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Chapter 1. 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 6.3. These changes include added or updated
procfs entries, sysfs default values, boot parameters, kernel configuration options, or any noticeable behavior changes. For more details on the features added and bugs fixed in the Red Hat Enterprise Linux 6.3 kernel, refer to the Kernel chapter in the 6.3 Release Notes, or Section 5.135.14, “ RHSA-2012:0862 — Moderate: Red Hat Enterprise Linux 6.3 kernel security, bug fix, and enhancement update ” in this book.
pci=use_crs- The
pci=use_crsboot parameter no longer needs to be specified to force PCI resource allocations to correspond to a specific host bridge the device resides on. It is now the default behavior. CONFIG_HPET_MMAP,hpet_mmap- The high-resolution timer's capacity to remap the HPET registers into the memory of a user process has been enabled via the
CONFIG_HPET_MMAPoption. Additionally, thehpet_mmapkernel parameter has been added. pcie_p=nomsi- The
pcie_p=nomsikernel parameter has been added to allow users to disable MSI/MSI-X for PCI Express Native Hotplug (that is, thepciehpdriver). When enabled all PCIe ports use INTx for hotplug services. msi_irqs- A per-PCI device subdirectory has been added to sysfs:
/sys/bus/pci/devices/<device>/msi_irqs. This subdirectory exports the set of MSI vectors allocated by a given PCI device, by creating a numbered subdirectory for each vector undermsi_irqs. For each vector, various attributes can be exported. Currently the only attribute, namedmode, tracks the operational mode of that vector (MSI versus MSI-X). CONFIG_PCI_DEBUG- When the
CONFIG_PCI_DEBUG=yoption is configured, the-DDEBUGflag is automatically added to theEXTRA_CFLAGScompilation flags. CONFIG_STRICT_DEVMEM- The
CONFIG_STRICT_DEVMEMoption is enabled by default for the PowerPC architecture. This option restricts access to the/dev/memdevice. If this option is disabled, userspace access to all memory is allowed, including kernel and userspace memory, and accidental memory (write) access could potentially be harmful. - kdump/kexec configuration options
- The following kernel configuration options were enabled for the kdump/kexec kernel dumping mechanism on IBM System z:
CONFIG_KEXEC_AUTO_RESERVE=y CONFIG_CRASH_DUMP=y CONFIG_PROC_VMCORE=y
KEXEC_AUTO_THRESHOLD- The default value for the
KEXEC_AUTO_THRESHOLDoption has been lowered to 2 GB. /proc/mounts- The
/proc/mountsfile now shows the following mount options for CIFS under thedir_mode=parameter:nostrictsync noperm backupuid backupgid
dmesg_restrict- Writing to the
/proc/sys/kernel/dmesg_restrictfile is only allowed for a root user that has theCAP_SYS_ADMINidentifier set. printk.always_kmsg_dump- A new kernel parameter,
printk.always_kmsg_dump, has been added to save the final kernel messages to the reboot, halt, poweroff, and emergency_restart paths. For usage information, refer to the/usr/share/doc/kernel-doc-<version>/Documentation/kernel-parameters.txtfile. - ulimit
- The default hard ulimit on the number of files has been increased to
4096:~]$
ulimit -Hn4096 soft_panic- A watchdog module parameter,
soft_panic, has been added. Whensoft_panicis set to1, it causes softdog to invoke kernel panic instead of a reboot when the softdog timer expires. By invoking kernel panic, the system executes kdump, if kdump is configured. Kdump then generates a vmcore which provides additional information on the reasons of the failure. - perf examples
- The
/usr/share/doc/perf-<version>/examples.txtdocumentation file has been added to the perf package. shm_rmid_forced- Support for the
shm_rmid_forcedsysctl option has been added. When set to1, all shared memory objects not referenced in current ipc namespace (with no tasks attached to it) will be automatically forced to use IPC_RMID. For more information refer to/usr/share/doc/kernel-doc-<version>/Documentation/sysctl/kernel.txtfile. - UV systems reduced boot time
- A number of patches have been applied to the kernel in Red Hat Enterprise Linux 6.3 to improve overall performance and reduce boot time on extremely large UV systems (patches were tested on a system with 2048 cores and 16 TB of memory). Additionally, boot messages for the SGI UV2 platform were updated.
accept_local- The
/proc/sys/net/ipv4/conf/*/accept_localsysctl setting has been added to allow a system to receive packets it sent itself. This is needed in order to work with certain load balancing solutions that load balance to themselves. CONFIG_VGA_SWITCHEROO- The
CONFIG_VGA_SWITCHEROOconfiguration option is now enabled by default to allow switching between two graphics cards. O_DIRECTin FUSE- Support for the
O_DIRECTflag for files in FUSE (File system in Userspace) has been added. CONFIG_IP_MROUTE_MULTIPLE_TABLES- The
CONFIG_IP_MROUTE_MULTIPLE_TABLES=yhas been added to enable support for multiple independent multicast routing instances. nfs.max_session_slots- The
nfs.max_session_slotsmodule/kernel boot parameter has been added. This parameter sets the maximum number of session slots that an NFS client attempts to negotiate with the server. - Default mount option for
/proc - In Red Hat Enterprise Linux 6.3, the default mount option of
/procduring boot up has been changed to:~]#
mount -t proc -o nosuid,noexec,nodev proc /procFor third party modules which create devices viaprocfs, please remountprocfswith the old option:~]#
mount -t proc /proc /proc
