Re: [PATCH, -tip] Make hierarchical RCU be the default

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

On Sat, Dec 27, 2008 at 03:27:31PM -0800, Paul E. McKenney wrote:
> On Sat, Dec 27, 2008 at 11:35:01AM +0100, Ingo Molnar wrote:
> > 
> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > > --- a/include/linux/hardirq.h
> > > +++ b/include/linux/hardirq.h
> > > @@ -118,7 +118,7 @@ static inline void account_system_vtime(struct task_struct *tsk)
> > >  }
> > >  #endif
> > >  
> > > -#if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU)
> > > +#if defined(CONFIG_NO_HZ) && !defined(CONFIG_FLAT_RCU)
> > 
> > this can be done in an even lazier fashion, via a ~5 lines patch.
> 
> I clearly need serious help on this laziness thing.  ;-)
> 
> > Just add FLAT_RCU as the new kconfig method, and make CONFIG_CLASSIC_RCU 
> > mirror it via something like:
> > 
> >   config CLASSIC_RCU
> >   bool
> >   default FLAT_RCU
> > 
> > that makes your patch only affect the Kconfig file - and makes it easily 
> > revertable, etc.
> 
> OK.  But don't I also need "select FLAT_RCU" so that all of the
> defconfig files containing "CLASSIC_RCU" do the right thing?  If I try
> that, "make defconfig" complains as follows:
> 
> 	init/Kconfig:941:error: found recursive dependency: FLAT_RCU -> CLASSIC_RCU -> FLAT_RCU -> <choice>
> 
> So unless I am missing a laziness opportunity in here somewhere, it
> seems to me that I need to either (1) change all the arch files so that
> what is now "CLASSIC_RCU" instead becomes "FLAT_RCU" or (2) change all
> in-code references to "CLASSIC_RCU" to instead reference "FLAT_RCU".
> 
> Unless there is some other way out, option #2 seems to me to be the
> lazier of the two, which is represented by the earlier patch.  This
> patch, updated to handle the RCU choice moving to init/Kconfig, appears
> below.  One consequence of this change is that RCU moves from the
> "Processor type and features" menu to the top level is xconfig and
> friends.
> 
> Which might be why Sam is suggesting HAVE_PREEMPT, which could allow it
> to stay where it is?
> 
> If so, another alternative would be to add kernel/Kconfig.rcu, and add
> it to all the arch/*/Kconfig files.  Then it would still appear in the
> "Processor type and features" menu, but would also be available to all
> arches.
> 
> Thoughts?

Silly me for assuming that all architectures had even vaguely similar
Kconfig menu layouts.  Ouch!!!

