안녕하세요 호서대학교 석사(과정) 이우영 입니다.

오늘은 cpu_idle 에서 잠깐 다뤘던 tickless 에 대해서 알아 보도록 하겠습니다.

tickless란?

tickless 모드는 전력 조모를 줄이기 위한 방법의 하나이다.

전통적으로 리눅스 커널은 각각의 CPU에 대한 정기적인 타이머를 사용한다.

이 주기적인 타이머 틱은 idle 상태에서도 쉬지 않고 동작 합니다.

idle 상태에서는 타이머가 동작할 이유가 없습니다.

그래서 이시간에는 타이머를 쉬게 하자 해서 나온것이 이 tickless 모드 입니다.

tickless 모드는 리눅스 커널 2.6.21 커널부터 mainstream에 포함되었다.

소스코드는 kernel/time/tick-sched.c 에  구현되어 있다.

tick_nohz_stop_sched_tick(1); tickless 모드로 진입한다.

/**
 * tick_nohz_stop_sched_tick - stop the idle tick from the idle task
 *
 * When the next event is more than a tick into the future, stop the idle tick
 * Called either from the idle loop or from irq_exit() when an idle period was
 * just interrupted by an interrupt which did not cause a reschedule.
 */
void tick_nohz_stop_sched_tick(int inidle)
{
        unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
        struct tick_sched *ts;
        ktime_t last_update, expires, now;
        struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
        int cpu;

        local_irq_save(flags);

        cpu = smp_processor_id();
        ts = &per_cpu(tick_cpu_sched, cpu);
        now = tick_nohz_start_idle(ts);

        /*
         * If this cpu is offline and it is the one which updates
         * jiffies, then give up the assignment and let it be taken by
         * the cpu which runs the tick timer next. If we don't drop
         * this here the jiffies might be stale and do_timer() never
         * invoked.
         */
        if (unlikely(!cpu_online(cpu))) {
                if (cpu == tick_do_timer_cpu)
                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;
        }

        if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
                goto end;

        if (!inidle && !ts->inidle)
                goto end;

        ts->inidle = 1;

        if (need_resched())
                goto end;

        if (unlikely(local_softirq_pending() && cpu_online(cpu))) {
                static int ratelimit;

                if (ratelimit < 10) {
                        printk(KERN_ERR "NOHZ: local_softirq_pending %02x\n",
                               local_softirq_pending());
                        ratelimit++;
                }
                goto end;
        }

        ts->idle_calls++;
        /* Read jiffies and the time when jiffies were updated last */
        do {
                seq = read_seqbegin(&xtime_lock);
                last_update = last_jiffies_update;
                last_jiffies = jiffies;
        } while (read_seqretry(&xtime_lock, seq));

        /* Get the next timer wheel timer */
        next_jiffies = get_next_timer_interrupt(last_jiffies);
        delta_jiffies = next_jiffies - last_jiffies;

        if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu))
                delta_jiffies = 1;
        /*
         * Do not stop the tick, if we are only one off
         * or if the cpu is required for rcu
         */
        if (!ts->tick_stopped && delta_jiffies == 1)
                goto out;

        /* Schedule the tick, if we are at least one jiffie off */
        if ((long)delta_jiffies >= 1) {

                /*
                * calculate the expiry time for the next timer wheel
                * timer
                */
                expires = ktime_add_ns(last_update, tick_period.tv64 *
                                   delta_jiffies);

                /*
                 * If this cpu is the one which updates jiffies, then
                 * give up the assignment and let it be taken by the
                 * cpu which runs the tick timer next, which might be
                 * this cpu as well. If we don't drop this here the
                 * jiffies might be stale and do_timer() never
                 * invoked.
                 */
                if (cpu == tick_do_timer_cpu)
                        tick_do_timer_cpu = TICK_DO_TIMER_NONE;

                if (delta_jiffies > 1)
                        cpumask_set_cpu(cpu, nohz_cpu_mask);

                /* Skip reprogram of event if its not changed */
                if (ts->tick_stopped && ktime_equal(expires, dev->next_event))
                        goto out;

