linux-kernel - [PATCH v3 tip/core/rcu 12/40] rcu: Default RCU_FANOUT

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Date:   Wed, 19 Apr 2017 09:58:09 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v3 tip/core/rcu 12/40] rcu: Default RCU_FANOUT_LEAF to 16 unless explicitly changed

If the RCU_EXPERT Kconfig option is not set (the default), then the
RCU_FANOUT_LEAF Kconfig option will not be defined, which will cause
the leaf-level rcu_node tree fanout to default to 32 on 32-bit systems
and 64 on 64-bit systems.  This can result in excessive lock contention.
This commit therefore changes the computation of the leaf-level rcu_node
tree fanout so that the result will be 16 unless an explicit Kconfig or
kernel-boot setting says otherwise.

Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree.h | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index d2f276fc2edc..376c01e539c7 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -52,11 +52,7 @@
 #ifdef CONFIG_RCU_FANOUT_LEAF
 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
 #else /* #ifdef CONFIG_RCU_FANOUT_LEAF */
-# ifdef CONFIG_64BIT
-# define RCU_FANOUT_LEAF 64
-# else
-# define RCU_FANOUT_LEAF 32
-# endif
+#define RCU_FANOUT_LEAF 16
 #endif /* #else #ifdef CONFIG_RCU_FANOUT_LEAF */
 
 #define RCU_FANOUT_1	      (RCU_FANOUT_LEAF)
-- 
2.5.2