Re: [GIT PULL rcu/next] RCU commits for 4.2

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 02, 2015 at 08:31:53AM +0200, Ingo Molnar wrote:
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > Hello, Ingo,
> > 
> > The changes in this series include the following:
> > 
> > 1.	Remove all uses of RCU-protected array indexes.  These were posted
> > 	to LKML at https://lkml.org/lkml/2015/5/12/827.
> > 
> > 2.	Documentation updates.  These were posted to LKML at
> > 	https://lkml.org/lkml/2015/5/12/848.
> > 
> > 3.	Miscellaneous fixes.  These were posted to LKML at
> > 	https://lkml.org/lkml/2015/5/12/876.
> > 
> > 4.	CPU-hotplug updates.  These were posted to LKML at
> > 	https://lkml.org/lkml/2015/5/12/880.
> > 
> > 5.	Initialization/Kconfig updates.  These were posted to LKML at
> > 	https://lkml.org/lkml/2015/5/12/918, however, the updates to
> > 	Tree RCU's initialization have been deferred to 4.3.
> > 
> > 6.	Updates to Tiny RCU.  These were posted to LKML at
> > 	https://lkml.org/lkml/2015/5/12/957.
> > 
> > 7.	Torture-testing updates.  These were posted to LKML at
> > 	https://lkml.org/lkml/2015/5/12/964.

[ . . . ]

> Pulled, thanks a lot Paul!
> 
> I am wondering about the following small detail. You have reworked the whole RCU 
> Kconfig machinery with the introduction of RCU_EXPERT and smart selection of 
> defaults. (very nice!)
> 
> There's a single interactive option left when a user configures RCU 'anew' and 
> keeps CONFIG_RCU_EXPERT disabled:
> 
>       Offload RCU callback processing from boot-selected CPUs (RCU_NOCB_CPU) [N/y/?] (NEW)
> 
> I'm wondering whether we could put this behind CONFIG_RCU_EXPERT as well, and 
> disable it by default? 
> 
> The argument for this is very simple IMHO: NO_HZ_FULL depends on nocb_cpu support, 
> and actually selects it, so there's no way to misconfigure it.
> 
> Without NO_HZ_FULL it might still make sense to use nocb_cpu, but it's very much 
> an advanced, expert option.
> 
> With this change we could make RCU essentially configuration free in the common 
> case! :-)

And the -rt folks could presumably add a select clause as well, where
needed.  And I cannot recall a specific reason for not doing so.  I have
queued the following patch, so let's see how it goes.  ;-)

							Thanx, Paul

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

    rcu: Hide RCU_NOCB_CPU behind RCU_EXPERT
    
    This commit prevents Kconfig from asking the user about RCU_NOCB_CPU
    unless the user really wants to be asked.
    
    Reported-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Cc: Thomas Gleixner <tglx@linutronix.de>

diff --git a/init/Kconfig b/init/Kconfig
index 5b8726c10685..67a0156fa091 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -697,6 +697,7 @@ config RCU_BOOST_DELAY
 config RCU_NOCB_CPU
 	bool "Offload RCU callback processing from boot-selected CPUs"
 	depends on TREE_RCU || PREEMPT_RCU
+	depends on RCU_EXPERT
 	default n
 	help
 	  Use this option to reduce OS jitter for aggressive HPC or