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Date: Sat, 27 Dec 2008 15:39:26 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Ingo Molnar <mingo@...e.hu>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de,
akpm@...ux-foundation.org, ralf@...ux-mips.org,
kernel@...tstofly.org, hskinnemoen@...el.com, cooloney@...nel.org,
tony.luck@...el.com, geert@...ux-m68k.org, zippel@...ux-m68k.org,
jwboyer@...ux.vnet.ibm.com, mporter@...nel.crashing.org,
grant.likely@...retlab.ca, paulus@...ba.org,
benh@...nel.crashing.org, dhowells@...hat.com,
yasutake.koichi@...panasonic.com, lethal@...ux-sh.org,
hpa@...or.com
Subject: Re: [PATCH, -tip] Make hierarchical RCU be the default
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@...ux.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@...e.hu>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.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
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