                /*
                 * nohz_stop_sched_tick can be called several times before
                 * the nohz_restart_sched_tick is called. This happens when
                 * interrupts arrive which do not cause a reschedule. In the
                 * first call we save the current tick time, so we can restart
                 * the scheduler tick in nohz_restart_sched_tick.
                 */
                if (!ts->tick_stopped) {
                        if (select_nohz_load_balancer(1)) {
                                /*
                                 * sched tick not stopped!
                                 */
                                cpumask_clear_cpu(cpu, nohz_cpu_mask);
                                goto out;
                        }

                        ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
                        ts->tick_stopped = 1;
                        ts->idle_jiffies = last_jiffies;
                        rcu_enter_nohz();
                }

                ts->idle_sleeps++;

                /*
                 * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
                 * there is no timer pending or at least extremly far
                 * into the future (12 days for HZ=1000). In this case
                 * we simply stop the tick timer:
                 */
                if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
                        ts->idle_expires.tv64 = KTIME_MAX;
                        if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
                                hrtimer_cancel(&ts->sched_timer);
                        goto out;
                }

                /* Mark expiries */
                ts->idle_expires = expires;

                if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
                        hrtimer_start(&ts->sched_timer, expires,
                                      HRTIMER_MODE_ABS);
                        /* Check, if the timer was already in the past */
                        if (hrtimer_active(&ts->sched_timer))
                                goto out;
                } else if (!tick_program_event(expires, 0))
                                goto out;
                /*
                 * We are past the event already. So we crossed a
                 * jiffie boundary. Update jiffies and raise the
                 * softirq.
                 */
                tick_do_update_jiffies64(ktime_get());
                cpumask_clear_cpu(cpu, nohz_cpu_mask);
        }
        raise_softirq_irqoff(TIMER_SOFTIRQ);
out:
        ts->next_jiffies = next_jiffies;
        ts->last_jiffies = last_jiffies;
        ts->sleep_length = ktime_sub(dev->next_event, now);
end:
        local_irq_restore(flags);
}

tick_nohz_restart_sched_tick();  tickless 모드에서 빠져 나온다.

/**
 * tick_nohz_restart_sched_tick - restart the idle tick from the idle task
 *
 * Restart the idle tick when the CPU is woken up from idle
 */
void tick_nohz_restart_sched_tick(void)
{
        int cpu = smp_processor_id();
        struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
#ifndef CONFIG_VIRT_CPU_ACCOUNTING
        unsigned long ticks;
#endif
        ktime_t now;

        local_irq_disable();
        tick_nohz_stop_idle(cpu);

        if (!ts->inidle || !ts->tick_stopped) {
                ts->inidle = 0;
                local_irq_enable();
                return;
        }

        ts->inidle = 0;

        rcu_exit_nohz();

        /* Update jiffies first */
        select_nohz_load_balancer(0);
        now = ktime_get();
        tick_do_update_jiffies64(now);
        cpumask_clear_cpu(cpu, nohz_cpu_mask);

#ifndef CONFIG_VIRT_CPU_ACCOUNTING
        /*
         * We stopped the tick in idle. Update process times would miss the
         * time we slept as update_process_times does only a 1 tick
         * accounting. Enforce that this is accounted to idle !
         */
        ticks = jiffies - ts->idle_jiffies;
        /*
         * We might be one off. Do not randomly account a huge number of ticks!
         */
        if (ticks && ticks < LONG_MAX)
                account_idle_ticks(ticks);
#endif

        touch_softlockup_watchdog();
        /*
         * Cancel the scheduled timer and restore the tick
         */
        ts->tick_stopped  = 0;
        ts->idle_exittime = now;

        tick_nohz_restart(ts, now);

        local_irq_enable();
}

그리고 tickless 모드를 사용하기 위해서는 커널 옵션에서 설정을 해주어야 한다.

s3c6410 에서는 idle 진입 할 때 tickless 모드도 지원한다.

오늘은 간단하게 끝냈습니다.

그럼 다음시간에 만나요~