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SubjectRe: [RFC PATCH 4/6] Preempt-RCU: Implementation
On Fri, Feb 29, 2008 at 01:38:15PM +0100, Roman Zippel wrote:
> Hi
>
> On Thu, 28 Feb 2008, Paul E. McKenney wrote:
>
> > > Why got this moved into init/Kconfig?
> >
> > Because there are some arches that don't have kernel/Kconfig.preempt,
> > its earlier home. Therefore, putting it into kernel/Kconfig.preempt
> > broke those arches' builds by supplying neither PREEMPT_RCU nor
> > CLASSIC_RCU.
> >
> > > Now it's somewhere in the root menu, not really belonging to anything.
> >
> > Do you have a suggested location?
> >
> > > Also why is this a choice? Are more RCU types planned?
> >
> > I don't expect additional drop-in replacements for RCU, though people
> > are certainly free to experiment if they wish. It is a choice because
> > this gives people a very clear idea of the two options and because
> > it makes the implementation a bit cleaner.
>
> I'd suggest to move PREEMPT_RCU back to Kconfig.preempt and if you really
> need the second symbol leave this behind (maybe with a comment):
>
> config CLASSIC_RCU
> def_bool !PREEMPT_RCU
>
> Once there are more options, we can still look for a better place...
>
> Also could you please add a proper dependency to RCU_TRACE on PREEMPT_RCU,
> so that this condition isn't needed anymore:
>
> ifeq ($(CONFIG_PREEMPT_RCU),y)
> obj-$(CONFIG_RCU_TRACE) += rcupreempt_trace.o
> endif

Is this what you had in mind? I don't have any way to test on a
system not supporting CONFIG_PREEMPT, but seems to work on x86.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

init/Kconfig | 34 +++-------------------------------
kernel/Kconfig.preempt | 15 +++++++++++++++
2 files changed, 18 insertions(+), 31 deletions(-)

diff -urpNa -X dontdiff linux-2.6.25-rc3/init/Kconfig linux-2.6.25-rc3-preempt_rcu/init/Kconfig
--- linux-2.6.25-rc3/init/Kconfig 2008-02-26 16:58:42.000000000 -0800
+++ linux-2.6.25-rc3-preempt_rcu/init/Kconfig 2008-03-01 11:30:59.000000000 -0800
@@ -860,38 +860,10 @@ source "block/Kconfig"
config PREEMPT_NOTIFIERS
bool

-choice
- prompt "RCU implementation type:"
- default CLASSIC_RCU
- help
- This allows you to choose either the classic RCU implementation
- that is designed for best read-side performance on non-realtime
- systems, or the preemptible RCU implementation for best latency
- on realtime systems. Note that some kernel preemption modes
- will restrict your choice.
-
- Select the default if you are unsure.
-
config CLASSIC_RCU
- bool "Classic RCU"
+ def_bool !PREEMPT_RCU
help
This option selects the classic RCU implementation that is
designed for best read-side performance on non-realtime
- systems.
-
- Say Y if you are unsure.
-
-config PREEMPT_RCU
- bool "Preemptible RCU"
- depends on PREEMPT
- help
- This option reduces the latency of the kernel by making certain
- RCU sections preemptible. Normally RCU code is non-preemptible, if
- this option is selected then read-only RCU sections become
- preemptible. This helps latency, but may expose bugs due to
- now-naive assumptions about each RCU read-side critical section
- remaining on a given CPU through its execution.
-
- Say N if you are unsure.
-
-endchoice
+ systems. Classic RCU is the default. Note that the
+ PREEMPT_RCU symbol is used to select/deselect this option.
diff -urpNa -X dontdiff linux-2.6.25-rc3/kernel/Kconfig.preempt linux-2.6.25-rc3-preempt_rcu/kernel/Kconfig.preempt
--- linux-2.6.25-rc3/kernel/Kconfig.preempt 2008-02-26 16:58:42.000000000 -0800
+++ linux-2.6.25-rc3-preempt_rcu/kernel/Kconfig.preempt 2008-03-01 11:35:39.000000000 -0800
@@ -52,8 +52,23 @@ config PREEMPT

endchoice

+config PREEMPT_RCU
+ bool "Preemptible RCU"
+ depends on PREEMPT
+ default n
+ help
+ This option reduces the latency of the kernel by making certain
+ RCU sections preemptible. Normally RCU code is non-preemptible, if
+ this option is selected then read-only RCU sections become
+ preemptible. This helps latency, but may expose bugs due to
+ now-naive assumptions about each RCU read-side critical section
+ remaining on a given CPU through its execution.
+
+ Say N if you are unsure.
+
config RCU_TRACE
bool "Enable tracing for RCU - currently stats in debugfs"
+ depends on PREEMPT_RCU
select DEBUG_FS
default y
help

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