Chapter 5. Important changes to external kernel parameters

With this kernel parameter, you can disable kernel mode setting. DRM drivers will not perform display-mode changes or accelerated rendering. Only the system frame buffer will be available for use if this was set-up by the firmware or boot loader.

nomodeset is useful as fallback, or for testing and debugging.

With this kernel parameter, you can disable console loglevel raise on oops, panic or lockdep-detected issues (only if lock debug is on). With an exception to setups with low baudrate on serial console, set this parameter to 0 to provide more debug information.

With this kernel parameter, you can set timeout for expedited RCU CPU stall warning messages. The value is in milliseconds and the maximum allowed value is 21000 milliseconds.

Note that this value is adjusted to an arch timer tick resolution. Setting this to zero causes the value from rcupdate.rcu_cpu_stall_timeout to be used (after conversion from seconds to milliseconds).

With this parameter, you can set initial timeout in jiffies for RCU task stall informational messages, which give some indication of the problem for those not patient enough to wait for ten minutes. Informational messages are only printed prior to the stall-warning message for a given grace period. Disable with a value less than or equal to zero.

This parameter is a multiplier for time interval between successive RCU task stall informational messages for a given RCU tasks grace period. This value is clamped to one through ten, inclusive.

It defaults to the value of three, so that the first informational message is printed 10 seconds into the grace period, the second at 40 seconds, the third at 160 seconds, and then the stall warning at 600 seconds would prevent a fourth at 640 seconds.

With this parameter, you can specify the period of time in milliseconds that smp_call_function() and friends will wait for a CPU to release the CSD lock. This is useful when diagnosing bugs involving CPUs disabling interrupts for extended periods of time.

With this parameter, you can specify the number of CPUs constituting a large system, such that srcu_struct structures should immediately allocate an srcu_node array.

With this parameter, you can specify under what conditions an SRCU tree srcu_struct structure will be converted to big form, that is, with an rcu_node tree:

With this parameter, you can specify the number of update-side contention events per jiffy will be tolerated before initiating a conversion of an srcu_struct structure to big form.

[KNL] Using kexec, Linux can switch to a crash kernel upon panic. This parameter reserves the physical memory region [offset, offset + size] for that kernel image. If @offset is omitted, then a suitable offset is selected automatically.

[KNL, X86-64, ARM64] Select a region under 4G first, and fall back to reserve region above 4G when @offset has not been specified.

For more details, see Documentation/admin-guide/kdump/kdump.rst.

With this parameter, you can set a timeout in seconds for deferred probe to give up waiting on dependencies to probe. Only specific dependencies (subsystems or drivers) that have opted in will be ignored.

A timeout of 0 will time out at the end of initcalls. If the time out has not expired, the option will be restarted by each successful driver registration. This option will also dump out devices still on the deferred probe list after retrying.

With this parameter, you can list of driver names to be probed asynchronously. * (the asterisk) matches with all driver names.

With this parameter, you can specify the size of a CMA area used for allocation of gigantic hugepages. Or using node format, the size of a CMA area per node.

Format: nn[KMGTPE] or (node format) <node>:nn[KMGTPE][,<node>:nn[KMGTPE]]

Reserve a CMA area of given size and allocate gigantic hugepages using the CMA allocator. If enabled, the boot-time allocation of gigantic hugepages is skipped.

With this parameter, you can specify the number of HugeTLB pages to allocate at boot.

This parameter requires CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP to be enabled. Allows heavy hugetlb users to free up some more memory (7 * PAGE_SIZE for each 2MB hugetlb page).

This parameter provides an override to the IOAPIC-ID <-> DEVICE-ID mapping provided in the IVRS ACPI table.

By default, PCI segment is 0, and can be omitted. For example,

This parameter provides an override to the HPET-ID <-> DEVICE-ID mapping provided in the IVRS ACPI table.

By default, PCI segment is 0, and can be omitted. For example:

This parameter provides an override to the ACPI-HID:UID <-> DEVICE-ID mapping provided in the IVRS ACPI table.

For example, to map UART-HID:UID AMD0020:0 to PCI segment 0x1 and PCI device ID 00:14.5, write the parameter as:

ivrs_acpihid[0001:00:14.5]=AMD0020:0

By default, PCI segment is 0, and can be omitted. For example, for the PCI device 00:14.5 write the parameter as:

ivrs_acpihid[00:14.5]=AMD0020:0

With this parameter, you can control whether or not KVM will try to proactively split all huge pages during dirty logging.

