RT-Scheduler/cgroups: Possible overuse of resources assigned via cpu.rt_period_us and cpu.rt_runtime_us

["Daniel K." <dk@uw.no>]

mkdir /dev/cgroup
mount -t cgroup -o cpu,cpuset cgroup /dev/cgroup

mkdir /dev/cgroup/0

echo 3 > /dev/cgroup/0/cpuset.cpus
echo 0 > /dev/cgroup/0/cpuset.mems
echo 100000 > /dev/cgroup/0/cpu.rt_period_us
echo   5000 > /dev/cgroup/0/cpu.rt_runtime_us

schedtool -R -p 1 -e burnP6 &
[1] 3309
echo 3309 > /dev/cgroup/0/tasks

At this point I'd expect the burnP6 task to use 5% of the available CPU
resources in the cgroup (5000/100000), but the real CPU usage, as
reported by top, is 20% This is 4 times the expected result, and as I
have 4 cores, I think there is a strong hint of correlation there.

Maybe with a 4 core system there really is 4 000 000 us available for
every 1 wall-time second?

However, I have only assigned one core (3) to _this_ cgroup, so I think
this cgroup is overusing its assigned resources.

What do you think?


Daniel K.