Selecting this option causes timing information to be included in printk output. This allows you to measure the interval between kernel operations, including bootup operations. This is useful for identifying long delays in kernel startup. Or add printk.time=1 at boot-time. See Documentation/kernel-parameters.txt
Default log level for printk statements with no specified priority. This was hard-coded to KERN_WARNING since at least 2.6.10 but folks that are auditing their logs closely may want to set it to a lower priority.
Enable the __deprecated logic in the kernel build. Disable this to suppress the "warning: 'foo' is deprecated (declared at kernel/power/somefile.c:1234)" messages.
Enable the __must_check logic in the kernel build. Disable this to suppress the "warning: ignoring return value of 'foo', declared with attribute warn_unused_result" messages.
Tell gcc to warn at build time for stack frames larger than this. Setting this too low will cause a lot of warnings. Setting it to 0 disables the warning. Requires gcc 4.4
If you say Y here, you will have some control over the system even if the system crashes for example during kernel debugging (e.g., you will be able to flush the buffer cache to disk, reboot the system immediately or dump some status information). This is accomplished by pressing various keys while holding SysRq (Alt+PrintScreen). It also works on a serial console (on PC hardware at least), if you send a BREAK and then within 5 seconds a command keypress. The keys are documented in <file:Documentation/sysrq.txt>. Don't say Y unless you really know what this hack does.
Strip internal assembler-generated symbols during a link (symbols that look like '.Lxxx') so they don't pollute the output of get_wchan() and suchlike.
Unused but exported symbols make the kernel needlessly bigger. For that reason most of these unused exports will soon be removed. This option is provided temporarily to provide a transition period in case some external kernel module needs one of these symbols anyway. If you encounter such a case in your module, consider if you are actually using the right API. (rationale: since nobody in the kernel is using this in a module, there is a pretty good chance it's actually the wrong interface to use). If you really need the symbol, please send a mail to the linux kernel mailing list mentioning the symbol and why you really need it, and what the merge plan to the mainline kernel for your module is.
debugfs is a virtual file system that kernel developers use to put debugging files into. Enable this option to be able to read and write to these files. For detailed documentation on the debugfs API, see Documentation/DocBook/filesystems. If unsure, say N.
This option will extract the user-visible kernel headers whenever building the kernel, and will run basic sanity checks on them to ensure that exported files do not attempt to include files which were not exported, etc. If you're making modifications to header files which are relevant for userspace, say 'Y', and check the headers exported to $(INSTALL_HDR_PATH) (usually 'usr/include' in your build tree), to make sure they're suitable.
The section mismatch analysis checks if there are illegal references from one section to another section. Linux will during link or during runtime drop some sections and any use of code/data previously in these sections will most likely result in an oops. In the code functions and variables are annotated with __init, __devinit etc. (see full list in include/linux/init.h) which results in the code/data being placed in specific sections. The section mismatch analysis is always done after a full kernel build but enabling this option will in addition do the following: - Add the option -fno-inline-functions-called-once to gcc When inlining a function annotated __init in a non-init function we would lose the section information and thus the analysis would not catch the illegal reference. This option tells gcc to inline less but will also result in a larger kernel. - Run the section mismatch analysis for each module/built-in.o When we run the section mismatch analysis on vmlinux.o we lose valueble information about where the mismatch was introduced. Running the analysis for each module/built-in.o file will tell where the mismatch happens much closer to the source. The drawback is that we will report the same mismatch at least twice. - Enable verbose reporting from modpost to help solving the section mismatches reported.
Say Y here if you are developing drivers or trying to debug and identify kernel problems.
Enable this to generate a spurious interrupt as soon as a shared interrupt handler is registered, and just before one is deregistered. Drivers ought to be able to handle interrupts coming in at those points; some don't and need to be caught.
