4.9. Kernel

Memory Type Range Register (MTRR) setup on some hyperthreaded machines may be incorrect following a suspend/resume cycle. This can cause graphics performance (specifically, scrolling) to slow considerably after a suspend/resume cycle.

To work around this issue, disable and then re-enable the hyperthreaded sibling CPUs around suspend/resume, for example:

#!/bin/sh
# Disable hyper-threading processor cores on suspend and hibernate, re-enable
# on resume.
# This file goes into /etc/pm/sleep.d/

case $1 in
        hibernate|suspend)
                echo 0 > /sys/devices/system/cpu/cpu1/online
                echo 0 > /sys/devices/system/cpu/cpu3/online
                ;;

        thaw|resume)
                echo 1 > /sys/devices/system/cpu/cpu1/online
                echo 1 > /sys/devices/system/cpu/cpu3/online
                ;;
esac

Loading the megaraid_sas driver on the kdump kernel will result in the insmod command being blocked, returning messages similar to:

INFO: task insmod:201 blocked for more than 120 seconds.

Refer to BZ#682110 for more information.

In Red Hat Enterprise Linux 6.1, the nmi_watchdog registers with the perf subsystem. Consequently, during boot, the perf subsystem grabs control of the performance counter registers, blocking oprofile from working. To resolve this, either boot with the nmi_watchdog=0 kernel parameter set, or run echo 0 > /proc/sys/kernel/nmi_watchdog to disable at run time. To re-enable the watchdog, use the command echo 1 > /proc/sys/kernel/nmi_watchdog.

Due to the way ftrace works when modifying the code during startup, the NMI watchdog causes too much noise and ftrace can not find a quiet period to instrument the code. Consequently, machines with more than 512 cpus will encounter issues with the NMI watchdog. Such issues will return error messages similar to "BUG: NMI Watchdog detected LOCKUP" and have either 'ftrace_modify_code' or 'ipi_handler' in the backtrace. To work around this issue, disable nmi_watchdog using the command:

nmi_watchdog=0

Creating many 'cpu' control groups (cgroups) on a system with a large number of CPUs will slow down the machine when the control groups feature is enabled. To work around this issue, disable control groups.

On 64-bit POWER systems the EHEA NIC driver will fail when attempting to dump a vmcore via NFS. To work around this issue, utilize other kdump facilities, for example dumping to the local filesystem, or dumping over SSH.

A BIOS emulated floppy disk might cause the installation or kernel boot process to hang. To avoid this, disable emulated floppy disk support in the BIOS.

The preferred method to enable nmi_watchdog on 32-bit x86 systems is to use either nmi_watchdog=2 or nmi_watchdog=lapic parameters. The parameter nmi_watchdog=1 is not supported.

The kernel parameter, pci=noioapicquirk, is required when installing the 32 bit variant of Red Hat Enterprise Linux 6 on HP xw9300 workstations. Note that the parameter change is not required when installing the 64 bit variant.

On IBM PowerPC systems, the exec-shield value in sysctl or /proc/sys/kernel/exec-shield parameter is not enforced.