Re: [PATCH v1 13/14] sched: psi: implement bpf_psi_create_trigger() kfunc

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Aug 18, 2025 at 10:06 AM Roman Gushchin
<roman.gushchin@linux.dev> wrote:
>
> Implement a new bpf_psi_create_trigger() bpf kfunc, which allows
> to create new psi triggers and attach them to cgroups or be
> system-wide.
>
> Created triggers will exist until the struct ops is loaded and
> if they are attached to a cgroup until the cgroup exists.
>
> Due to a limitation of 5 arguments, the resource type and the "full"
> bit are squeezed into a single u32.
>
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  kernel/sched/bpf_psi.c | 84 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 84 insertions(+)
>
> diff --git a/kernel/sched/bpf_psi.c b/kernel/sched/bpf_psi.c
> index 2ea9d7276b21..94b684221708 100644
> --- a/kernel/sched/bpf_psi.c
> +++ b/kernel/sched/bpf_psi.c
> @@ -156,6 +156,83 @@ static const struct bpf_verifier_ops bpf_psi_verifier_ops = {
>         .is_valid_access = bpf_psi_ops_is_valid_access,
>  };
>
> +__bpf_kfunc_start_defs();
> +
> +/**
> + * bpf_psi_create_trigger - Create a PSI trigger
> + * @bpf_psi: bpf_psi struct to attach the trigger to
> + * @cgroup_id: cgroup Id to attach the trigger; 0 for system-wide scope
> + * @resource: resource to monitor (PSI_MEM, PSI_IO, etc) and the full bit.
> + * @threshold_us: threshold in us
> + * @window_us: window in us
> + *
> + * Creates a PSI trigger and attached is to bpf_psi. The trigger will be
> + * active unless bpf struct ops is unloaded or the corresponding cgroup
> + * is deleted.
> + *
> + * Resource's most significant bit encodes whether "some" or "full"
> + * PSI state should be tracked.
> + *
> + * Returns 0 on success and the error code on failure.
> + */
> +__bpf_kfunc int bpf_psi_create_trigger(struct bpf_psi *bpf_psi,
> +                                      u64 cgroup_id, u32 resource,
> +                                      u32 threshold_us, u32 window_us)
> +{
> +       enum psi_res res = resource & ~BPF_PSI_FULL;
> +       bool full = resource & BPF_PSI_FULL;
> +       struct psi_trigger_params params;
> +       struct cgroup *cgroup __maybe_unused = NULL;
> +       struct psi_group *group;
> +       struct psi_trigger *t;
> +       int ret = 0;
> +
> +       if (res >= NR_PSI_RESOURCES)
> +               return -EINVAL;
> +
> +#ifdef CONFIG_CGROUPS
> +       if (cgroup_id) {
> +               cgroup = cgroup_get_from_id(cgroup_id);
> +               if (IS_ERR_OR_NULL(cgroup))
> +                       return PTR_ERR(cgroup);
> +
> +               group = cgroup_psi(cgroup);
> +       } else
> +#endif
> +               group = &psi_system;

just a drive-by comment while skimming through the patch set: can't
you use IS_ENABLED(CONFIG_CGROUPS) and have a proper if/else with
proper {} ?

> +
> +       params.type = PSI_BPF;
> +       params.bpf_psi = bpf_psi;
> +       params.privileged = capable(CAP_SYS_RESOURCE);
> +       params.res = res;
> +       params.full = full;
> +       params.threshold_us = threshold_us;
> +       params.window_us = window_us;
> +
> +       t = psi_trigger_create(group, &params);
> +       if (IS_ERR(t))
> +               ret = PTR_ERR(t);
> +       else
> +               t->cgroup_id = cgroup_id;
> +
> +#ifdef CONFIG_CGROUPS
> +       if (cgroup)
> +               cgroup_put(cgroup);
> +#endif
> +
> +       return ret;
> +}
> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(bpf_psi_kfuncs)
> +BTF_ID_FLAGS(func, bpf_psi_create_trigger, KF_TRUSTED_ARGS)
> +BTF_KFUNCS_END(bpf_psi_kfuncs)
> +
> +static const struct btf_kfunc_id_set bpf_psi_kfunc_set = {
> +       .owner          = THIS_MODULE,
> +       .set            = &bpf_psi_kfuncs,
> +};
> +
>  static int bpf_psi_ops_reg(void *kdata, struct bpf_link *link)
>  {
>         struct bpf_psi_ops *ops = kdata;
> @@ -238,6 +315,13 @@ static int __init bpf_psi_struct_ops_init(void)
>         if (!bpf_psi_wq)
>                 return -ENOMEM;
>
> +       err = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
> +                                       &bpf_psi_kfunc_set);

would this make kfunc callable from any struct_ops, not just this psi one?

> +       if (err) {
> +               pr_warn("error while registering bpf psi kfuncs: %d", err);
> +               goto err;
> +       }
> +
>         err = register_bpf_struct_ops(&bpf_psi_bpf_ops, bpf_psi_ops);
>         if (err) {
>                 pr_warn("error while registering bpf psi struct ops: %d", err);
> --
> 2.50.1
>
>