Helper function `bpf_timer_start`

This helper starts a timer.

Definition

Copyright (c) 2015 The Libbpf Authors. All rights reserved.

Set timer expiration N nanoseconds from the current time. The configured callback will be invoked in soft irq context on some cpu and will not repeat unless another bpf_timer_start() is made. In such case the next invocation can migrate to a different cpu. Since struct bpf_timer is a field inside map element the map owns the timer. The bpf_timer_set_callback() will increment refcnt of BPF program to make sure that callback_fn code stays valid. When user space reference to a map reaches zero all timers in a map are cancelled and corresponding program's refcnts are decremented. This is done to make sure that Ctrl-C of a user process doesn't leave any timers running. If map is pinned in bpffs the callback_fn can re-arm itself indefinitely. bpf_map_update/delete_elem() helpers and user space sys_bpf commands cancel and free the timer in the given map element. The map can contain timers that invoke callback_fn-s from different programs. The same callback_fn can serve different timers from different maps if key/value layout matches across maps. Every bpf_timer_set_callback() can have different callback_fn.

flags can be one of:

BPF_F_TIMER_ABS

Start the timer in absolute expire value instead of the default relative one.

BPF_F_TIMER_CPU_PIN

Timer will be pinned to the CPU of the caller.

Returns

0 on success. -EINVAL if timer was not initialized with bpf_timer_init() earlier or invalid flags are passed.

static long (* const bpf_timer_start)(struct bpf_timer *timer, __u64 nsecs, __u64 flags) = (void *) 171;

Usage

Docs could be improved

This part of the docs is incomplete, contributions are very welcome

Example

#include <linux/bpf.h>
#include <time.h>
#include <stdbool.h>
#include <errno.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>

struct elem {
    struct bpf_timer t;
};

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __uint(max_entries, 1);
    __type(key, int);
    __type(value, struct elem);
} hmap SEC(".maps");

static int timer_callback(void* hmap, int* key, struct bpf_timer *timer)
{
        bpf_printk("Callback was invoked do something useful");
    return 0;
}

SEC("cgroup_skb/egress")
int bpf_prog1(void *ctx)
{
    struct bpf_timer *timer;
    int err, key = 0;
    struct elem init;
    struct elem* ele;

    __builtin_memset(&init, 0, sizeof(struct elem));
    bpf_map_update_elem(&hmap, &key, &init, BPF_ANY);

    ele = bpf_map_lookup_elem(&hmap, &key);
    if (!ele)
        return 1;

    timer = &ele->t;
    err = bpf_timer_init(timer, &hmap, CLOCK_MONOTONIC);
    if (err && err != -EBUSY)
        return 1;

    bpf_timer_set_callback(timer, timer_callback);
    bpf_timer_start(timer, 0, 0);
    bpf_timer_cancel(timer);

    return 0;
}

char _license[] SEC("license") = "GPL";

Helper function bpf_timer_start