Function sched_pctcpu_update

freebsd/kern/sched_ule.c:1721–1741

* Update the percent cpu tracking information when it is requested or * the total history exceeds the maximum. We keep a sliding history of * tick counts that slowly decays. This is less precise than the 4BSD * mechanism since it happens with less regular and frequent events. */

 * mechanism since it happens with less regular and frequent events.
 */
static void
sched_pctcpu_update(struct td_sched *ts, int run)
{
	int t = ticks;

	/*
	 * The signed difference may be negative if the thread hasn't run for
	 * over half of the ticks rollover period.
	 */
	if ((u_int)(t - ts->ts_ltick) >= SCHED_TICK_TARG) {
		ts->ts_ticks = 0;
		ts->ts_ftick = t - SCHED_TICK_TARG;
	} else if (t - ts->ts_ftick >= SCHED_TICK_MAX) {
		ts->ts_ticks = (ts->ts_ticks / (ts->ts_ltick - ts->ts_ftick)) *
		    (ts->ts_ltick - (t - SCHED_TICK_TARG));
		ts->ts_ftick = t - SCHED_TICK_TARG;
	}
	if (run)
		ts->ts_ticks += (t - ts->ts_ltick) << SCHED_TICK_SHIFT;
	ts->ts_ltick = t;
}

/*
 * Adjust the priority of a thread.  Move it to the appropriate run-queue