xref: /openbmc/linux/init/Kconfig (revision 80daa560)

config ARCH
	string
	option env="ARCH"

config KERNELVERSION
	string
	option env="KERNELVERSION"

config DEFCONFIG_LIST
	string
	depends on !UML
	option defconfig_list
	default "/lib/modules/$UNAME_RELEASE/.config"
	default "/etc/kernel-config"
	default "/boot/config-$UNAME_RELEASE"
	default "arch/$ARCH/defconfig"

menu "General setup"

config EXPERIMENTAL
	bool "Prompt for development and/or incomplete code/drivers"
	---help---
	  Some of the various things that Linux supports (such as network
	  drivers, file systems, network protocols, etc.) can be in a state
	  of development where the functionality, stability, or the level of
	  testing is not yet high enough for general use. This is usually
	  known as the "alpha-test" phase among developers. If a feature is
	  currently in alpha-test, then the developers usually discourage
	  uninformed widespread use of this feature by the general public to
	  avoid "Why doesn't this work?" type mail messages. However, active
	  testing and use of these systems is welcomed. Just be aware that it
	  may not meet the normal level of reliability or it may fail to work
	  in some special cases. Detailed bug reports from people familiar
	  with the kernel internals are usually welcomed by the developers
	  (before submitting bug reports, please read the documents
	  <file:README>, <file:MAINTAINERS>, <file:REPORTING-BUGS>,
	  <file:Documentation/BUG-HUNTING>, and
	  <file:Documentation/oops-tracing.txt> in the kernel source).

	  This option will also make obsoleted drivers available. These are
	  drivers that have been replaced by something else, and/or are
	  scheduled to be removed in a future kernel release.

	  Unless you intend to help test and develop a feature or driver that
	  falls into this category, or you have a situation that requires
	  using these features, you should probably say N here, which will
	  cause the configurator to present you with fewer choices. If
	  you say Y here, you will be offered the choice of using features or
	  drivers that are currently considered to be in the alpha-test phase.

config BROKEN
	bool

config BROKEN_ON_SMP
	bool
	depends on BROKEN || !SMP
	default y

config LOCK_KERNEL
	bool
	depends on SMP || PREEMPT
	default y

config INIT_ENV_ARG_LIMIT
	int
	default 32 if !UML
	default 128 if UML
	help
	  Maximum of each of the number of arguments and environment
	  variables passed to init from the kernel command line.


config LOCALVERSION
	string "Local version - append to kernel release"
	help
	  Append an extra string to the end of your kernel version.
	  This will show up when you type uname, for example.
	  The string you set here will be appended after the contents of
	  any files with a filename matching localversion* in your
	  object and source tree, in that order.  Your total string can
	  be a maximum of 64 characters.

config LOCALVERSION_AUTO
	bool "Automatically append version information to the version string"
	default y
	help
	  This will try to automatically determine if the current tree is a
	  release tree by looking for git tags that belong to the current
	  top of tree revision.

	  A string of the format -gxxxxxxxx will be added to the localversion
	  if a git-based tree is found.  The string generated by this will be
	  appended after any matching localversion* files, and after the value
	  set in CONFIG_LOCALVERSION.

	  (The actual string used here is the first eight characters produced
	  by running the command:

	    $ git rev-parse --verify HEAD

	  which is done within the script "scripts/setlocalversion".)

config SWAP
	bool "Support for paging of anonymous memory (swap)"
	depends on MMU && BLOCK
	default y
	help
	  This option allows you to choose whether you want to have support
	  for so called swap devices or swap files in your kernel that are
	  used to provide more virtual memory than the actual RAM present
	  in your computer.  If unsure say Y.

config SYSVIPC
	bool "System V IPC"
	---help---
	  Inter Process Communication is a suite of library functions and
	  system calls which let processes (running programs) synchronize and
	  exchange information. It is generally considered to be a good thing,
	  and some programs won't run unless you say Y here. In particular, if
	  you want to run the DOS emulator dosemu under Linux (read the
	  DOSEMU-HOWTO, available from <http://www.tldp.org/docs.html#howto>),
	  you'll need to say Y here.

