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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Sat, 22 Sep 2018 04:10:51 -0400
Message-ID: <CAEXW_YTgSFr5r3Qkr0+0_wxKGQV1KKP_8X8uQKAnHmhvem8XtA@mail.gmail.com>
Subject: Re: [PATCH RFC] Documentation: RCU: Clarify comment about fanout
To:     paulmck@linux.ibm.com
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-doc@vger.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Steven Rostedt <rostedt@goodmis.org>
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On Fri, Sep 21, 2018 at 9:14 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
>
> 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@joelfernandes.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!

Thanks!

> 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?

Yes, the edit looks good and improved the changes ;-)

I have a few more as well, I'll send them out soon.

thanks!

 - Joel


>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 9870c5b0f7b41a7b6085c20c22b320681800dabc
> Author: Joel Fernandes (Google) <joel@joelfernandes.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@joelfernandes.org>
>     Signed-off-by: Paul E. McKenney <paulmck@linux.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
>