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

[Tao Cui <cui.tao@linux.dev>]

在 2026/8/15 06:23, Tejun Heo 写道:
> 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?
> 
Hi, tejun

Thanks for the detailed walkthrough. I was chasing the clamp, but the
runnable-task limit and the window granting behavior explain it much
better, learned a lot from your analysis.

I sent a fix based on your suggestion, and saw it's been applied to
your tree:

  sched_ext/scx_flatcg: Fix cvtime true-up on slice overrun

  https://lore.kernel.org/all/20260815045905.3431991-1-cui.tao@linux.dev/

Kept your approach (s64 delta, keep dividends positive) and added
A/B numbers from the same VM setup in the commit message.

Looking forward to more of this while digging through sched_ext.

Thanks,
Tao

> Thanks.
> 
> --
> tejun