How to enable and disable a cpu on a live Linux system?

For squeezing out a bit of extra battery time, I usually disables one of cpus of my laptop.

This option is only available from RHEL5.

I do have two cores in my system.

# grep processor /proc/cpuinfo
processor : 0
processor : 1

/sys/devices/system/cpu/cpuX/online decides if a cpu is enabled or disabled.

# cat /sys/devices/system/cpu/cpu1/online
1

To disable the cpu1:

# echo 0 > /sys/devices/system/cpu/cpu1/online

Now only one cpu is up.

# grep processor /proc/cpuinfo
processor : 0

To re-enable the cpu:

# echo 1 > /sys/devices/system/cpu/cpu1/online

I will not be able to disable cpu0. Even online file is not availabe.

# ll /sys/devices/system/cpu/cpu0/
total 0
drwxr-xr-x 5 root root 0 2009-03-10 16:50 cache
drwxr-xr-x 3 root root 0 2009-03-10 16:38 cpufreq
drwxr-xr-x 5 root root 0 2009-03-10 16:50 cpuidle
-r-------- 1 root root 4096 2009-03-10 16:50 crash_notes
drwxr-xr-x 2 root root 0 2009-03-10 16:50 topology

Some architectures (i386, X86_64) does have some special dependency on a certain CPU. I have not tested it in any other architectures.

How to create rpm packages with non stripped binaries in Red Hat Enterprise Linux?

Usually binaries are stripped of the debug information for making them compact. Non-stripped binaries are binaries which have debug information built into them. Non stripped binaries are generally build by passing -g option to gcc.

Rpms shipped with Red Hat Enterprise Linux provides stripped binaries. A separate debuginfo package is provided with all debugging information. Debuginfo rpms for Red Hat Enterprise Linux could be found in following url.

http://people.redhat.com/anderson/debuginfo.html

'file' command could tell if a binary is stripped or not.

# file /usr/bin/python
/usr/bin/python: ELF 64-bit LSB executable, AMD x86-64, version 1 (SYSV), for GNU/Linux 2.4.0, dynamically linked (uses shared libs), stripped

Some times a non stripped binary will be useful for profiling and debugging.

Below steps could be followed to create an rpm package which provides a non stripped binary. Following is an example of building a python rpm package with non stripped binary.

1. Download the source rpm for the package from rhn.redhat.com.

2. Install the source rpm in the system with following command.

# rpm -ivh python-2.3.4-14.2.src.rpm

3. Edit /etc/rpm/macros and add following line.

%debug_package %{nil}

4. Build the rpm using rpmbuild command.

# rpmbuild -bb /usr/src/redhat/SPECS/python.spec

'-bb' option is given so that a binary package is only created. '-ba' option could build binary and source packages

Binary package will be created under /usr/src/redhat/RPMS/x86_64/

5. Now remove the current rpm with stripped binary and install the new binary rpm package win non stripped binary.

# rpm -e python --nodeps

# rpm -ivh /usr/src/redhat/RPMS/x86_64/python-2.3.4-14.2.x86_64.rpm
Preparing... ########################################### [100%]
1:python ########################################### [100%]

# file /usr/bin/python
/usr/bin/python: ELF 64-bit LSB executable, AMD x86-64, version 1 (SYSV), for GNU/Linux 2.4.0, dynamically linked (uses shared libs), not stripped

6. For listing symbols from the non stripped binary 'nm' command could be used.

# nm /usr/bin/python

I used RHEL in this example because of familiarity. This should work with almost all rpm based operating system flavors.