linux-kernel - [PATCH tip/core/rcu 02/20] doc: Clarify RCU data-structure comment about rcu

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Date:   Sun, 11 Nov 2018 11:56:27 -0800
From:   "Paul E. McKenney" <paulmck@...ux.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, joel@...lfernandes.org,
        kernel-team@...roid.com,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 02/20] doc: Clarify RCU data-structure comment about rcu_tree fanout

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>

RCU Data-Structures document describes a trick to test RCU with small
number of CPUs but with a taller 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.  This commit therefore
provides the needed clarification.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Cc: <kernel-team@...roid.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 .../RCU/Design/Data-Structures/Data-Structures.html       | 8 +++++---
 1 file changed, 5 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..476b1ac38e4c 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -127,9 +127,11 @@ 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 both <tt>CONFIG_RCU_FANOUT</tt> and
+<tt>CONFIG_RCU_FANOUT_LEAF</tt> to be as small as 2, which would result
+in a 16-CPU test using a 4-level tree.
+This can be useful for testing large-system capabilities on small test
+machines.
 
 </p><p>This multi-level combining tree allows us to get most of the
 performance and scalability
-- 
2.17.1