linux-kernel - Re: [PATCH v2] rcu: Allow to eliminate softirq processing from rcutree

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Date:   Wed, 20 Mar 2019 10:30:01 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, tglx@...utronix.de,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH v2] rcu: Allow to eliminate softirq processing from
 rcutree

On Wed, Mar 20, 2019 at 05:35:32PM +0100, Sebastian Andrzej Siewior wrote:
> On 2019-03-20 09:15:00 [-0700], Paul E. McKenney wrote:
> > I am considering making it a module_param() to avoid namespace pollution,
> > as it would become something like rcutree.nosoftirq.
> > 
> > Thoughts?
> 
> nope, perfect.

Please see below for an untested patch.  Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d377a2166b79..767cdea30a1c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3672,6 +3672,10 @@
 			the propagation of recent CPU-hotplug changes up
 			the rcu_node combining tree.
 
+	rcutree.nosoftirq=	[KNL]
+			If set, move all RCU_SOFTIRQ processing to per-CPU
+			rcuc kthreads.  Defaults to using RCU_SOFTIRQ.
+
 	rcutree.rcu_fanout_exact= [KNL]
 			Disable autobalancing of the rcu_node combining
 			tree.  This is used by rcutorture, and might
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 961dbc7b8949..e4baba8800f3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -98,6 +98,9 @@ struct rcu_state rcu_state = {
 /* Dump rcu_node combining tree at boot to verify correct setup. */
 static bool dump_tree;
 module_param(dump_tree, bool, 0444);
+/* Move RCU_SOFTIRQ to rcuc kthreads. */
+static bool nosoftirq;
+module_param(nosoftirq, bool, 0444);
 /* Control rcu_node-tree auto-balancing at boot time. */
 static bool rcu_fanout_exact;
 module_param(rcu_fanout_exact, bool, 0444);
@@ -2330,15 +2333,6 @@ static void rcu_wake_cond(struct task_struct *t, int status)
 		wake_up_process(t);
 }
 
-static bool rcu_softirq_enabled = true;
-
-static int __init rcunosoftirq_setup(char *str)
-{
-	rcu_softirq_enabled = false;
-	return 0;
-}
-__setup("rcunosoftirq", rcunosoftirq_setup);
-
 /*
  * Wake up this CPU's rcuc kthread to do RCU core processing.
  */
@@ -2349,7 +2343,7 @@ static void invoke_rcu_core(void)
 
 	if (!cpu_online(smp_processor_id()))
 		return;
-	if (rcu_softirq_enabled) {
+	if (!nosoftirq) {
 		raise_softirq(RCU_SOFTIRQ);
 	} else {
 		local_irq_save(flags);
@@ -2424,7 +2418,7 @@ static int __init rcu_spawn_core_kthreads(void)
 
 	for_each_possible_cpu(cpu)
 		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
-	if (!IS_ENABLED(CONFIG_RCU_BOOST) && rcu_softirq_enabled)
+	if (!IS_ENABLED(CONFIG_RCU_BOOST) && !nosoftirq)
 		return 0;
 	WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__);
 	return 0;
@@ -3467,7 +3461,7 @@ void __init rcu_init(void)
 	rcu_init_one();
 	if (dump_tree)
 		rcu_dump_rcu_node_tree();
-	if (rcu_softirq_enabled)
+	if (!nosoftirq)
 		open_softirq(RCU_SOFTIRQ, rcu_core_si);
 
 	/*
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index eb99e750a930..c5a2acb2c7af 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -72,6 +72,8 @@ static void __init rcu_bootup_announce_oddness(void)
 		pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
 	if (gp_cleanup_delay)
 		pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_cleanup_delay);
+	if (nosoftirq)
+		pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
 	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
 		pr_info("\tRCU debug extended QS entry/exit.\n");
 	rcupdate_announce_bootup_oddness();