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rcutorture.shutdown_secs= [KNL]
Set time (s) after boot system shutdown. This
is useful for hands-off automated testing.

rcutorture.stall_cpu= [KNL]
Duration of CPU stall (s) to test RCU CPU stall
warnings, zero to disable.

rcutorture.stall_cpu_holdoff= [KNL]
Time to wait (s) after boot before inducing stall.

rcutorture.stall_cpu_irqsoff= [KNL]
Disable interrupts while stalling if set.

rcutorture.stat_interval= [KNL]
Time (s) between statistics printk()s.

rcutorture.stutter= [KNL]
Time (s) to stutter testing, for example, specifying
five seconds causes the test to run for five seconds,
wait for five seconds, and so on. This tests RCU's
ability to transition abruptly to and from idle.

rcutorture.test_boost= [KNL]
Test RCU priority boosting? 0=no, 1=maybe, 2=yes.
"Maybe" means test if the RCU implementation
under test support RCU priority boosting.

rcutorture.test_boost_duration= [KNL]
Duration (s) of each individual boost test.

rcutorture.test_boost_interval= [KNL]
Interval (s) between each boost test.

rcutorture.test_no_idle_hz= [KNL]
Test RCU's dyntick-idle handling. See also the
rcutorture.shuffle_interval parameter.

rcutorture.torture_type= [KNL]
Specify the RCU implementation to test.

rcutorture.verbose= [KNL]
Enable additional printk() statements.

rcupdate.rcu_cpu_stall_suppress= [KNL]
Suppress RCU CPU stall warning messages.

rcupdate.rcu_cpu_stall_timeout= [KNL]
Set timeout for RCU CPU stall warning messages.

rcupdate.rcu_expedited= [KNL]
Use expedited grace-period primitives, for
example, synchronize_rcu_expedited() instead
of synchronize_rcu(). This reduces latency,
but can increase CPU utilization, degrade
real-time latency, and degrade energy efficiency.
No effect on CONFIG_TINY_RCU kernels.

rcupdate.rcu_normal= [KNL]
Use only normal grace-period primitives,
for example, synchronize_rcu() instead of
synchronize_rcu_expedited(). This improves
real-time latency, CPU utilization, and
energy efficiency, but can expose users to
increased grace-period latency. This parameter
overrides rcupdate.rcu_expedited. No effect on

rcupdate.rcu_normal_after_boot= [KNL]
Once boot has completed (that is, after
rcu_end_inkernel_boot() has been invoked), use
only normal grace-period primitives. No effect
on CONFIG_TINY_RCU kernels.

rcupdate.rcu_task_stall_timeout= [KNL]
Set timeout in jiffies for RCU task stall warning
messages. Disable with a value less than or equal
to zero.

rcupdate.rcu_self_test= [KNL]
Run the RCU early boot self tests

rcupdate.rcu_self_test_bh= [KNL]
Run the RCU bh early boot self tests

rcupdate.rcu_self_test_sched= [KNL]
Run the RCU sched early boot self tests

rdinit= [KNL]
Format: <full_path>
Run specified binary instead of /init from the ramdisk,
used for early userspace startup. See initrd.

rdt= [HW,X86,RDT]
Turn on/off individual RDT features. List is:
cmt, mbmtotal, mbmlocal, l3cat, l3cdp, l2cat, l2cdp,
E.g. to turn on cmt and turn off mba use:

reboot= [KNL]
Format (x86 or x86_64):
[w[arm] | c[old] | h[ard] | s[oft] | g[pio]] \
[[,]s[mp]#### \
[[,]b[ios] | a[cpi] | k[bd] | t[riple] | e[fi] | p[ci]] \
Where reboot_mode is one of warm (soft) or cold (hard) or gpio,
reboot_type is one of bios, acpi, kbd, triple, efi, or pci,
reboot_force is either force or not specified,
reboot_cpu is s[mp]#### with #### being the processor
to be used for rebooting.

[KNL, SMP] Set scheduler's default relax_domain_level.
See Documentation/cgroup-v1/cpusets.txt.

reserve= [KNL,BUGS] Force kernel to ignore I/O ports or memory
Format: <base1>,<size1>[,<base2>,<size2>,...]
Reserve I/O ports or memory so the kernel won't use
them. If <base> is less than 0x10000, the region
is assumed to be I/O ports; otherwise it is memory.

reservetop= [X86-32]
Format: nn[KMG]
Reserves a hole at the top of the kernel virtual
address space.

reservelow= [X86]
Format: nn[K]
Set the amount of memory to reserve for BIOS at
the bottom of the address space.

reset_devices [KNL] Force drivers to reset the underlying device
during initialization.

