"luwei (O)" [off-list ref] writes:

在 2021/7/2 3:20 AM, Alexei Starovoitov 写道:

quoted

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);

[...]

quoted

+static enum hrtimer_restart bpf_timer_cb(struct hrtimer *hrtimer)
+{
+	struct bpf_hrtimer *t = container_of(hrtimer, struct bpf_hrtimer, timer);
+	struct bpf_map *map = t->map;
+	void *value = t->value;
+	struct bpf_timer_kern *timer = value + map->timer_off;
+	struct bpf_prog *prog;
+	void *callback_fn;
+	void *key;
+	u32 idx;
+	int ret;
+
+	/* The triple underscore bpf_spin_lock is a direct call
+	 * to BPF_CALL_1(bpf_spin_lock) which does irqsave.
+	 */
+	____bpf_spin_lock(&timer->lock);
+	/* callback_fn and prog need to match. They're updated together
+	 * and have to be read under lock.
+	 */
+	prog = t->prog;
+	callback_fn = t->callback_fn;
+
+	/* wrap bpf subprog invocation with prog->refcnt++ and -- to make
+	 * sure that refcnt doesn't become zero when subprog is executing.
+	 * Do it under lock to make sure that bpf_timer_start doesn't drop
+	 * prev prog refcnt to zero before timer_cb has a chance to bump it.
+	 */
+	bpf_prog_inc(prog);
+	____bpf_spin_unlock(&timer->lock);
+
+	/* bpf_timer_cb() runs in hrtimer_run_softirq. It doesn't migrate and
+	 * cannot be preempted by another bpf_timer_cb() on the same cpu.
+	 * Remember the timer this callback is servicing to prevent
+	 * deadlock if callback_fn() calls bpf_timer_cancel() on the same timer.
+	 */
+	this_cpu_write(hrtimer_running, t);
+	if (map->map_type == BPF_MAP_TYPE_ARRAY) {
+		struct bpf_array *array = container_of(map, struct bpf_array, map);
+
+		/* compute the key */
+		idx = ((char *)value - array->value) / array->elem_size;
+		key = &idx;
+	} else { /* hash or lru */
+		key = value - round_up(map->key_size, 8);
+	}
+
+	ret = BPF_CAST_CALL(callback_fn)((u64)(long)map,
+					 (u64)(long)key,
+					 (u64)(long)value, 0, 0);
+	WARN_ON(ret != 0); /* Next patch moves this check into the verifier */
+	bpf_prog_put(prog);
+
+	this_cpu_write(hrtimer_running, NULL);
+	return HRTIMER_NORESTART;
+}
+

Your patch is awesome, and I tried your example and it works, my 
call_back() is called after a certained time.  But  I am confued that 
the bpf_timer_cb() function only returns HRTIMER_NORESTART,

how to trigger a periodic task, that means I want to call my call_back() 
periodically, not just for one time ?

The callback just needs to reschedule itself by calling
bpf_timer_start() again before exiting... :)

-Toke