	  You can find documentation about IPC with "info ipc" and also in
	  section 6.4 of the Linux Programmer's Guide, available from
	  <http://www.tldp.org/guides.html>.

config SYSVIPC_SYSCTL
	bool
	depends on SYSVIPC
	depends on SYSCTL
	default y

config POSIX_MQUEUE
	bool "POSIX Message Queues"
	depends on NET && EXPERIMENTAL
	---help---
	  POSIX variant of message queues is a part of IPC. In POSIX message
	  queues every message has a priority which decides about succession
	  of receiving it by a process. If you want to compile and run
	  programs written e.g. for Solaris with use of its POSIX message
	  queues (functions mq_*) say Y here.

	  POSIX message queues are visible as a filesystem called 'mqueue'
	  and can be mounted somewhere if you want to do filesystem
	  operations on message queues.

	  If unsure, say Y.

config BSD_PROCESS_ACCT
	bool "BSD Process Accounting"
	help
	  If you say Y here, a user level program will be able to instruct the
	  kernel (via a special system call) to write process accounting
	  information to a file: whenever a process exits, information about
	  that process will be appended to the file by the kernel.  The
	  information includes things such as creation time, owning user,
	  command name, memory usage, controlling terminal etc. (the complete
	  list is in the struct acct in <file:include/linux/acct.h>).  It is
	  up to the user level program to do useful things with this
	  information.  This is generally a good idea, so say Y.

config BSD_PROCESS_ACCT_V3
	bool "BSD Process Accounting version 3 file format"
	depends on BSD_PROCESS_ACCT
	default n
	help
	  If you say Y here, the process accounting information is written
	  in a new file format that also logs the process IDs of each
	  process and it's parent. Note that this file format is incompatible
	  with previous v0/v1/v2 file formats, so you will need updated tools
	  for processing it. A preliminary version of these tools is available
	  at <http://www.physik3.uni-rostock.de/tim/kernel/utils/acct/>.

config TASKSTATS
	bool "Export task/process statistics through netlink (EXPERIMENTAL)"
	depends on NET
	default n
	help
	  Export selected statistics for tasks/processes through the
	  generic netlink interface. Unlike BSD process accounting, the
	  statistics are available during the lifetime of tasks/processes as
	  responses to commands. Like BSD accounting, they are sent to user
	  space on task exit.

	  Say N if unsure.

config TASK_DELAY_ACCT
	bool "Enable per-task delay accounting (EXPERIMENTAL)"
	depends on TASKSTATS
	help
	  Collect information on time spent by a task waiting for system
	  resources like cpu, synchronous block I/O completion and swapping
	  in pages. Such statistics can help in setting a task's priorities
	  relative to other tasks for cpu, io, rss limits etc.

	  Say N if unsure.

config TASK_XACCT
	bool "Enable extended accounting over taskstats (EXPERIMENTAL)"
	depends on TASKSTATS
	help
	  Collect extended task accounting data and send the data
	  to userland for processing over the taskstats interface.

	  Say N if unsure.

config TASK_IO_ACCOUNTING
	bool "Enable per-task storage I/O accounting (EXPERIMENTAL)"
	depends on TASK_XACCT
	help
	  Collect information on the number of bytes of storage I/O which this
	  task has caused.

	  Say N if unsure.

config USER_NS
	bool "User Namespaces (EXPERIMENTAL)"
	default n
	depends on EXPERIMENTAL
	help
	  Support user namespaces.  This allows containers, i.e.
	  vservers, to use user namespaces to provide different
	  user info for different servers.  If unsure, say N.

config PID_NS
	bool "PID Namespaces (EXPERIMENTAL)"
	default n
	depends on EXPERIMENTAL
	help
	  Suport process id namespaces.  This allows having multiple
	  process with the same pid as long as they are in different
	  pid namespaces.  This is a building block of containers.

