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Message-ID: <20080301193903.GC15887@linux.vnet.ibm.com> Date: Sat, 1 Mar 2008 11:39:03 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Roman Zippel <zippel@...ux-m68k.org> Cc: ego@...ibm.com, linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org, 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> Subject: Re: [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@...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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