Say Y here to enable the kernel to act as a watchdog to detect hard and soft lockups. Softlockups are bugs that cause the kernel to loop in kernel mode for more than 60 seconds, without giving other tasks a chance to run. The current stack trace is displayed upon detection and the system will stay locked up. Hardlockups are bugs that cause the CPU to loop in kernel mode for more than 60 seconds, without letting other interrupts have a chance to run. The current stack trace is displayed upon detection and the system will stay locked up. The overhead should be minimal. A periodic hrtimer runs to generate interrupts and kick the watchdog task every 10-12 seconds. An NMI is generated every 60 seconds or so to check for hardlockups.
Say Y here to enable the kernel to panic on "hard lockups", which are bugs that cause the kernel to loop in kernel mode with interrupts disabled for more than 60 seconds. Say N if unsure.
Say Y here to enable the kernel to panic on "soft lockups", which are bugs that cause the kernel to loop in kernel mode for more than 60 seconds, without giving other tasks a chance to run. The panic can be used in combination with panic_timeout, to cause the system to reboot automatically after a lockup has been detected. This feature is useful for high-availability systems that have uptime guarantees and where a lockup must be resolved ASAP. Say N if unsure.
Say Y here to enable the kernel to detect "hung tasks", which are bugs that cause the task to be stuck in uninterruptible "D" state indefinitiley. When a hung task is detected, the kernel will print the current stack trace (which you should report), but the task will stay in uninterruptible state. If lockdep is enabled then all held locks will also be reported. This feature has negligible overhead.
This option controls the default timeout (in seconds) used to determine when a task has become non-responsive and should be considered hung. It can be adjusted at runtime via the kernel.hung_task_timeout sysctl or by writing a value to /proc/sys/kernel/hung_task_timeout. A timeout of 0 disables the check. The default is two minutes. Keeping the default should be fine in most cases.
Say Y here to enable the kernel to panic on "hung tasks", which are bugs that cause the kernel to leave a task stuck in uninterruptible "D" state. The panic can be used in combination with panic_timeout, to cause the system to reboot automatically after a hung task has been detected. This feature is useful for high-availability systems that have uptime guarantees and where a hung tasks must be resolved ASAP. Say N if unsure.
If you say Y here, the /proc/sched_debug file will be provided that can help debug the scheduler. The runtime overhead of this option is minimal.
If you say Y here, additional code will be inserted into the scheduler and related routines to collect statistics about scheduler behavior and provide them in /proc/schedstat. These stats may be useful for both tuning and debugging the scheduler If you aren't debugging the scheduler or trying to tune a specific application, you can say N to avoid the very slight overhead this adds.
If you say Y here, additional code will be inserted into the timer routines to collect statistics about kernel timers being reprogrammed. The statistics can be read from /proc/timer_stats. The statistics collection is started by writing 1 to /proc/timer_stats, writing 0 stops it. This feature is useful to collect information about timer usage patterns in kernel and userspace. This feature is lightweight if enabled in the kernel config but not activated (it defaults to deactivated on bootup and will only be activated if some application like powertop activates it explicitly).
If you say Y here, additional code will be inserted into the kernel to track the life time of various objects and validate the operations on those objects.
This enables the selftest of the object debug code.
This enables checks whether a k/v free operation frees an area which contains an object which has not been deactivated properly. This can make kmalloc/kfree-intensive workloads much slower.
If you say Y here, additional code will be inserted into the timer routines to track the life time of timer objects and validate the timer operations.
If you say Y here, additional code will be inserted into the work queue routines to track the life time of work objects and validate the work operations.
Enable this to turn on debugging of RCU list heads (call_rcu() usage).
If you say Y here, additional code will be inserted into the percpu counter routines to track the life time of percpu counter objects and validate the percpu counter operations.
Debug objects boot parameter default value
Say Y here to have the kernel do limited verification on memory allocation as well as poisoning memory on free to catch use of freed memory. This can make kmalloc/kfree-intensive workloads much slower.
Boot with debugging on by default. SLUB boots by default with the runtime debug capabilities switched off. Enabling this is equivalent to specifying the "slub_debug" parameter on boot. There is no support for more fine grained debug control like possible with slub_debug=xxx. SLUB debugging may be switched off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying "slub_debug=-".