	  Unless you want to work with an experimental feature
	  say N here.

config AUDIT
	bool "Auditing support"
	depends on NET
	help
	  Enable auditing infrastructure that can be used with another
	  kernel subsystem, such as SELinux (which requires this for
	  logging of avc messages output).  Does not do system-call
	  auditing without CONFIG_AUDITSYSCALL.

config AUDITSYSCALL
	bool "Enable system-call auditing support"
	depends on AUDIT && (X86 || PPC || PPC64 || S390 || IA64 || UML || SPARC64|| SUPERH)
	default y if SECURITY_SELINUX
	help
	  Enable low-overhead system-call auditing infrastructure that
	  can be used independently or with another kernel subsystem,
	  such as SELinux.  To use audit's filesystem watch feature, please
	  ensure that INOTIFY is configured.

config AUDIT_TREE
	def_bool y
	depends on AUDITSYSCALL && INOTIFY

config IKCONFIG
	tristate "Kernel .config support"
	---help---
	  This option enables the complete Linux kernel ".config" file
	  contents to be saved in the kernel. It provides documentation
	  of which kernel options are used in a running kernel or in an
	  on-disk kernel.  This information can be extracted from the kernel
	  image file with the script scripts/extract-ikconfig and used as
	  input to rebuild the current kernel or to build another kernel.
	  It can also be extracted from a running kernel by reading
	  /proc/config.gz if enabled (below).

config IKCONFIG_PROC
	bool "Enable access to .config through /proc/config.gz"
	depends on IKCONFIG && PROC_FS
	---help---
	  This option enables access to the kernel configuration file
	  through /proc/config.gz.

config LOG_BUF_SHIFT
	int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
	range 12 21
	default 17 if S390 || LOCKDEP
	default 16 if X86_NUMAQ || IA64
	default 15 if SMP
	default 14
	help
	  Select kernel log buffer size as a power of 2.
	  Defaults and Examples:
	  	     17 => 128 KB for S/390
		     16 => 64 KB for x86 NUMAQ or IA-64
	             15 => 32 KB for SMP
	             14 => 16 KB for uniprocessor
		     13 =>  8 KB
		     12 =>  4 KB

config CGROUPS
	bool "Control Group support"
	help
	  This option will let you use process cgroup subsystems
	  such as Cpusets

	  Say N if unsure.

config CGROUP_DEBUG
	bool "Example debug cgroup subsystem"
	depends on CGROUPS
	help
	  This option enables a simple cgroup subsystem that
	  exports useful debugging information about the cgroups
	  framework

	  Say N if unsure

config CGROUP_NS
        bool "Namespace cgroup subsystem"
        depends on CGROUPS
        help
          Provides a simple namespace cgroup subsystem to
          provide hierarchical naming of sets of namespaces,
          for instance virtual servers and checkpoint/restart
          jobs.

config CPUSETS
	bool "Cpuset support"
	depends on SMP && CGROUPS
	help
	  This option will let you create and manage CPUSETs which
	  allow dynamically partitioning a system into sets of CPUs and
	  Memory Nodes and assigning tasks to run only within those sets.
	  This is primarily useful on large SMP or NUMA systems.

	  Say N if unsure.

config FAIR_GROUP_SCHED
	bool "Fair group CPU scheduler"
	default y
	help
	  This feature lets CPU scheduler recognize task groups and control CPU
	  bandwidth allocation to such task groups.

choice
	depends on FAIR_GROUP_SCHED
	prompt "Basis for grouping tasks"
	default FAIR_USER_SCHED

config FAIR_USER_SCHED
	bool "user id"
	help
	  This option will choose userid as the basis for grouping
	  tasks, thus providing equal CPU bandwidth to each user.

config FAIR_CGROUP_SCHED
	bool "Control groups"
 	depends on CGROUPS
 	help
	  This option allows you to create arbitrary task groups
	  using the "cgroup" pseudo filesystem and control
	  the cpu bandwidth allocated to each such task group.
	  Refer to Documentation/cgroups.txt for more information
	  on "cgroup" pseudo filesystem.

endchoice

config CGROUP_CPUACCT
	bool "Simple CPU accounting cgroup subsystem"
	depends on CGROUPS
	help
	  Provides a simple Resource Controller for monitoring the
	  total CPU consumed by the tasks in a cgroup

config SYSFS_DEPRECATED
	bool "Create deprecated sysfs files"
	depends on SYSFS
	default y
	help
	  This option creates deprecated symlinks such as the
	  "device"-link, the <subsystem>:<name>-link, and the
	  "bus"-link. It may also add deprecated key in the
	  uevent environment.
	  None of these features or values should be used today, as
	  they export driver core implementation details to userspace
	  or export properties which can't be kept stable across kernel
	  releases.

