linux-kernel - [PATCH RFC] Documentation: RCU: Clarify comment about fanout

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Message-Id: <20180921223153.114728-1-joel@joelfernandes.org>
Date:   Fri, 21 Sep 2018 18:31:53 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH RFC] Documentation: RCU: Clarify comment about fanout

RCU Data-Structures document describes a trick to test RCU with small
number of CPUs but with a larger tree. It wasn't immediately clear how
the document arrived at 16 CPUs which also requires setting the
FANOUT_LEAF to 2 instead of the default of 16. Clarify that.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 .../RCU/Design/Data-Structures/Data-Structures.html         | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index 1d2051c0c3fc..791348977da7 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -127,9 +127,9 @@ CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
 </p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
 accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
 32-bit systems.
-On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
-as small as 2 if you wish, which would permit only 16 CPUs, which
-is useful for testing.
+On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be as small as 2
+and set <tt>CONFIG_RCU_FANOUT_LEAF</tt> to 2 if you wish, which would permit
+only 16 CPUs in a 4-level tree. This can be useful for testing.
 
 </p><p>This multi-level combining tree allows us to get most of the
 performance and scalability
-- 
2.19.0.444.g18242da7ef-goog