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Message-ID: <20080304202207.GA10926@linux.vnet.ibm.com> Date: Tue, 4 Mar 2008 12:22:07 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org Cc: ego@...ibm.com, Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>, Dipankar Sarma <dipankar@...ibm.com>, Ted Tso <tytso@...ibm.com>, dvhltc@...ibm.com, Oleg Nesterov <oleg@...sign.ru>, Andrew Morton <akpm@...ux-foundation.org>, bunk@...nel.org, Josh Triplett <josh@...edesktop.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, zippel@...ux-m68k.org Subject: [PATCH] move PREEMPT_RCU config option back under PREEMPT Hello! 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. I have tested this, but sadly do not have access to a machine that does not support CONFIG_PREEMPT. However, I did edit my config in an attempt to simulate this situation. Signed-off-by: Paul E. McKenney <paulmck@...ux.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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