	  If enabled, this option will also move any device structures
	  that belong to a class, back into the /sys/class hierarchy, in
	  order to support older versions of udev.

	  If you are using a distro that was released in 2006 or later,
	  it should be safe to say N here.

config PROC_PID_CPUSET
	bool "Include legacy /proc/<pid>/cpuset file"
	depends on CPUSETS
	default y

config RELAY
	bool "Kernel->user space relay support (formerly relayfs)"
	help
	  This option enables support for relay interface support in
	  certain file systems (such as debugfs).
	  It is designed to provide an efficient mechanism for tools and
	  facilities to relay large amounts of data from kernel space to
	  user space.

	  If unsure, say N.

config BLK_DEV_INITRD
	bool "Initial RAM filesystem and RAM disk (initramfs/initrd) support"
	depends on BROKEN || !FRV
	help
	  The initial RAM filesystem is a ramfs which is loaded by the
	  boot loader (loadlin or lilo) and that is mounted as root
	  before the normal boot procedure. It is typically used to
	  load modules needed to mount the "real" root file system,
	  etc. See <file:Documentation/initrd.txt> for details.

	  If RAM disk support (BLK_DEV_RAM) is also included, this
	  also enables initial RAM disk (initrd) support and adds
	  15 Kbytes (more on some other architectures) to the kernel size.

	  If unsure say Y.

if BLK_DEV_INITRD

source "usr/Kconfig"

endif

config CC_OPTIMIZE_FOR_SIZE
	bool "Optimize for size (Look out for broken compilers!)"
	default y
	depends on ARM || H8300 || SUPERH || EXPERIMENTAL
	help
	  Enabling this option will pass "-Os" instead of "-O2" to gcc
	  resulting in a smaller kernel.

	  WARNING: some versions of gcc may generate incorrect code with this
	  option.  If problems are observed, a gcc upgrade may be needed.

	  If unsure, say N.

config SYSCTL
	bool

menuconfig EMBEDDED
	bool "Configure standard kernel features (for small systems)"
	help
	  This option allows certain base kernel options and settings
          to be disabled or tweaked. This is for specialized
          environments which can tolerate a "non-standard" kernel.
          Only use this if you really know what you are doing.

config UID16
	bool "Enable 16-bit UID system calls" if EMBEDDED
	depends on ARM || BLACKFIN || CRIS || FRV || H8300 || X86_32 || M68K || (S390 && !64BIT) || SUPERH || SPARC32 || (SPARC64 && SPARC32_COMPAT) || UML || (X86_64 && IA32_EMULATION)
	default y
	help
	  This enables the legacy 16-bit UID syscall wrappers.

config SYSCTL_SYSCALL
	bool "Sysctl syscall support" if EMBEDDED
	default y
	select SYSCTL
	---help---
	  sys_sysctl uses binary paths that have been found challenging
	  to properly maintain and use.  The interface in /proc/sys
	  using paths with ascii names is now the primary path to this
	  information.

	  Almost nothing using the binary sysctl interface so if you are
	  trying to save some space it is probably safe to disable this,
	  making your kernel marginally smaller.

	  If unsure say Y here.

config KALLSYMS
	 bool "Load all symbols for debugging/ksymoops" if EMBEDDED
	 default y
	 help
	   Say Y here to let the kernel print out symbolic crash information and
	   symbolic stack backtraces. This increases the size of the kernel
	   somewhat, as all symbols have to be loaded into the kernel image.

config KALLSYMS_ALL
	bool "Include all symbols in kallsyms"
	depends on DEBUG_KERNEL && KALLSYMS
	help
	   Normally kallsyms only contains the symbols of functions, for nicer
	   OOPS messages.  Some debuggers can use kallsyms for other
	   symbols too: say Y here to include all symbols, if you need them
	   and you don't care about adding 300k to the size of your kernel.

