Re: [PATCH rfc 09/12] sched: psi: bpf hook to handle psi events

[Suren Baghdasaryan <surenb@google.com>]

On Sun, Apr 27, 2025 at 8:37 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> Introduce a bpf hook to handle psi events. The primary intended
> purpose of this hook is to declare OOM events based on the reaching
> a certain memory pressure level, similar to what systemd-oomd and oomd
> are doing in userspace.

It's a bit awkward that this requires additional userspace action to
create PSI triggers. I have almost no experience with BPF, so this
might be a stupid question, but maybe we could provide a bpf kfunc for
the BPF handler to register its PSI trigger(s) upon handler
registration?


>
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  kernel/sched/psi.c | 36 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 35 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 1396674fa722..4c4eb4ead8f6 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -176,6 +176,32 @@ static void psi_avgs_work(struct work_struct *work);
>
>  static void poll_timer_fn(struct timer_list *t);
>
> +#ifdef CONFIG_BPF_SYSCALL
> +__bpf_hook_start();
> +
> +__weak noinline int bpf_handle_psi_event(struct psi_trigger *t)
> +{
> +       return 0;
> +}
> +
> +__bpf_hook_end();
> +
> +BTF_KFUNCS_START(bpf_psi_hooks)
> +BTF_ID_FLAGS(func, bpf_handle_psi_event, KF_SLEEPABLE)
> +BTF_KFUNCS_END(bpf_psi_hooks)
> +
> +static const struct btf_kfunc_id_set bpf_psi_hook_set = {
> +       .owner = THIS_MODULE,
> +       .set   = &bpf_psi_hooks,
> +};
> +
> +#else
> +static inline int bpf_handle_psi_event(struct psi_trigger *t)
> +{
> +       return 0;
> +}
> +#endif
> +
>  static void group_init(struct psi_group *group)
>  {
>         int cpu;
> @@ -489,6 +515,7 @@ static void update_triggers(struct psi_group *group, u64 now,
>
>                 /* Generate an event */
>                 if (cmpxchg(&t->event, 0, 1) == 0) {
> +                       bpf_handle_psi_event(t);
>                         if (t->of)
>                                 kernfs_notify(t->of->kn);
>                         else
> @@ -1655,6 +1682,8 @@ static const struct proc_ops psi_irq_proc_ops = {
>
>  static int __init psi_proc_init(void)
>  {
> +       int err = 0;
> +
>         if (psi_enable) {
>                 proc_mkdir("pressure", NULL);
>                 proc_create("pressure/io", 0666, NULL, &psi_io_proc_ops);
> @@ -1662,9 +1691,14 @@ static int __init psi_proc_init(void)
>                 proc_create("pressure/cpu", 0666, NULL, &psi_cpu_proc_ops);
>  #ifdef CONFIG_IRQ_TIME_ACCOUNTING
>                 proc_create("pressure/irq", 0666, NULL, &psi_irq_proc_ops);
> +#endif
> +#ifdef CONFIG_BPF_SYSCALL
> +               err = register_btf_fmodret_id_set(&bpf_psi_hook_set);
> +               if (err)
> +                       pr_err("error while registering bpf psi hooks: %d", err);
>  #endif
>         }
> -       return 0;
> +       return err;
>  }
>  module_init(psi_proc_init);
>
> --
> 2.49.0.901.g37484f566f-goog
>