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rcu: move PREEMPT_RCU config option back under PREEMPT
The original preemptible-RCU patch put the choice between classic and preemptible RCU into kernel/Kconfig.preempt, which resulted in build failures on machines not supporting CONFIG_PREEMPT. This choice was therefore moved to init/Kconfig, which worked, but placed the choice between classic and preemptible RCU at the top level, a very obtuse choice indeed. This patch changes from the Kconfig "choice" mechanism to a pair of booleans, only one of which (CONFIG_PREEMPT_RCU) is user-visible, and is located in kernel/Kconfig.preempt, where one would expect it to be. The other (CONFIG_CLASSIC_RCU) is in init/Kconfig so that it is available to all architectures, hopefully avoiding build breakage. Thanks to Roman Zippel for suggesting this approach. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Ingo Molnar <mingo@elte.hu> Acked-by: Steven Rostedt <rostedt@goodmis.org> Cc: Dipankar Sarma <dipankar@in.ibm.com> Cc: Josh Triplett <josh@freedesktop.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Sam Ravnborg <sam@ravnborg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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init/Kconfig
34
init/Kconfig
@ -865,38 +865,10 @@ source "block/Kconfig"
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config PREEMPT_NOTIFIERS
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bool
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choice
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prompt "RCU implementation type:"
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default CLASSIC_RCU
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help
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This allows you to choose either the classic RCU implementation
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that is designed for best read-side performance on non-realtime
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systems, or the preemptible RCU implementation for best latency
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on realtime systems. Note that some kernel preemption modes
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will restrict your choice.
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Select the default if you are unsure.
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config CLASSIC_RCU
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bool "Classic RCU"
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def_bool !PREEMPT_RCU
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help
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This option selects the classic RCU implementation that is
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designed for best read-side performance on non-realtime
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systems.
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Say Y if you are unsure.
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config PREEMPT_RCU
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bool "Preemptible RCU"
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depends on PREEMPT
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help
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This option reduces the latency of the kernel by making certain
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RCU sections preemptible. Normally RCU code is non-preemptible, if
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this option is selected then read-only RCU sections become
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preemptible. This helps latency, but may expose bugs due to
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now-naive assumptions about each RCU read-side critical section
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remaining on a given CPU through its execution.
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Say N if you are unsure.
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endchoice
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systems. Classic RCU is the default. Note that the
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PREEMPT_RCU symbol is used to select/deselect this option.
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@ -52,8 +52,23 @@ config PREEMPT
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endchoice
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config PREEMPT_RCU
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bool "Preemptible RCU"
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depends on PREEMPT
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default n
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help
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This option reduces the latency of the kernel by making certain
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RCU sections preemptible. Normally RCU code is non-preemptible, if
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this option is selected then read-only RCU sections become
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preemptible. This helps latency, but may expose bugs due to
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now-naive assumptions about each RCU read-side critical section
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remaining on a given CPU through its execution.
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Say N if you are unsure.
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config RCU_TRACE
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bool "Enable tracing for RCU - currently stats in debugfs"
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depends on PREEMPT_RCU
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select DEBUG_FS
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default y
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help
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