atom_link_hb
callout_reset(&ntb->heartbeat_timer, 0, atom_link_hb, ntb);
callout_reset(&ntb->heartbeat_timer, timo, atom_link_hb, ntb);
callout_reset(&ntb->heartbeat_timer, NTB_HB_TIMEOUT * hz, atom_link_hb,
static void atom_link_hb(void *arg);