resume= [SWSUSP]
Specify the partition device for software suspend
{/dev/<dev> | PARTUUID=<uuid> | <int>:<int> | <hex>}

resume_offset= [SWSUSP]
Specify the offset from the beginning of the partition
given by "resume=" at which the swap header is located,
in <PAGE_SIZE> units (needed only for swap files).
See Documentation/power/swsusp-and-swap-files.txt

resumedelay= [HIBERNATION] Delay (in seconds) to pause before attempting to
read the resume files

resumewait [HIBERNATION] Wait (indefinitely) for resume device to show up.
Useful for devices that are detected asynchronously
(e.g. USB and MMC devices).

hibernate= [HIBERNATION]
noresume Don't check if there's a hibernation image
present during boot.
nocompress Don't compress/decompress hibernation images.
no Disable hibernation and resume.
protect_image Turn on image protection during restoration
(that will set all pages holding image data
during restoration read-only).

retain_initrd [RAM] Keep initrd memory after extraction

0 "airplane mode". All wifi, bluetooth, wimax, gps, fm,
etc. communication is blocked by default.
1 Unblocked.

0 The "airplane mode" button does nothing.
1 The "airplane mode" button toggles between everything
blocked and the previous configuration.
2 The "airplane mode" button toggles between everything
blocked and everything unblocked.

rhash_entries= [KNL,NET]
Set number of hash buckets for route cache

[KNL] Disable ring 3 MONITOR/MWAIT feature on supported

ro [KNL] Mount root device read-only on boot

rodata= [KNL]
on Mark read-only kernel memory as read-only (default).
off Leave read-only kernel memory writable for debugging.

Enable the uart passthrough on the designated usb port
on Rockchip SoCs. When active, the signals of the
debug-uart get routed to the D+ and D- pins of the usb
port and the regular usb controller gets disabled.

root= [KNL] Root filesystem
See name_to_dev_t comment in init/do_mounts.c.

rootdelay= [KNL] Delay (in seconds) to pause before attempting to
mount the root filesystem

rootflags= [KNL] Set root filesystem mount option string

rootfstype= [KNL] Set root filesystem type

rootwait [KNL] Wait (indefinitely) for root device to show up.
Useful for devices that are detected asynchronously
(e.g. USB and MMC devices).

[KNL,ARM,CMA] Remoteproc physical memory block.
Memory area to be used by remote processor image,
managed by CMA.

rw [KNL] Mount root device read-write on boot

S [KNL] Run init in single mode

s390_iommu= [HW,S390]
Set s390 IOTLB flushing mode
With strict flushing every unmap operation will result in
an IOTLB flush. Default is lazy flushing before reuse,
which is faster.

sa1100ir [NET]
See drivers/net/irda/sa1100_ir.c.

sbni= [NET] Granch SBNI12 leased line adapter

sched_debug [KNL] Enables verbose scheduler debug messages.

schedstats= [KNL,X86] Enable or disable scheduled statistics.
Allowed values are enable and disable. This feature
incurs a small amount of overhead in the scheduler
but is useful for debugging and performance tuning.

skew_tick= [KNL] Offset the periodic timer tick per cpu to mitigate
xtime_lock contention on larger systems, and/or RCU lock
contention on all systems with CONFIG_MAXSMP set.
Format: { "0" | "1" }
0 -- disable. (may be 1 via CONFIG_CMDLINE="skew_tick=1"
1 -- enable.
Note: increases power consumption, thus should only be
enabled if running jitter sensitive (HPC/RT) workloads.

security= [SECURITY] Choose a security module to enable at boot.
If this boot parameter is not specified, only the first
security module asking for security registration will be
loaded. An invalid security module name will be treated
as if no module has been chosen.

selinux= [SELINUX] Disable or enable SELinux at boot time.
Format: { "0" | "1" }
See security/selinux/Kconfig help text.
0 -- disable.
1 -- enable.
Default value is set via kernel config option.
If enabled at boot time, /selinux/disable can be used
later to disable prior to initial policy load.

apparmor= [APPARMOR] Disable or enable AppArmor at boot time
Format: { "0" | "1" }
See security/apparmor/Kconfig help text
0 -- disable.
1 -- enable.
Default value is set via kernel config option.

serialnumber [BUGS=X86-32]

shapers= [NET]
Maximal number of shapers.

simeth= [IA-64]

slram= [HW,MTD]

slab_nomerge [MM]
Disable merging of slabs with similar size. May be
necessary if there is some reason to distinguish
allocs to different slabs, especially in hardened
environments where the risk of heap overflows and
layout control by attackers can usually be
frustrated by disabling merging. This will reduce
most of the exposure of a heap attack to a single
cache (risks via metadata attacks are mostly
unchanged). Debug options disable merging on their
For more information see Documentation/vm/slub.txt.

slab_max_order= [MM, SLAB]
Determines the maximum allowed order for slabs.
A high setting may cause OOMs due to memory
fragmentation. Defaults to 1 for systems with
more than 32MB of RAM, 0 otherwise.

