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Suppose I limited the cores a set of processes is permitted to use with cgroup/cpuset option.

I need to know if I have too many threads allocated to that cgroup, if the threads are experiencing excessive competition for the available cores.

How can I do that?

Without cgropups, I'd simply use load average in top with the rule of thumb that load average should be less than the number of cores. Is there something similart that takes cgroup/cpuset into account?

Depending on what exactly you care about, there are two options:

If you care about how many threads/processes are in the cgroup, but not the 'load average':

Simply count the lines in the tasks file for the cgroup. This file will automatically update whenever a thread or process is created or removed in the cgroup.

If you care about actual utilization:

This is a bit trickier. You can get a list of processes/threads in the cgroup from the above mentioned tasks file, and then check usage for each PID using /proc, but that approach has so many race conditions you could monetize it as a competitive sport. Alternatively, if you're using version 2 cgroups, you can check the cpu.stat file in the cgroup. This file will have a couple of lines counting microseconds of CPU time consumed by the cgroup, so you can get a rough estimate of usage by checking it two times one second apart and dividing the difference by 10 to get a utilization percentage (100% for full utilization of one core, 20o% for full utilization of two, etc). A similar approach can be done with version one cgroups with the cpuacct controller (create a cgroup under that controller mirroring the one you created under the cpuset controller).