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

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Message-Id: <20180922011428.GU4222@linux.ibm.com>
Date:   Fri, 21 Sep 2018 18:14:28 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.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>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] Documentation: RCU: Clarify comment about fanout

On Fri, Sep 21, 2018 at 06:31:53PM -0400, Joel Fernandes (Google) wrote:
> 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>

Good catch, applied, thank you!  You know, those two Kconfig options
are so closely linked in my mind that I doubt if I would ever have
noticed this issue.  So good show!

I of course could not resist doing a bit of editing.  Could you please
check the following to make sure that I didn't mess something up?

							Thanx, Paul

------------------------------------------------------------------------

commit 9870c5b0f7b41a7b6085c20c22b320681800dabc
Author: Joel Fernandes (Google) <joel@...lfernandes.org>
Date:   Fri Sep 21 18:31:53 2018 -0400

    doc: Clarify RCU data-structure comment about rcu_tree fanout
    
    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>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>

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