 
 	put_online_cpus();
@@ -1475,7 +1475,7 @@ static void __init rcu_spawn_boost_kthreads(void)
 	BUG_ON(smpboot_register_percpu_thread(&rcu_cpu_thread_spec));
 	rnp = rcu_get_root(rcu_state_p);
 	(void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
-	if (NUM_RCU_NODES > 1) {
+	if (rcu_num_nodes > 1) {
 		rcu_for_each_leaf_node(rcu_state_p, rnp)
 			(void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
 	}
-- 
1.9.1


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