							Thanx, Paul

> ------------------------------------------------------------------------
> 
> This patch makes the new TREE_RCU be the default, pointing the old
> CLASSIC_RCU Kconfig symbol at it and introducing a new FLAT_RCU symbol
> to allow the old version to be selected.
> 
> This patch is -not- yet suitable for inclusion, only for testing in
> -tip, -next, and the like.
> 
> Suggested-by: Ingo Molnar <mingo@elte.hu>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  include/linux/hardirq.h  |    4 ++--
>  include/linux/pagemap.h  |    4 ++--
>  include/linux/rcupdate.h |    4 ++--
>  init/Kconfig             |   14 ++++++++++----
>  kernel/Makefile          |    2 +-
>  lib/Kconfig.debug        |    2 +-
>  6 files changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index 9b70b92..0afd180 100644
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -118,7 +118,7 @@ static inline void account_system_vtime(struct task_struct *tsk)
>  }
>  #endif
>  
> -#if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU)
> +#if defined(CONFIG_NO_HZ) && !defined(CONFIG_FLAT_RCU)
>  extern void rcu_irq_enter(void);
>  extern void rcu_irq_exit(void);
>  extern void rcu_nmi_enter(void);
> @@ -128,7 +128,7 @@ extern void rcu_nmi_exit(void);
>  # define rcu_irq_exit() do { } while (0)
>  # define rcu_nmi_enter() do { } while (0)
>  # define rcu_nmi_exit() do { } while (0)
> -#endif /* #if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU) */
> +#endif /* #if defined(CONFIG_NO_HZ) && !defined(CONFIG_FLAT_RCU) */
>  
>  /*
>   * It is safe to do non-atomic ops on ->hardirq_context,
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 709742b..5cfb133 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -140,7 +140,7 @@ static inline int page_cache_get_speculative(struct page *page)
>  {
>  	VM_BUG_ON(in_interrupt());
>  
> -#if !defined(CONFIG_SMP) && defined(CONFIG_CLASSIC_RCU)
> +#if !defined(CONFIG_SMP) && defined(CONFIG_FLAT_RCU)
>  # ifdef CONFIG_PREEMPT
>  	VM_BUG_ON(!in_atomic());
>  # endif
> @@ -178,7 +178,7 @@ static inline int page_cache_add_speculative(struct page *page, int count)
>  {
>  	VM_BUG_ON(in_interrupt());
>  
> -#if !defined(CONFIG_SMP) && defined(CONFIG_CLASSIC_RCU)
> +#if !defined(CONFIG_SMP) && defined(CONFIG_FLAT_RCU)
>  # ifdef CONFIG_PREEMPT
>  	VM_BUG_ON(!in_atomic());
>  # endif
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index bfd289a..819bf83 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -52,7 +52,7 @@ struct rcu_head {
>  	void (*func)(struct rcu_head *head);
>  };
>  
> -#if defined(CONFIG_CLASSIC_RCU)
> +#if defined(CONFIG_FLAT_RCU)
>  #include <linux/rcuclassic.h>
>  #elif defined(CONFIG_TREE_RCU)
>  #include <linux/rcutree.h>
> @@ -60,7 +60,7 @@ struct rcu_head {
>  #include <linux/rcupreempt.h>
>  #else
>  #error "Unknown RCU implementation specified to kernel configuration"
> -#endif /* #else #if defined(CONFIG_CLASSIC_RCU) */
> +#endif /* #else #if defined(CONFIG_FLAT_RCU) */
>  
>  #define RCU_HEAD_INIT 	{ .next = NULL, .func = NULL }
>  #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
> diff --git a/init/Kconfig b/init/Kconfig
> index 6b0fded..58d6575 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -928,16 +928,22 @@ source "block/Kconfig"
>  config PREEMPT_NOTIFIERS
>  	bool
>  
> +config CLASSIC_RCU
> +	bool
> +	select TREE_RCU
> +
>  choice
>  	prompt "RCU Implementation"
> -	default CLASSIC_RCU
> +	default TREE_RCU
>  
> -config CLASSIC_RCU
> -	bool "Classic RCU"
> +config FLAT_RCU
> +	bool "Flat (AKA classic) RCU"
>  	help
>  	  This option selects the classic RCU implementation that is
>  	  designed for best read-side performance on non-realtime
> -	  systems.
> +	  systems with modest numbers of CPUs.  Its flat bit-map
> +	  implementation makes it unsuitable for systems with hundreds
> +	  or thousands of CPUs.
>  
>  	  Select this option if you are unsure.
>  
> diff --git a/kernel/Makefile b/kernel/Makefile
> index b4fdbbf..73e9a94 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -73,7 +73,7 @@ obj-$(CONFIG_DETECT_SOFTLOCKUP) += softlockup.o
>  obj-$(CONFIG_GENERIC_HARDIRQS) += irq/
>  obj-$(CONFIG_SECCOMP) += seccomp.o
>  obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
> -obj-$(CONFIG_CLASSIC_RCU) += rcuclassic.o
> +obj-$(CONFIG_FLAT_RCU) += rcuclassic.o
>  obj-$(CONFIG_TREE_RCU) += rcutree.o
>  obj-$(CONFIG_PREEMPT_RCU) += rcupreempt.o
>  obj-$(CONFIG_TREE_RCU_TRACE) += rcutree_trace.o
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 465d822..a036720 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -621,7 +621,7 @@ config RCU_CPU_STALL_DETECTOR
>  
>  config RCU_CPU_STALL_DETECTOR
>  	bool "Check for stalled CPUs delaying RCU grace periods"
> -	depends on CLASSIC_RCU || TREE_RCU
> +	depends on TREE_RCU || FLAT_RCU
>  	default n
>  	help
>  	  This option causes RCU to printk information on which