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sched_ext/scx_flatcg: expire cached hweights on weight changes

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fcg_cgroup_set_weight() updates cgc->weight and the parent's
child_weight_sum but doesn't bump hweight_gen, so the hweights cached by
cgrp_refresh_hweight() stay stale until some task activation bumps the
generation. For cgroups whose tasks never go through a 0->n runnable
transition (e.g. persistently busy ones), a cpu.weight change never
propagates to scheduling at all.

Bump hweight_gen on weight changes so the next refresh recomputes with
the new weight.

Verified on a flatcg VM: a live cpu.weight 100->800 change on a busy
cgroup leaves HWT update at 0 and the distribution unchanged; with it,
hweight_gen increments and the refresh recomputes.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Tao Cui 2026-08-14 22:41:16 +08:00 committed by Tejun Heo
parent 03506edca6
commit 94c20e1fb3
1 changed files with 3 additions and 0 deletions
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3
tools/sched_ext/scx_flatcg.bpf.c
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@ -605,6 +605,9 @@ void BPF_STRUCT_OPS(fcg_cgroup_set_weight, struct cgroup *cgrp, u32 weight)
pcgc->child_weight_sum += (s64)weight - cgc->weight;
cgc->weight = weight;
bpf_spin_unlock(&cgv_tree_lock);
/* expire cached hweights so the new weight propagates */
__sync_fetch_and_add(&hweight_gen, 1);
}
static bool try_pick_next_cgroup(u64 *cgidp)