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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 enable intel_pstate as the default scaling driver for the supported processors.
  • [NEW] active - Use intel_pstate driver to bypass the scaling governors layer of cpufreq and provides it own algorithms for p-state selection. There are two P-state selection algorithms provided by intel_pstate in 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 - Use intel_pstate as a scaling driver, but configure it to work with generic cpufreq governors (instead of enabling its internal governor). This mode cannot be used along with the hardware-managed P-states (HWP) feature.
  • force - Enable intel_pstate on systems that prohibit it by default in favor of acpi-cpufreq. Forcing the intel_pstate driver instead of acpi-cpufreq might 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 the intel_pstate driver or on platforms that use pcc-cpufreq instead of acpi-cpufreq.
  • no_hwp - Do not enable hardware P state control (HWP) if available.
  • hwp_only - Only load intel_pstate on systems that support hardware P state control (HWP) if available.
  • support_acpi_ppc - Enforce ACPI _PPC performance 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 the cpufreq sysfs interface.
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 cmt and turn off mba use: 

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 do irq accounting. Used to run time disable IRQ_TIME_ACCOUNTING on 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_watchdog is set to 1). 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.

  • A value of 0 means no change.
  • The default value is 200 meaning the NMI watchdog is set to 30s (based on watchdog_thresh equal to 10).
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.
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