Enabling Linux* Kernel Analysis

Enabling Kernel Modules Resolution

To provide accurate performance statistics for the Linux kernel, the VTune Amplifier requires kernel modules information provided in the /proc/kallsyms file. Make sure the /proc/sys/kernel/kptr_restrict file contains values that enable reading /proc/kallsyms and providing non-zero addresses for the kernel pointers:

• If the kptr_restrict value is 0, kernel addresses are provided without limitations (recommended).

• If the kptr_restict value is 1, addresses are provided if the current user has a CAP_SYSLOG capability.

• If the kptr_restrict value is 2, the kernel addresses are hidden regardless of privileges the current user has.

See more details at: http://lwn.net/Articles/420403/, http://man7.org/linux/man-pages/man7/capabilities.7.html.

If kernel pointers information was explicitly hidden by setting the kptr_restrict to a non-zero value, hardware event-based analysis results may not contain functions from kernel modules. As a result, you may see the CPU time associated with the [Outside any known module] item. To workaround this problem for the current session, set the contents of the /proc/sys/kernel/kptr_restrict file to 0 before starting the VTune Amplifier as follows:

> sysctl -w kernel.kptr_restrict=0

To resolve symbols for the Linux kernel, the VTune Amplifier also uses the System.map file created during the kernel build and shipped with the system by default. If the file is located in a non-default directory, you may add it to the list of search directories in the Binary/Symbol Search dialog box when configuring your target properties.

Downloading and Installing Available Debug Kernel Versions

After installing the Linux operating system, the kernel is contained in vmlinux, or vmlinuz, or bzImage in /boot. Linux vendors typically release compressed kernel files stripped of symbols (vmlinuz or bzImage). vmlinux is the uncompressed Linux kernel, but it does not include debug information. So, by default the VTune Amplifier cannot retrieve kernel function information from these kernels and presents all hot addresses captured in the kernel as a unique function or module named [vmlinux]. However, some vendors have released special debug versions of their kernels that are suitable for performance analysis.

1. Use the uname -r command to identify the running Linux kernel version.

2. Download and install two RPMs matching your system: kernel-debug-debuginfo-*.rpm and kernel-debuginfo-common-*.rpm. To do this, use any of the following options:

   • Browse through the RPMs on your installation CDs or DVDs. For example, for SuSE Linux Enterprise* 9, 10, and 11 distros, SuSE provides "debug" kernel RPMs (kernel-debug-*.rpm) available on the install CD or from the website. After installing the RPM, the debug version of the kernel file is located under /boot/vmlinux-*-debug or under /boot/vmlinuz-*-debug. You need to manually uncompress this kernel file using the gunzip program.

   • Browse through the OS vendor FTP site and download the packages. For example: look at ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os to get packages for Redhat* Enterprise Server.

   • Look for other sources on the internet. For example, for Red Hat Enterprise* Linux 3, 4 and 5 distros, Red Hat provides debuginfo RPMs at http://people.redhat.com/duffy/debuginfo/. After installing the RPM, the debug version of the kernel file is located under /usr/lib/debug/boot (EL 3) or /usr/lib/debug/lib/modules (EL 4, 5).

3. Use the following commands to install the RPMs:

   > rpm -ivh kernel-debuginfo-common-*.rpm
   > rpm -ivh kernel-debug-debuginfo-*.rpm

   For some operating systems, you can use yum to install packages directly, for example:

   > yum --enablerepo=rhel-debuginfo install kernel-debuginfo

4. Verify that the packages have been installed, for example:

   > rpm -qa|grep kernel

5. Modify the VTune Amplifier target properties and specify the path to the uncompressed kernel binary in the Dialog Box: Binary/Symbol Search , for example: /usr/lib/debug/lib/modules/2.6.18-128.el5debug/.

Building the Linux Kernel with Debug Information

1. Configure the kernel sources.

2. Edit the kernel source top-level Makefile and add the -g option to the following variables:

   CFLAGS_KERNEL := -g

   CFLAGS := -g

3. Run make clean; make to create the vmlinux kernel file with debug information. Once a debug version of the kernel is created or obtained, specify that kernel file as the one to use during performance analysis.

As soon as the debug information is available for your kernel modules, any future analysis runs will display the kernel functions appropriately. To resolve the previously collected data against this new symbol information, update the project Search Directories and click the Re-resolve button to apply the changes.