On Fri, Sep 21, 2018 at 9:14 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxx> > 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@xxxxxxxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> > > 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 >
