[PATCH RFC] sched_ext: warn when cpu.max is set but the BPF scheduler doesn't implement bandwidth control

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

From: Tao Cui <cuitao@kylinos.cn>

The kernel stores cpu.max bandwidth parameters in the task_group and
passes them to the BPF scheduler via ops.cgroup_set_bandwidth() and
scx_cgroup_init_args, but does not enforce the quota itself. If the
loaded BPF scheduler doesn't implement the callback, cpu.max is
silently ignored -- the cgroup gets unlimited CPU regardless of the
configured quota.

Of the example schedulers, only scx_qmap implements the callback --
and only to bpf_printk() the parameters, so no in-tree scheduler
actually enforces the quota. Measured with scx_simple: a
cgroup with cpu.max = "50000 100000" (50% of one CPU) and one
busy task used 9946ms of CPU in 10 seconds with nr_throttled
remaining 0.

Print a one-time warning when a finite quota is configured on a
cgroup while the active scheduler lacks the callback, so users and
container orchestrators know the quota is not enforced.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 kernel/sched/ext/ext.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 10af28a9f2c0..de786d0b928a 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4953,6 +4953,12 @@ void scx_group_set_bandwidth(struct task_group *tg,
 	     tg->scx.bw_burst_us != burst_us))
 		SCX_CALL_OP(sch, cgroup_set_bandwidth, NULL,
 			    tg_cgrp(tg), period_us, quota_us, burst_us);
+	else if (scx_cgroup_enabled && sch &&
+		 !SCX_HAS_OP(sch, cgroup_set_bandwidth) &&
+		 quota_us != RUNTIME_INF)
+		pr_warn_once("sched_ext: BPF scheduler \"%s\" does not implement "
+			     "ops.cgroup_set_bandwidth(); cpu.max will not be enforced\n",
+			     sch->ops.name);
 
 	tg->scx.bw_period_us = period_us;
 	tg->scx.bw_quota_us = quota_us;
-- 
2.43.0