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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The new warning for unenforced `cpu.max` is missing during BPF scheduler initialization, meaning preexisting cgroup quotas are silently ignored without notification.
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--- Patch [1]: [PATCH RFC] sched_ext: warn when cpu.max is set but the BPF scheduler doesn't implement bandwidth control ---
commit 88bd45e8c02c048f00992a1d03614c8ef709c738
Author: Tao Cui <cuitao@kylinos.cn>

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

This commit adds a one-time warning when a finite cpu.max quota is
configured on a cgroup but the active BPF scheduler lacks the
cgroup_set_bandwidth callback. The intent is to notify users and
orchestrators that the quota is silently ignored and not enforced by the
scheduler.

> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 18183062f751c..1067d271f6b27 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -4565,6 +4565,12 @@ 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);

[Severity: Medium]
Will this warning also catch preexisting cgroup bandwidth configurations?

If a user configures a finite cpu.max limit on a cgroup before the BPF
scheduler is loaded, the initialization path in scx_cgroup_init() passes
the existing quota to the scheduler:

kernel/sched/ext/ext.c:scx_cgroup_init() {
    ...
		struct scx_cgroup_init_args args = {
			.weight = tg->scx.weight,
			.bw_period_us = tg->scx.bw_period_us,
			.bw_quota_us = tg->scx.bw_quota_us,
			.bw_burst_us = tg->scx.bw_burst_us,
		};
    ...
		ret = SCX_CALL_OP_RET(sch, cgroup_init, NULL,
				      css->cgroup, &args);
    ...
}

Should scx_cgroup_init() also check for an unenforced quota and emit this
warning, to ensure users are notified when preexisting configurations are
silently ignored?

>  
>  	tg->scx.bw_period_us = period_us;
>  	tg->scx.bw_quota_us = quota_us;

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Sashiko AI review · https://sashiko.dev/#/patchset/20260818135328.174152-1-cui.tao@linux.dev?part=1