Re: [PATCH v2 1/2] cgroup: add BPF kfuncs to read a cpu cgroup's stats

[Tejun Heo <tj@kernel.org>]

On Mon, Aug 17, 2026 at 05:24:49PM -0700, Ziyang Men wrote:
> +++ b/kernel/cgroup/bpf_cpu.c

Probably not the best file name. bpf_cgroup.c or maybe just put it in
cgroup.c?

> +/**
> + * bpf_css_to_task_group - Cast a CPU controller css to its task group
> + * @css: CPU controller css
> + *
> + * Must be called under RCU.
> + * A C cast does not give the verifier a task_group pointer. This kfunc
> + * preserves the task_group and per-CPU types needed to read cfs_rq.

The fact that this is used for per-CPU types now probably won't age well if
this grows more usages in the future.

> + *
> + * Return: The task group, or NULL if @css belongs to another controller.
> + */
> +__bpf_kfunc struct task_group *
> +bpf_css_to_task_group(struct cgroup_subsys_state *css)
> +{
> +	if (css->ss != &cpu_cgrp_subsys)

unlikely()?

> +		return NULL;
> +
> +	/* task_group embeds css at offset zero. */
> +	return (struct task_group *)css;

container_of()?

> +/**
> + * bpf_css_flush_rstat - Flush a cgroup subsystem's rstat data
> + * @css: cgroup subsystem state to flush
> + */
> +__bpf_kfunc void bpf_css_flush_rstat(struct cgroup_subsys_state *css)
> +{
> +	css_rstat_flush(css);
> +}

Why is this necessary? Isn't css_rstat_flush() already exposed as a kfunc?

> +/**
> + * bpf_cgroup_base_stat - Read a cgroup's base statistics
> + * @cgrp: cgroup to read from
> + * @out: zero-initialized output in nanoseconds
> + *
> + * CPU time is adjusted as for cpu.stat.
> + */
> +__bpf_kfunc void bpf_cgroup_base_stat(struct cgroup *cgrp,
> +				      struct cgroup_base_stat *out)
> +{
> +	if (cgroup_parent(cgrp)) {
> +		__css_rstat_lock(&cgrp->self, -1);
> +		*out = cgrp->bstat;
> +		cputime_adjust(&cgrp->bstat.cputime, &cgrp->prev_cputime,
> +			       &out->cputime.utime, &out->cputime.stime);
> +		__css_rstat_unlock(&cgrp->self, -1);
> +	} else {
> +		root_cgroup_cputime(out);
> +	}
> +}
> +
> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(bpf_rstat_common_kfunc_ids)
> +BTF_ID_FLAGS(func, bpf_css_flush_rstat, KF_SLEEPABLE)
> +BTF_ID_FLAGS(func, bpf_cgroup_base_stat, KF_SLEEPABLE)

Why are these SLEEPABLE?

Thanks.

-- 
tejun