Re: [PATCH] sched_ext/scx_flatcg: expire cached hweights on weight changes

[Tejun Heo]

Hello,

On Fri, Aug 14, 2026 at 10:48:35PM +0800, Tao Cui wrote:
> Is this the intended behavior of the budget clamping, or should the
> steady-state distribution converge to the compounded shares over time?
> The header comment's model doesn't seem to hold in this scenario.

The clamp isn't the culprit. I reproduced your setup (3 busy tasks per
leaf, 4 CPUs) and dug in.

The dominant factor is that D is runnable-task limited. At weight 800
its compounded share is 3.55 CPUs but it only has 3 tasks, so 75% is
the best it can do. Below that cap, the window granting loses more:
whenever all three of D's tasks are already running and another CPU
picks D, the pop from D's DSQ comes up empty, the cgv_node gets
stashed and that CPU grants a full cgrp_slice_ns window to another
cgroup. The loss scales with the window size. Measured D shares at
weight 800:

	3 tasks/leaf			55-59%
	3 tasks/leaf, 100ms slices	52%
	5 tasks/leaf			84-86%

Disabling the clamp entirely is the 59% above, so it barely matters.
With enough runnable tasks per cgroup, the distribution converges to
the documented compounding. The model holds, but only when no cgroup
is runnable-task limited, and the window granting degrades sooner than
per-task fair queueing would as that limit is approached. That's an
inherent simplification of this example scheduler.

Separately, while digging into this, I found that the true-up in
fcg_dispatch() is broken:

	__sync_fetch_and_add(&cgc->cvtime_delta,
			     (cpuc->cur_at + cgrp_slice_ns - now) *
			     FCG_HWEIGHT_ONE / (cgc->hweight ?: 1));

In the CNS_EXPIRE case, now is past cur_at + cgrp_slice_ns, so the u64
subexpression wraps. The multiplication preserves the two's complement
encoding but the unsigned division by hweight destroys it, adding about
2^64 / hweight per expiry instead of a small correction. The sign is
also inverted. The true-up should be actual minus charged, so the
expiry overrun should be added and the CNS_EMPTY unused portion
subtracted. Under saturation the budget clamp mostly masks the garbage,
which is why the numbers above barely move with it fixed (the 86% in
the 5 tasks/leaf row), but the accounting is broken all the same. The
following fixes it and tests fine (BPF division is unsigned, keep the
dividends positive):

	s64 delta = now - cpuc->cur_at - cgrp_slice_ns;

	if (delta >= 0)
		__sync_fetch_and_add(&cgc->cvtime_delta,
				     (u64)delta * FCG_HWEIGHT_ONE /
				     (cgc->hweight ?: 1));
	else
		__sync_fetch_and_sub(&cgc->cvtime_delta,
				     (u64)-delta * FCG_HWEIGHT_ONE /
				     (cgc->hweight ?: 1));

Care to send a patch?

Thanks.

--
tejun