	   Say N.

config KALLSYMS_EXTRA_PASS
	bool "Do an extra kallsyms pass"
	depends on KALLSYMS
	help
	   If kallsyms is not working correctly, the build will fail with
	   inconsistent kallsyms data.  If that occurs, log a bug report and
	   turn on KALLSYMS_EXTRA_PASS which should result in a stable build.
	   Always say N here unless you find a bug in kallsyms, which must be
	   reported.  KALLSYMS_EXTRA_PASS is only a temporary workaround while
	   you wait for kallsyms to be fixed.


config HOTPLUG
	bool "Support for hot-pluggable devices" if EMBEDDED
	default y
	help
	  This option is provided for the case where no hotplug or uevent
	  capabilities is wanted by the kernel.  You should only consider
	  disabling this option for embedded systems that do not use modules, a
	  dynamic /dev tree, or dynamic device discovery.  Just say Y.

config PRINTK
	default y
	bool "Enable support for printk" if EMBEDDED
	help
	  This option enables normal printk support. Removing it
	  eliminates most of the message strings from the kernel image
	  and makes the kernel more or less silent. As this makes it
	  very difficult to diagnose system problems, saying N here is
	  strongly discouraged.

config BUG
	bool "BUG() support" if EMBEDDED
	default y
	help
          Disabling this option eliminates support for BUG and WARN, reducing
          the size of your kernel image and potentially quietly ignoring
          numerous fatal conditions. You should only consider disabling this
          option for embedded systems with no facilities for reporting errors.
          Just say Y.

config ELF_CORE
	default y
	bool "Enable ELF core dumps" if EMBEDDED
	help
	  Enable support for generating core dumps. Disabling saves about 4k.

config BASE_FULL
	default y
	bool "Enable full-sized data structures for core" if EMBEDDED
	help
	  Disabling this option reduces the size of miscellaneous core
	  kernel data structures. This saves memory on small machines,
	  but may reduce performance.

config FUTEX
	bool "Enable futex support" if EMBEDDED
	default y
	select RT_MUTEXES
	help
	  Disabling this option will cause the kernel to be built without
	  support for "fast userspace mutexes".  The resulting kernel may not
	  run glibc-based applications correctly.

config ANON_INODES
	bool

config EPOLL
	bool "Enable eventpoll support" if EMBEDDED
	default y
	select ANON_INODES
	help
	  Disabling this option will cause the kernel to be built without
	  support for epoll family of system calls.

config SIGNALFD
	bool "Enable signalfd() system call" if EMBEDDED
	select ANON_INODES
	default y
	help
	  Enable the signalfd() system call that allows to receive signals
	  on a file descriptor.

	  If unsure, say Y.

config TIMERFD
	bool "Enable timerfd() system call" if EMBEDDED
	select ANON_INODES
	depends on BROKEN
	default y
	help
	  Enable the timerfd() system call that allows to receive timer
	  events on a file descriptor.

	  If unsure, say Y.

config EVENTFD
	bool "Enable eventfd() system call" if EMBEDDED
	select ANON_INODES
	default y
	help
	  Enable the eventfd() system call that allows to receive both
	  kernel notification (ie. KAIO) or userspace notifications.

	  If unsure, say Y.

config SHMEM
	bool "Use full shmem filesystem" if EMBEDDED
	default y
	depends on MMU
	help
	  The shmem is an internal filesystem used to manage shared memory.
	  It is backed by swap and manages resource limits. It is also exported
	  to userspace as tmpfs if TMPFS is enabled. Disabling this
	  option replaces shmem and tmpfs with the much simpler ramfs code,
	  which may be appropriate on small systems without swap.

config VM_EVENT_COUNTERS
	default y
	bool "Enable VM event counters for /proc/vmstat" if EMBEDDED
	help
	  VM event counters are needed for event counts to be shown.
	  This option allows the disabling of the VM event counters
	  on EMBEDDED systems.  /proc/vmstat will only show page counts
	  if VM event counters are disabled.

config SLUB_DEBUG
	default y
	bool "Enable SLUB debugging support" if EMBEDDED
	depends on SLUB
	help
	  SLUB has extensive debug support features. Disabling these can
	  result in significant savings in code size. This also disables
	  SLUB sysfs support. /sys/slab will not exist and there will be
	  no support for cache validation etc.

choice
	prompt "Choose SLAB allocator"
	default SLUB
	help
	   This option allows to select a slab allocator.

config SLAB
	bool "SLAB"
	help
	  The regular slab allocator that is established and known to work
	  well in all environments. It organizes cache hot objects in
	  per cpu and per node queues. SLAB is the default choice for
	  a slab allocator.

