linux-kernel - [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew

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Message-Id: <1492016149-18834-4-git-send-email-paulmck@linux.vnet.ibm.com>
Date:   Wed, 12 Apr 2017 09:55:40 -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 tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

If you set RCU_FANOUT_LEAF too high, you can get lock contention
on the leaf rcu_node, and you should boot with the skew_tick kernel
parameter set in order to avoid this lock contention.  This commit
therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state
this.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 init/Kconfig | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index a92f27da4a27..946e561e67b7 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF
 	  initialization.  These systems tend to run CPU-bound, and thus
 	  are not helped by synchronized interrupts, and thus tend to
 	  skew them, which reduces lock contention enough that large
-	  leaf-level fanouts work well.
+	  leaf-level fanouts work well.  That said, setting leaf-level
+	  fanout to a large number will likely cause problematic
+	  lock contention on the leaf-level rcu_node structures unless
+	  you boot with the skew_tick kernel parameter.
 
 	  Select a specific number if testing RCU itself.
 
-	  Select the maximum permissible value for large systems.
+	  Select the maximum permissible value for large systems, but
+	  	please understand that you may also need to set the
+		skew_tick kernel boot parameter to avoid contention
+		on the rcu_node structure's locks.
 
 	  Take the default if unsure.
 
-- 
2.5.2