slub_debug[=options[,slabs]] [MM, SLUB]
Enabling slub_debug allows one to determine the
culprit if slab objects become corrupted. Enabling
slub_debug can create guard zones around objects and
may poison objects when not in use. Also tracks the
last alloc / free. For more information see

slub_memcg_sysfs= [MM, SLUB]
Determines whether to enable sysfs directories for
memory cgroup sub-caches. 1 to enable, 0 to disable.
The default is determined by CONFIG_SLUB_MEMCG_SYSFS_ON.
Enabling this can lead to a very high number of debug
directories and files being created under

slub_max_order= [MM, SLUB]
Determines the maximum allowed order for slabs.
A high setting may cause OOMs due to memory
fragmentation. For more information see

slub_min_objects= [MM, SLUB]
The minimum number of objects per slab. SLUB will
increase the slab order up to slub_max_order to
generate a sufficiently large slab able to contain
the number of objects indicated. The higher the number
of objects the smaller the overhead of tracking slabs
and the less frequently locks need to be acquired.
For more information see Documentation/vm/slub.txt.

slub_min_order= [MM, SLUB]
Determines the minimum page order for slabs. Must be
lower than slub_max_order.
For more information see Documentation/vm/slub.txt.

slub_nomerge [MM, SLUB]
Same with slab_nomerge. This is supported for legacy.
See slab_nomerge for more information.

smart2= [HW]
Format: <io1>[,<io2>[,...,<io8>]]

smsc-ircc2.nopnp [HW] Don't use PNP to discover SMC devices
smsc-ircc2.ircc_cfg= [HW] Device configuration I/O port
smsc-ircc2.ircc_sir= [HW] SIR base I/O port
smsc-ircc2.ircc_fir= [HW] FIR base I/O port
smsc-ircc2.ircc_irq= [HW] IRQ line
smsc-ircc2.ircc_dma= [HW] DMA channel
smsc-ircc2.ircc_transceiver= [HW] Transceiver type:
0: Toshiba Satellite 1800 (GP data pin select)
1: Fast pin select (default)
2: ATC IRMode

smt [KNL,S390] Set the maximum number of threads (logical
CPUs) to use per physical CPU on systems capable of
symmetric multithreading (SMT). Will be capped to the
actual hardware limit.
Format: <integer>
Default: -1 (no limit)

[KNL] Should the soft-lockup detector generate panics.
Format: <integer>

A nonzero value instructs the soft-lockup detector
to panic the machine when a soft-lockup occurs. This
which is the respective build-time switch to that

[KNL] Should the soft-lockup detector generate
backtraces on all cpus.
Format: <integer>

sonypi.*= [HW] Sony Programmable I/O Control Device driver
See Documentation/laptops/sonypi.txt

spectre_v2= [X86] Control mitigation of Spectre variant 2
(indirect branch speculation) vulnerability.

on - unconditionally enable
off - unconditionally disable
auto - kernel detects whether your CPU model is

Selecting 'on' will, and 'auto' may, choose a
mitigation method at run time according to the
CPU, the available microcode, the setting of the
CONFIG_RETPOLINE configuration option, and the
compiler with which the kernel was built.

Specific mitigations can also be selected manually:

retpoline - replace indirect branches
retpoline,generic - google's original retpoline
retpoline,amd - AMD-specific minimal thunk

Not specifying this option is equivalent to

spia_io_base= [HW,MTD]

srcutree.counter_wrap_check [KNL]
Specifies how frequently to check for
grace-period sequence counter wrap for the
srcu_data structure's ->srcu_gp_seq_needed field.
The greater the number of bits set in this kernel
parameter, the less frequently counter wrap will
be checked for. Note that the bottom two bits
are ignored.

srcutree.exp_holdoff [KNL]
Specifies how many nanoseconds must elapse
since the end of the last SRCU grace period for
a given srcu_struct until the next normal SRCU
grace period will be considered for automatic
expediting. Set to zero to disable automatic

stack_guard_gap= [MM]
override the default stack gap protection. The value
is in page units and it defines how many pages prior
to (for stacks growing down) resp. after (for stacks
growing up) the main stack are reserved for no other
mapping. Default value is 256 pages.

stacktrace [FTRACE]
Enabled the stack tracer on boot up.

[FTRACE] Limit the functions that the stack tracer
will trace at boot up. function-list is a comma separated
list of functions. This list can be changed at run
time by the stack_trace_filter file in the debugfs
tracing directory. Note, this enables stack tracing
and the stacktrace above is not needed.

sti= [PARISC,HW]
Format: <num>
Set the STI (builtin display/keyboard on the HP-PARISC
machines) console (graphic card) which should be used
as the initial boot-console.
See also comment in drivers/video/console/sticore.c.

sti_font= [HW]
See comment in drivers/video/console/sticore.c.

stifb= [HW]
Format: bpp:<bpp1>[:<bpp2>[:<bpp3>...]]

SunRPC servers often require that client requests
originate from a privileged port (i.e. a port in the
range 0 < portnr < 1024).
An administrator who wishes to reserve some of these
ports for other uses may adjust the range that the
kernel's sunrpc client considers to be privileged
using these two parameters to set the minimum and
maximum port values.