config SLUB
	bool "SLUB (Unqueued Allocator)"
	help
	   SLUB is a slab allocator that minimizes cache line usage
	   instead of managing queues of cached objects (SLAB approach).
	   Per cpu caching is realized using slabs of objects instead
	   of queues of objects. SLUB can use memory efficiently
	   and has enhanced diagnostics.

config SLOB
	depends on EMBEDDED
	bool "SLOB (Simple Allocator)"
	help
	   SLOB replaces the SLAB allocator with a drastically simpler
	   allocator.  SLOB is more space efficient than SLAB but does not
	   scale well (single lock for all operations) and is also highly
	   susceptible to fragmentation. SLUB can accomplish a higher object
	   density. It is usually better to use SLUB instead of SLOB.

endchoice

endmenu		# General setup

config SLABINFO
	bool
	depends on PROC_FS
	depends on SLAB || SLUB
	default y

config RT_MUTEXES
	boolean
	select PLIST

config TINY_SHMEM
	default !SHMEM
	bool

config BASE_SMALL
	int
	default 0 if BASE_FULL
	default 1 if !BASE_FULL

menuconfig MODULES
	bool "Enable loadable module support"
	help
	  Kernel modules are small pieces of compiled code which can
	  be inserted in the running kernel, rather than being
	  permanently built into the kernel.  You use the "modprobe"
	  tool to add (and sometimes remove) them.  If you say Y here,
	  many parts of the kernel can be built as modules (by
	  answering M instead of Y where indicated): this is most
	  useful for infrequently used options which are not required
	  for booting.  For more information, see the man pages for
	  modprobe, lsmod, modinfo, insmod and rmmod.

	  If you say Y here, you will need to run "make
	  modules_install" to put the modules under /lib/modules/
	  where modprobe can find them (you may need to be root to do
	  this).

	  If unsure, say Y.

config MODULE_UNLOAD
	bool "Module unloading"
	depends on MODULES
	help
	  Without this option you will not be able to unload any
	  modules (note that some modules may not be unloadable
	  anyway), which makes your kernel slightly smaller and
	  simpler.  If unsure, say Y.

config MODULE_FORCE_UNLOAD
	bool "Forced module unloading"
	depends on MODULE_UNLOAD && EXPERIMENTAL
	help
	  This option allows you to force a module to unload, even if the
	  kernel believes it is unsafe: the kernel will remove the module
	  without waiting for anyone to stop using it (using the -f option to
	  rmmod).  This is mainly for kernel developers and desperate users.
	  If unsure, say N.

config MODVERSIONS
	bool "Module versioning support"
	depends on MODULES
	help
	  Usually, you have to use modules compiled with your kernel.
	  Saying Y here makes it sometimes possible to use modules
	  compiled for different kernels, by adding enough information
	  to the modules to (hopefully) spot any changes which would
	  make them incompatible with the kernel you are running.  If
	  unsure, say N.

config MODULE_SRCVERSION_ALL
	bool "Source checksum for all modules"
	depends on MODULES
	help
	  Modules which contain a MODULE_VERSION get an extra "srcversion"
	  field inserted into their modinfo section, which contains a
    	  sum of the source files which made it.  This helps maintainers
	  see exactly which source was used to build a module (since
	  others sometimes change the module source without updating
	  the version).  With this option, such a "srcversion" field
	  will be created for all modules.  If unsure, say N.

config KMOD
	bool "Automatic kernel module loading"
	depends on MODULES
	help
	  Normally when you have selected some parts of the kernel to
	  be created as kernel modules, you must load them (using the
	  "modprobe" command) before you can use them. If you say Y
	  here, some parts of the kernel will be able to load modules
	  automatically: when a part of the kernel needs a module, it
	  runs modprobe with the appropriate arguments, thereby
	  loading the module if it is available.  If unsure, say Y.

config STOP_MACHINE
	bool
	default y
	depends on (SMP && MODULE_UNLOAD) || HOTPLUG_CPU
	help
	  Need stop_machine() primitive.

source "block/Kconfig"

config PREEMPT_NOTIFIERS
	bool

choice
	prompt "RCU implementation type:"
	default CLASSIC_RCU

config CLASSIC_RCU
	bool "Classic 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