Eager page splitting reduces interruptions to vCPU execution by eliminating the write-protection faults and MMU lock contention that would otherwise be required to split huge pages lazily. VM workloads that rarely perform writes or that write only to a small region of VM memory may benefit from disabling eager page splitting to allow huge pages to still be used for reads.

The behavior of eager page splitting depends on whether KVM_DIRTY_LOG_INITIALLY_SET is enabled or disabled.

With this parameter, you can select one of KVM/arm64’s modes of operation.

With this parameter, you can disable SMEP (Supervisor Mode Execution Prevention) even if it is supported by processor.

Format: pci=option[,option…​] [PCI] various_PCI_subsystem_options

Some options herein operate on a specific device or a set of devices (<pci_dev>). These are specified in one of the following formats:

[<domain>:]<bus>:<dev>.<func>[/<dev>.<func>]*
pci:<vendor>:<device>[:<subvendor>:<subdevice>]

With this parameter, you can set timeout in jiffies for RCU task stall warning messages. Disable with a value less than or equal to zero.

Defaults to 10 minutes.

A change in value does not take effect until the beginning of the next grace period.

With this parameter, you can control mitigation of RETBleed (Arbitrary Speculative Code Execution with Return Instructions) vulnerability.

AMD-based UNRET and IBPB mitigations alone do not stop sibling threads from influencing the predictions of other sibling threads. For that reason, STIBP is used on processors that support it, and mitigate SMT on processors that do not.

Format: { <int> [,<int>] | force | noforce }

This factor represents the percentage added to watchdog_thresh when calculating the NMI watchdog timeout during an LPM. The soft lockup timeout is not impacted. Use this factor to apply to the NMI watchdog timeout (only when nmi_watchdog is set to 1).

With this parameter, you can control default hash rethink behavior on listening socket when the SO_TXREHASH option is set to SOCK_TXREHASH_DEFAULT (that is, not overridden by setsockopt).

With this extension, you can enable or disable extension of Stream Reconfiguration functionality specified in RFC6525. This extension provides the ability to "reset" a stream and includes the parameters of Outgoing/Incoming SSN Reset, SSN/TSN Reset and Add Outgoing/Incoming Streams.

With this extension, you can enable or disable extension of User Message Interleaving functionality specified in RFC8260. This extension allows the interleaving of user messages sent on different streams. With this feature enabled, I-DATA chunk will replace DATA chunk to carry user messages if also supported by the peer. Note that to use this feature, you must set this option to 1 and also set socket options SCTP_FRAGMENT_INTERLEAVE to 2 and SCTP_INTERLEAVING_SUPPORTED to 1.

With this extension, you can control use of Explicit Congestion Notification (ECN) by SCTP. Like in TCP, ECN is used only when both ends of the SCTP connection indicate support for it. This feature is useful in avoiding losses due to congestion by allowing supporting routers to signal congestion before having to drop packets.

This knob is not available when the memory_hotplug.memmap_on_memory kernel parameter is configured or the size of struct page (a structure defined in include/linux/mm_types.h) is not power of two (an unusual system configuration could result in this).

You can enable (set to 1) or disable (set to 0) the feature of optimizing vmemmap pages associated with each HugeTLB page.

With this parameter, you can enable, disable, and configure automatic page fault based NUMA memory balancing. Memory is moved automatically to nodes that access it often. The value to set can be the result of ORing the following:

= =================================
0 NUMA_BALANCING_DISABLED
1 NUMA_BALANCING_NORMAL
2 NUMA_BALANCING_MEMORY_TIERING
= =================================

Or NUMA_BALANCING_NORMAL to optimize page placement among different NUMA nodes to reduce remote accessing. On NUMA machines, there is a performance penalty if remote memory is accessed by a CPU. When this feature is enabled the kernel samples what task thread is accessing memory by periodically unmapping pages and later trapping a page fault. At the time of the page fault, it is determined if the data being accessed should be migrated to a local memory node.

Or NUMA_BALANCING_MEMORY_TIERING to optimize page placement among different types of memory (represented as different NUMA nodes) to place the hot pages in the fast memory. This is implemented based on unmapping and page fault, too.

This tunable previously was documented as not having any effect. Now, only the first value (min) is used, default and max are ignored.