SLUB statistics are useful to debug SLUBs allocation behavior in order find ways to optimize the allocator. This should never be enabled for production use since keeping statistics slows down the allocator by a few percentage points. The slabinfo command supports the determination of the most active slabs to figure out which slabs are relevant to a particular load. Try running: slabinfo -DA
Say Y here if you want to enable the memory leak detector. The memory allocation/freeing is traced in a way similar to the Boehm's conservative garbage collector, the difference being that the orphan objects are not freed but only shown in /sys/kernel/debug/kmemleak. Enabling this feature will introduce an overhead to memory allocations. See Documentation/kmemleak.txt for more details. Enabling DEBUG_SLAB or SLUB_DEBUG may increase the chances of finding leaks due to the slab objects poisoning. In order to access the kmemleak file, debugfs needs to be mounted (usually at /sys/kernel/debug).
Kmemleak must track all the memory allocations to avoid reporting false positives. Since memory may be allocated or freed before kmemleak is initialised, an early log buffer is used to store these actions. If kmemleak reports "early log buffer exceeded", please increase this value.
This option enables a module that explicitly leaks memory. If unsure, say N.
Say Y here to disable kmemleak by default. It can then be enabled on the command line via kmemleak=on.
If you say Y here then the kernel will use a debug variant of the commonly used smp_processor_id() function and will print warnings if kernel code uses it in a preemption-unsafe way. Also, the kernel will detect preemption count underflows.
This allows rt mutex semantics violations and rt mutex related deadlocks (lockups) to be detected and reported automatically.
This option enables a rt-mutex tester.
Say Y here and build SMP to catch missing spinlock initialization and certain other kinds of spinlock errors commonly made. This is best used in conjunction with the NMI watchdog so that spinlock deadlocks are also debuggable.
This feature allows mutex semantics violations to be detected and reported.
This feature will check whether any held lock (spinlock, rwlock, mutex or rwsem) is incorrectly freed by the kernel, via any of the memory-freeing routines (kfree(), kmem_cache_free(), free_pages(), vfree(), etc.), whether a live lock is incorrectly reinitialized via spin_lock_init()/mutex_init()/etc., or whether there is any lock held during task exit.
This feature enables the kernel to prove that all locking that occurs in the kernel runtime is mathematically correct: that under no circumstance could an arbitrary (and not yet triggered) combination of observed locking sequences (on an arbitrary number of CPUs, running an arbitrary number of tasks and interrupt contexts) cause a deadlock. In short, this feature enables the kernel to report locking related deadlocks before they actually occur. The proof does not depend on how hard and complex a deadlock scenario would be to trigger: how many participant CPUs, tasks and irq-contexts would be needed for it to trigger. The proof also does not depend on timing: if a race and a resulting deadlock is possible theoretically (no matter how unlikely the race scenario is), it will be proven so and will immediately be reported by the kernel (once the event is observed that makes the deadlock theoretically possible). If a deadlock is impossible (i.e. the locking rules, as observed by the kernel, are mathematically correct), the kernel reports nothing. NOTE: this feature can also be enabled for rwlocks, mutexes and rwsems - in which case all dependencies between these different locking variants are observed and mapped too, and the proof of observed correctness is also maintained for an arbitrary combination of these separate locking variants. For more details, see Documentation/lockdep-design.txt.
This feature enables lockdep extensions that check for correct use of RCU APIs. This is currently under development. Say Y if you want to debug RCU usage or help work on the PROVE_RCU feature. Say N if you are unsure.
By itself, PROVE_RCU will disable checking upon issuing the first warning (or "splat"). This feature prevents such disabling, allowing multiple RCU-lockdep warnings to be printed on a single reboot. Say Y to allow multiple RCU-lockdep warnings per boot. Say N if you are unsure.
This feature enables the __rcu sparse annotation for RCU-protected pointers. This annotation will cause sparse to flag any non-RCU used of annotated pointers. This can be helpful when debugging RCU usage. Please note that this feature is not intended to enforce code cleanliness; it is instead merely a debugging aid. Say Y to make sparse flag questionable use of RCU-protected pointers Say N if you are unsure.
