sched/fair: Add cgroup_mode switch

The effective task weight (W_t') for a task in cgroup g on CPU n is given by:

                                 W_t
	W_t' = W_g * F_g_n * ----------
                             \Sum W_t_n

Where W_g is the group's weight (cpu.weight), F_g_n is the fraction of the
group weight for CPU n and W_t/W is the relative weight of this task against
all other tasks in the same group on the same CPU.

Furthermore, this makes:

                \Sum W_t_n
	F_g_n = ----------
	         \Sum W_t

The fraction of weight inside the group of CPU n against the whole group.

The problem is with F_g_n, the primary goal of this fraction is to make sure
that the relative weight of tasks, when distributed over CPUs is maintained.
For example, consider 4 (equal weight) tasks and 2 CPUs with a 1:3
distribution, then if F_g_n would simply be 1 (no weight re-distribution) the
effective relative weights (W_t') of the tasks in our group would be:

	CPU0	CPU1
        W_g     W_g/3
	        W_g/3
		W_g/3

IOW, the lucky task on CPU0 would get an equal amount of weight as all 3 tasks
on CPU1 combined. However, with the weight redistribution, this becomes:

	CPU0	CPU1
        W_g/4   W_g/4
	        W_g/4
		W_g/4

All tasks are equal weight (as intended). However, as is already evident from
this example, the more CPUs you add, the smaller F_g_n becomes, which creates a
disparity against tasks not in our group.

Specifically:

	avg(F_g_n) ~ 1/N

This leads to a weight mismatch in the hierarchy. IOW tasks cannot compete
fairly across hierarchy levels.

*Notably*, what is meant by avg(F_g_n) being proportional to 1/N is that when
there are at least N runnable tasks, the average of this fraction tends to 1/N.

For a hierarchy of depth d, this gets even worse, since that gets terms on the
order of:

	avg(F_g_n)^d ~ 1/(N^d)

Given fixed point arithmetic, this also leads to numerical trouble.

However, the meaning of "cpu.weight" is simple and intiutive: the total weight
of the cgroup. But as explored above, there is deception in this simplicity.

Prepare to add a few alternative methods for distributing weight.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260605124051.338602724%40infradead.org

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