dc_tick
static void dc_tick(void *);
callout_reset(&sc->dc_stat_ch, hz/10, dc_tick, sc);
callout_reset(&sc->dc_stat_ch, hz, dc_tick, sc);
callout_reset(&sc->dc_stat_ch, hz/10, dc_tick, sc);
callout_reset(&sc->dc_stat_ch, hz, dc_tick, sc);