This feature enables tracking lock contention points For more details, see Documentation/lockstat.txt This also enables lock events required by "perf lock", subcommand of perf. If you want to use "perf lock", you also need to turn on CONFIG_EVENT_TRACING. CONFIG_LOCK_STAT defines "contended" and "acquired" lock events. (CONFIG_LOCKDEP defines "acquire" and "release" events.)
If you say Y here, the lock dependency engine will do additional runtime checks to debug itself, at the price of more runtime overhead.
Enables hooks to interrupt enabling and disabling for either tracing or lock debugging.
If you say Y here, various routines which may sleep will become very noisy if they are called with a spinlock held.
Say Y here if you want the kernel to run a short self-test during bootup. The self-test checks whether common types of locking bugs are detected by debugging mechanisms or not. (if you disable lock debugging then those bugs wont be detected of course.) The following locking APIs are covered: spinlocks, rwlocks, mutexes and rwsems.
Enables the display of the minimum amount of free stack which each task has ever had available in the sysrq-T and sysrq-P debug output. This option will slow down process creation somewhat.
If you say Y here, some extra kobject debugging messages will be sent to the syslog.
This options enables addition error checking for high memory systems. Disable for production systems.
Say Y here to make BUG() panics output the file name and line number of the BUG call as well as the EIP and oops trace. This aids debugging but costs about 70-100K of memory.
If you say Y here the resulting kernel image will include debugging info resulting in a larger kernel image. This adds debug symbols to the kernel and modules (gcc -g), and is needed if you intend to use kernel crashdump or binary object tools like crash, kgdb, LKCD, gdb, etc on the kernel. Say Y here only if you plan to debug the kernel. If unsure, say N.
If you say Y here gcc is instructed to generate less debugging information for structure types. This means that tools that need full debugging information (like kgdb or systemtap) won't be happy. But if you merely need debugging information to resolve line numbers there is no loss. Advantage is that build directory object sizes shrink dramatically over a full DEBUG_INFO build and compile times are reduced too. Only works with newer gcc versions.
Enable this to turn on extended checks in the virtual-memory system that may impact performance. If unsure, say N.
Enable some costly sanity checks in virtual to page code. This can catch mistakes with virt_to_page() and friends. If unsure, say N.
This option causes the global tree of anonymous and private mapping regions to be regularly checked for invalid topology.
Enable this to catch wrong use of the writers count in struct vfsmount. This will increase the size of each file struct by 32 bits. If unsure, say N.
Enable this for additional checks during memory initialisation. The sanity checks verify aspects of the VM such as the memory model and other information provided by the architecture. Verbose information will be printed at KERN_DEBUG loglevel depending on the mminit_loglevel= command-line option. If unsure, say Y
Enable this to turn on extended checks in the linked-list walking routines. If unsure, say N.
Enable this to turn on 'list_sort()' function test. This test is executed only once during system boot, so affects only boot time. If unsure, say N.
Enable this to turn on checks on scatter-gather tables. This can help find problems with drivers that do not properly initialize their sg tables. If unsure, say N.
Enable this to turn on sanity checking for notifier call chains. This is most useful for kernel developers to make sure that modules properly unregister themselves from notifier chains. This is a relatively cheap check but if you care about maximum performance, say N.
Enable this to turn on some debug checking for credential management. The additional code keeps track of the number of pointers from task_structs to any given cred struct, and checks to see that this number never exceeds the usage count of the cred struct. Furthermore, if SELinux is enabled, this also checks that the security pointer in the cred struct is never seen to be invalid. If unsure, say N.
