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From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     linux-kernel@vger.kernel.org
Cc:     mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
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        fweisbec@gmail.com, oleg@redhat.com, joel.opensrc@gmail.com,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 03/22] rcu: Don't allocate rcu_nocb_mask if no one needs it
Date:   Sun, 22 Apr 2018 19:32:08 -0700
In-Reply-To: <20180423023150.GA21533@linux.vnet.ibm.com>
References: <20180423023150.GA21533@linux.vnet.ibm.com>
Message-Id: <1524450747-22778-3-git-send-email-paulmck@linux.vnet.ibm.com>
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Commit 44c65ff2e3b0 ("rcu: Eliminate NOCBs CPU-state Kconfig options")
made allocation of rcu_nocb_mask depend only on the rcu_nocbs=,
nohz_full=, or isolcpus= kernel boot parameters.  However, it failed
to change the initial value of rcu_init_nohz()'s local variable
need_rcu_nocb_mask to false, which can result in useless allocation
of an all-zero rcu_nocb_mask.  This commit therefore fixes this bug by
changing the initial value of need_rcu_nocb_mask to false.

While we are in the area, also correct the error message that is printed
when someone specifies that can-never-exist CPUs should be NOCBs CPUs.

Reported-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Byungchul Park <byungchul.park@lge.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 6c5df18bbf2f..3566e4f6dfcc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2283,7 +2283,7 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
 void __init rcu_init_nohz(void)
 {
 	int cpu;
-	bool need_rcu_nocb_mask = true;
+	bool need_rcu_nocb_mask = false;
 	struct rcu_state *rsp;
 
 #if defined(CONFIG_NO_HZ_FULL)
@@ -2306,7 +2306,7 @@ void __init rcu_init_nohz(void)
 #endif /* #if defined(CONFIG_NO_HZ_FULL) */
 
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
-		pr_info("\tNote: kernel parameter 'rcu_nocbs=' contains nonexistent CPUs.\n");
+		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
 			    rcu_nocb_mask);
 	}
-- 
2.5.2