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Chapter 5. Important changes to external kernel parameters
This chapter provides system administrators with a summary of significant changes in the kernel shipped with Red Hat Enterprise Linux 8.9. These changes could include for example added or updated proc entries, sysctl, and sysfs default values, boot parameters, kernel configuration options, or any noticeable behavior changes.
New kernel parameters
- gather_data_sampling=[X86,INTEL]
With this kernel parameter, you can control the Gather Data Sampling (GDS) mitigation.
(GDS) is a hardware vulnerability that allows unprivileged speculative access to data that was previously stored in vector registers.
This issue is mitigated by default in updated microcode. The mitigation might have a performance impact but can be disabled. On systems without the microcode mitigation disabling AVX serves as a mitigation. Available values include:
-
force: Disable AVX to mitigate systems without microcode mitigation. No effect if the microcode mitigation is present. Known to cause crashes in userspace with buggy AVX enumeration. -
off: Disable GDS mitigation.
-
- rdrand=[X86]
With this kernel parameter, you can hide the advertisement of RDRAND support. This affects certain AMD processors because of buggy BIOS support, specifically around the suspend or resume path.
-
force: Override the decision by the kernel to hide the advertisement of RDRAND support.
-
Updated kernel parameters
- intel_pstate=[X86]
You can use this kernel parameter for CPU performance scaling. Available values include:
-
disable- Do not enableintel_pstateas the default scaling driver for the supported processors. -
[NEW]
active- Useintel_pstatedriver to bypass the scaling governors layer ofcpufreqand provides it own algorithms for p-state selection. There are two P-state selection algorithms provided byintel_pstatein the active mode: powersave and performance. The way they both operate depends on whether or not the hardware managed P-states (HWP) feature has been enabled in the processor and possibly on the processor model. -
passive- Useintel_pstateas a scaling driver, but configure it to work with genericcpufreqgovernors (instead of enabling its internal governor). This mode cannot be used along with the hardware-managed P-states (HWP) feature. -
force- Enableintel_pstateon systems that prohibit it by default in favor ofacpi-cpufreq. Forcing theintel_pstatedriver instead ofacpi-cpufreqmight disable platform features, such as thermal controls and power capping, that rely on ACPI P-States information being indicated to OSPM and therefore should be used with caution. This option does not work with processors that are not supported by theintel_pstatedriver or on platforms that usepcc-cpufreqinstead ofacpi-cpufreq. -
no_hwp- Do not enable hardware P state control (HWP) if available. -
hwp_only- Only loadintel_pstateon systems that support hardware P state control (HWP) if available. -
support_acpi_ppc- EnforceACPI _PPCperformance limits. If the Fixed ACPI Description Table specifies preferred power management profile as "Enterprise Server" or "Performance Server", then this feature is turned on by default. -
per_cpu_perf_limits- Allow per-logical-CPU P-State performance control limits using thecpufreq sysfsinterface.
-
- rdt=[HW,X86,RDT]
With this kernel parameter, you can turn on or off individual RDT features. The list includes:
cmt,mbmtotal,mbmlocal,l3cat,l3cdp,l2cat,l2cdp,mba, [NEW]smba, [NEW]bmec.For example, to turn on
cmtand turn offmbause:rdt=cmt,!mba
- tsc=[x86]
With this kernel parameter, you can disable clocksource stability checks for TSC. This parameter takes the format of:
<string>.-
reliable: mark tsc clocksource as reliable, this disables clocksource verification at runtime, as well as the stability checks done at bootup. Used to enable high-resolution timer mode on older hardware, and in virtualized environment. -
noirqtime: Do not use TSC to doirqaccounting. Used to run time disableIRQ_TIME_ACCOUNTINGon any platforms where RDTSC is slow and this accounting can add overhead. -
unstable: mark the TSC clocksource as unstable, this marks the TSC unconditionally unstable at bootup and avoids any further wobbles once the TSC watchdog notices. -
nowatchdog: disable clocksource watchdog. Used in situations with strict latency requirements (where interruptions from clocksource watchdog are not acceptable). -
recalibrate: force recalibration against a HW timer (HPET or PM timer) on systems whose TSC frequency was obtained from HW or FW using either an MSR or CPUID(0x15). Warn if the difference is more than 500 ppm.
-
New sysctl parameters
- nmi_wd_lpm_factor=(PPC only)
Factor to apply to the NMI watchdog timeout (only when
nmi_watchdogis set to1). This factor represents the percentage added towatchdog_threshwhen calculating the NMI watchdog timeout during an LPM. The soft lockup timeout is not impacted.-
A value of
0means no change. -
The default value is
200meaning the NMI watchdog is set to 30s (based onwatchdog_threshequal to 10).
-
A value of
- txrehash
With this kernel parameter, you can control default hash rethink behaviour on socket.
-
If set to
1(default), hash rethink is performed on listening socket. -
If set to
0, hash rethink is not performed.
-
If set to