If you say Y here the resulting kernel image will be slightly larger and slower, but it gives very useful debugging information in case of kernel bugs. (precise oopses/stacktraces/warnings)
This build option allows you to read kernel boot messages by inserting a short delay after each one. The delay is specified in milliseconds on the kernel command line, using "boot_delay=N". It is likely that you would also need to use "lpj=M" to preset the "loops per jiffie" value. See a previous boot log for the "lpj" value to use for your system, and then set "lpj=M" before setting "boot_delay=N". NOTE: Using this option may adversely affect SMP systems. I.e., processors other than the first one may not boot up. BOOT_PRINTK_DELAY also may cause DETECT_SOFTLOCKUP to detect what it believes to be lockup conditions.
This option provides a kernel module that runs torture tests on the RCU infrastructure. The kernel module may be built after the fact on the running kernel to be tested, if desired. Say Y here if you want RCU torture tests to be built into the kernel. Say M if you want the RCU torture tests to build as a module. Say N if you are unsure.
This option provides a way to build the RCU torture tests directly into the kernel without them starting up at boot time. You can use /proc/sys/kernel/rcutorture_runnable to manually override this setting. This /proc file is available only when the RCU torture tests have been built into the kernel. Say Y here if you want the RCU torture tests to start during boot (you probably don't). Say N here if you want the RCU torture tests to start only after being manually enabled via /proc.
If a given RCU grace period extends more than the specified number of seconds, a CPU stall warning is printed. If the RCU grace period persists, additional CPU stall warnings are printed at more widely spaced intervals.
This option causes RCU to printk detailed per-task information for any tasks that are stalling the current RCU grace period. Say N if you are unsure. Say Y if you want to enable such checks.
This option provides for testing basic kprobes functionality on boot. A sample kprobe, jprobe and kretprobe are inserted and verified for functionality. Say N if you are unsure.
This option provides a kernel module that can be used to test the kernel stack backtrace code. This option is not useful for distributions or general kernels, but only for kernel developers working on architecture code. Note that if you want to also test saved backtraces, you will have to enable STACKTRACE as well. Say N if you are unsure.
BIG FAT WARNING: ENABLING THIS OPTION MIGHT BREAK BOOTING ON SOME DISTRIBUTIONS. DO NOT ENABLE THIS UNLESS YOU KNOW WHAT YOU ARE DOING. Distros, please enable this and fix whatever is broken. Conventionally, block device numbers are allocated from predetermined contiguous area. However, extended block area may introduce non-contiguous block device numbers. This option forces most block device numbers to be allocated from the extended space and spreads them to discover kernel or userland code paths which assume predetermined contiguous device number allocation. Note that turning on this debug option shuffles all the device numbers for all IDE and SCSI devices including libata ones, so root partition specified using device number directly (via rdev or root=MAJ:MIN) won't work anymore. Textual device names (root=/dev/sdXn) will continue to work. Say N if you are unsure.
s390 and alpha require percpu variables in modules to be defined weak to work around addressing range issue which puts the following two restrictions on percpu variable definitions. 1. percpu symbols must be unique whether static or not 2. percpu variables can't be defined inside a function To ensure that generic code follows the above rules, this option forces all percpu variables to be defined as weak.
Say Y to verify that the per_cpu map being accessed has been set up. This adds a fair amount of code to kernel memory and decreases performance. Say N if unsure.
This module enables testing of the different dumping mechanisms by inducing system failures at predefined crash points. If you don't need it: say N Choose M here to compile this code as a module. The module will be called lkdtm. Documentation on how to use the module can be found in Documentation/fault-injection/provoke-crashes.txt
This option provides a kernel module that can be used to test the error handling of the cpu notifiers To compile this code as a module, choose M here: the module will be called cpu-notifier-error-inject. If unsure, say N.
Provide fault-injection framework. For more details, see Documentation/fault-injection/.
Provide fault-injection capability for kmalloc.
Provide fault-injection capability for alloc_pages().
Provide fault-injection capability for disk IO.
Provide fault-injection capability on end IO handling. This will make the block layer "forget" an interrupt as configured, thus exercising the error handling. Only works with drivers that use the generic timeout handling, for others it wont do anything.
Enable configuration of fault-injection capabilities via debugfs.
