sched/fair: Add cgroup_mode: max

In order to avoid the average CPU fraction avg(F_g_n) becoming tiny '1/N',
assume each cgroup is maximally concurrent and distrubute 'N*weight', such
that:

	F_g_n' = N * F_g_n

Giving:

	avg(F_g_n') = N*avg(F_g_n) ~ N * 1/N = 1

And while this sounds like it solves things, remember what that ~ meant. There
is the corner case when a cgroup is minimally loaded, eg a single runnable
task, therefore limit the CPU fraction to that of a nice -20 task to avoid
getting too much load.

This last bit is what makes it different from a previous proposal to allow
raising cpu.weight to '100 * N', that would not limit the mininal concurrency
case and results in a very large F_g_n. And just like F_g_n << 1 is
problematic, so is F_g_n >> 1 for the exact same reasons (it would drown the
kthreads, but it also risks overflowing the load values).

So while this might appear to be a better scheme than the current default
scheme, it doesn't really handle less than maximal concurrency nicely -- it
clips and introduces artificially large weights. So where the traditional SMP
mode works well when nr_tasks << nr_cpus, MAX doesn't work well in that regime
and vice-versa.

The meaning of "cpu.weight" would be: weight per allowed CPU.

Included for completeness (and infrastructure).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260605124051.589618504%40infradead.org

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