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

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

From: Tao Cui <cuitao@kylinos.cn>

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>
---
 tools/sched_ext/scx_flatcg.bpf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
index 0fd214cc61da..2d178c2ecacc 100644
--- a/tools/sched_ext/scx_flatcg.bpf.c
+++ b/tools/sched_ext/scx_flatcg.bpf.c
@@ -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)
-- 
2.43.0