Re: [PATCH v6 bpf-next 03/11] bpf: Introduce bpf timers.

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

On Tue, Jul 13, 2021 at 6:05 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> Introduce 'struct bpf_timer { __u64 :64; __u64 :64; };' that can be embedded
> in hash/array/lru maps as a regular field and helpers to operate on it:
>
> // Initialize the timer.
> // First 4 bits of 'flags' specify clockid.
> // Only CLOCK_MONOTONIC, CLOCK_REALTIME, CLOCK_BOOTTIME are allowed.
> long bpf_timer_init(struct bpf_timer *timer, struct bpf_map *map, int flags);
>
> // Configure the timer to call 'callback_fn' static function.
> long bpf_timer_set_callback(struct bpf_timer *timer, void *callback_fn);
>
> // Arm the timer to expire 'nsec' nanoseconds from the current time.
> long bpf_timer_start(struct bpf_timer *timer, u64 nsec, u64 flags);
>
> // Cancel the timer and wait for callback_fn to finish if it was running.
> long bpf_timer_cancel(struct bpf_timer *timer);
>
> Here is how BPF program might look like:
> struct map_elem {
>     int counter;
>     struct bpf_timer timer;
> };
>
> struct {
>     __uint(type, BPF_MAP_TYPE_HASH);
>     __uint(max_entries, 1000);
>     __type(key, int);
>     __type(value, struct map_elem);
> } hmap SEC(".maps");
>
> static int timer_cb(void *map, int *key, struct map_elem *val);
> /* val points to particular map element that contains bpf_timer. */
>
> SEC("fentry/bpf_fentry_test1")
> int BPF_PROG(test1, int a)
> {
>     struct map_elem *val;
>     int key = 0;
>
>     val = bpf_map_lookup_elem(&hmap, &key);
>     if (val) {
>         bpf_timer_init(&val->timer, &hmap, CLOCK_REALTIME);
>         bpf_timer_set_callback(&val->timer, timer_cb);
>         bpf_timer_start(&val->timer, 1000 /* call timer_cb2 in 1 usec */, 0);
>     }
> }
>
> This patch adds helper implementations that rely on hrtimers
> to call bpf functions as timers expire.
> The following patches add necessary safety checks.
>
> Only programs with CAP_BPF are allowed to use bpf_timer.
>
> The amount of timers used by the program is constrained by
> the memcg recorded at map creation time.
>
> The bpf_timer_init() helper needs explicit 'map' argument because inner maps
> are dynamic and not known at load time. While the bpf_timer_set_callback() is
> receiving hidden 'aux->prog' argument supplied by the verifier.
>
> The prog pointer is needed to do refcnting of bpf program to make sure that
> program doesn't get freed while the timer is armed. This approach relies on
> "user refcnt" scheme used in prog_array that stores bpf programs for
> bpf_tail_call. The bpf_timer_set_callback() will increment the prog refcnt which is
> paired with bpf_timer_cancel() that will drop the prog refcnt. The
> ops->map_release_uref is responsible for cancelling the timers and dropping
> prog refcnt when user space reference to a map reaches zero.
> This uref approach is done to make sure that Ctrl-C of user space process will
> not leave timers running forever unless the user space explicitly pinned a map
> that contained timers in bpffs.
>
> bpf_timer_init() and bpf_timer_set_callback() will return -EPERM if map doesn't
> have user references (is not held by open file descriptor from user space and
> not pinned in bpffs).
>
> The bpf_map_delete_elem() and bpf_map_update_elem() operations cancel
> and free the timer if given map element had it allocated.
> "bpftool map update" command can be used to cancel timers.
>
> The 'struct bpf_timer' is explicitly __attribute__((aligned(8))) because
> '__u64 :64' has 1 byte alignment of 8 byte padding.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> Acked-by: Martin KaFai Lau <kafai@fb.com>
> ---
>  include/linux/bpf.h            |   3 +
>  include/uapi/linux/bpf.h       |  73 ++++++++
>  kernel/bpf/helpers.c           | 325 +++++++++++++++++++++++++++++++++
>  kernel/bpf/verifier.c          | 109 +++++++++++
>  kernel/trace/bpf_trace.c       |   2 +-
>  scripts/bpf_doc.py             |   2 +
>  tools/include/uapi/linux/bpf.h |  73 ++++++++
>  7 files changed, 586 insertions(+), 1 deletion(-)
>

[...]

> +       if (!atomic64_read(&(map->usercnt))) {
> +               /* maps with timers must be either held by user space
> +                * or pinned in bpffs.
> +                */
> +               ret = -EPERM;
> +               goto out;
> +       }
> +       /* allocate hrtimer via map_kmalloc to use memcg accounting */
> +       t = bpf_map_kmalloc_node(map, sizeof(*t), GFP_ATOMIC, NUMA_NO_NODE);

I wonder if it would make sense to use map->numa_node here to keep map
value and timer data in the same NUMA node?

> +       if (!t) {
> +               ret = -ENOMEM;
> +               goto out;
> +       }

[...]

> +
> +/* This function is called by map_delete/update_elem for individual element.
> + * By ops->map_release_uref when the user space reference to a map reaches zero
> + * and by ops->map_free when the kernel reference reaches zero.

is ops->map_free part still valid?

> + */
> +void bpf_timer_cancel_and_free(void *val)
> +{
> +       struct bpf_timer_kern *timer = val;
> +       struct bpf_hrtimer *t;
> +

[...]