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SubjectRe: [RFC PATCH 4/6] Preempt-RCU: Implementation

On Sat, 1 Mar 2008, Paul E. McKenney wrote:
> 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.

Actually Paul, you don't need to make this an "option". Simply the default
if PREEMPT_RCU is not selected. In otherwords, no prompt.

config CLASSIC_RCU
bool
depends on !PREEMPT_RCU
default y

Then, have the PREEMPT_RCU defined down below (as you already did), and
have it be the only option. That is, you can have normal RCU or select
to have PREEMPT_RCU.

If the user selects PREEMPT_RCU, CLASSIC_RCU will be unselected.

-- Steve

> -
> - 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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