Provide stacktrace filter for fault-injection capabilities
Enable this option if you want to use the LatencyTOP tool to find out which userspace is blocking on what kernel operations.
sys_sysctl uses binary paths that have been found challenging to properly maintain and use. This enables checks that help you to keep things correct.
If you want to debug problems which hang or crash the kernel early on boot and the crashing machine has a FireWire port, you can use this feature to remotely access the memory of the crashed machine over FireWire. This employs remote DMA as part of the OHCI1394 specification which is now the standard for FireWire controllers. With remote DMA, you can monitor the printk buffer remotely using firescope and access all memory below 4GB using fireproxy from gdb. Even controlling a kernel debugger is possible using remote DMA. Usage: If ohci1394_dma=early is used as boot parameter, it will initialize all OHCI1394 controllers which are found in the PCI config space. As all changes to the FireWire bus such as enabling and disabling devices cause a bus reset and thereby disable remote DMA for all devices, be sure to have the cable plugged and FireWire enabled on the debugging host before booting the debug target for debugging. This code (~1k) is freed after boot. By then, the firewire stack in charge of the OHCI-1394 controllers should be used instead. See Documentation/debugging-via-ohci1394.txt for more information.
This option lets you use the FireWire bus for remote debugging with help of the firewire-ohci driver. It enables unfiltered remote DMA in firewire-ohci. See Documentation/debugging-via-ohci1394.txt for more information. If unsure, say N.
This option attempts to build objects from the source files in the kernel Documentation/ tree. Say N if you are unsure.
Compiles debug level messages into the kernel, which would not otherwise be available at runtime. These messages can then be enabled/disabled based on various levels of scope - per source file, function, module, format string, and line number. This mechanism implicitly enables all pr_debug() and dev_dbg() calls. The impact of this compile option is a larger kernel text size of about 2%. Usage: Dynamic debugging is controlled via the 'dynamic_debug/control' file, which is contained in the 'debugfs' filesystem. Thus, the debugfs filesystem must first be mounted before making use of this feature. We refer the control file as: <debugfs>/dynamic_debug/control. This file contains a list of the debug statements that can be enabled. The format for each line of the file is: filename:lineno [module]function flags format filename : source file of the debug statement lineno : line number of the debug statement module : module that contains the debug statement function : function that contains the debug statement flags : 'p' means the line is turned 'on' for printing format : the format used for the debug statement From a live system: nullarbor:~ # cat <debugfs>/dynamic_debug/control # filename:lineno [module]function flags format fs/aio.c:222 [aio]__put_ioctx - "__put_ioctx:\040freeing\040%p\012" fs/aio.c:248 [aio]ioctx_alloc - "ENOMEM:\040nr_events\040too\040high\012" fs/aio.c:1770 [aio]sys_io_cancel - "calling\040cancel\012" Example usage: // enable the message at line 1603 of file svcsock.c nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' > <debugfs>/dynamic_debug/control // enable all the messages in file svcsock.c nullarbor:~ # echo -n 'file svcsock.c +p' > <debugfs>/dynamic_debug/control // enable all the messages in the NFS server module nullarbor:~ # echo -n 'module nfsd +p' > <debugfs>/dynamic_debug/control // enable all 12 messages in the function svc_process() nullarbor:~ # echo -n 'func svc_process +p' > <debugfs>/dynamic_debug/control // disable all 12 messages in the function svc_process() nullarbor:~ # echo -n 'func svc_process -p' > <debugfs>/dynamic_debug/control See Documentation/dynamic-debug-howto.txt for additional information.
Enable this option to debug the use of the DMA API by device drivers. With this option you will be able to detect common bugs in device drivers like double-freeing of DMA mappings or freeing mappings that were never allocated. This option causes a performance degredation. Use only if you want to debug device drivers. If unsure, say N.
Enable this option to test the atomic64_t functions at boot. If unsure, say N.
This is a one-shot self test that permutes through the recovery of all the possible two disk failure scenarios for a N-disk array. Recovery is performed with the asynchronous raid6 recovery routines, and will optionally use an offload engine if one is available. If unsure, say N.