Spoil CPU Usage

This repository contains a tool used to spoil CPU usage computation on linux system with RT_PREEMPT patch enabled.

Build

cmake -B build
cmake --build build

Problem

There is a good description of the problem available at https://docs.kernel.org/admin-guide/cpu-load.html. Following this, all tools that measure CPU usage using /proc/stat such as htop may display incorrect values.

Other Ideas

cgroup v1

Note

cpuacct was removed in cgroup v2

Notes on /proc/schedstat

In order to enable /proc/schedstat the kernel option CONFIG_SCHEDSTATS=y must active.

Some docs suggest to also enable scheduler statistics at runtime using the following command:

sudo sysctl -w kernel.sched_schedstats=1

But, as observation shows, this is not necessary to gather basic CPU usage information. The required field #7 of /proc/schedstat is updated even if kernel.sched_schedstats=0 is set (not sure if this is true for all linux system / kernel versions).

Security Considerations

Note

There is some criticism about /proc/schedstat when it is used in security-aware environments.

The source of this criticism are papers about side channel attacs. Some of them are linked below for reference. Note that those papers use various information exposed by /proc filesystem. In fact, /proc/schedstat is only mentioned in very few of them directrly and only as side note.

If security is a need, there is more required than disabling /proc/schedstat, e.g. use the mount option hidepid=2 when mounting the /proc filesystem.

In order to prevent other users than root to see the contents of /proc/schedstat a start script can change the visibility of the file on bootup.

chmod 0400 /proc/schedstat

Runtime Overhead

There are voices (not confirmed yet, since there is no clear reference found yet) that /proc/schedstat comes with a runtime overhead of 1-2% CPU usage.

This overhead occurs when kernel.sched_schedstats is set to 1, which may not be necessary as stated above.

References

• https://docs.kernel.org/admin-guide/cpu-load.html
• https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/resource_management_guide/sec-cpuacct
• https://www.man7.org/linux/man-pages/man7/cgroups.7.html
• https://docs.kernel.org/admin-guide/cgroup-v1/cgroups.html
• https://docs.kernel.org/admin-guide/cgroup-v2.html
• https://www.kernel.org/doc/html/latest/filesystems/proc.html

/proc/schedstat related

• https://docs.kernel.org/scheduler/sched-stats.html
• https://www.kernel.org/doc/html/v6.2/security/self-protection.html
• https://www.kicksecure.com/wiki/Security-misc
• https://gruss.cc/files/procharvester.pdf
• https://vmonaco.com/papers/SoK-%20Keylogging%20Side%20Channels.pdf
• https://yinqian.org/papers/ccs15.pdf