sched/fair: Add cgroup_mode switch

Browse source

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

...

This commit is contained in:

Peter Zijlstra 2026-03-12 14:48:44 +01:00

parent 4f166adb5c

commit 4161cb2d86

1 changed files with 74 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

74

kernel/sched/debug.c

View file