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Subject[patch 14/19] dynticks: core code
From: Thomas Gleixner <tglx@linutronix.de>

dynamic ticks core code.

This is an extension to the per-cpu sched_tick timer of the high resolution
timer functionality. The sched_tick timer is reprogrammed to a longer timeout
before going idle, when no timer events are due in the next tick. The
periodic tick is resumed when the CPU leaves the idle state. If a non-timer
IRQ hits the idle task jiffies are updated from irq_enter before calling the
interrupt code, otherwise the interrupt handler would eventually deal with a
stale jiffy value.

The per-cpu idle statistics information can be used to optimize power
management decisions.

More detailed information is available in Documentation/hrtimer/highres.txt

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Index: linux-2.6.19-rc5-mm1/include/linux/hardirq.h
===================================================================
--- linux-2.6.19-rc5-mm1.orig/include/linux/hardirq.h 2006-11-09 20:14:41.000000000 +0100
+++ linux-2.6.19-rc5-mm1/include/linux/hardirq.h 2006-11-09 20:16:11.000000000 +0100
@@ -106,6 +106,16 @@ static inline void account_system_vtime(
* always balanced, so the interrupted value of ->hardirq_context
* will always be restored.
*/
+#define __irq_enter() \
+ do { \
+ account_system_vtime(current); \
+ add_preempt_count(HARDIRQ_OFFSET); \
+ trace_hardirq_enter(); \
+ } while (0)
+
+/*
+ * Enter irq context (on NO_HZ, update jiffies):
+ */
extern void irq_enter(void);

/*
@@ -123,7 +133,7 @@ extern void irq_enter(void);
*/
extern void irq_exit(void);

-#define nmi_enter() do { lockdep_off(); irq_enter(); } while (0)
+#define nmi_enter() do { lockdep_off(); __irq_enter(); } while (0)
#define nmi_exit() do { __irq_exit(); lockdep_on(); } while (0)

#endif /* LINUX_HARDIRQ_H */
Index: linux-2.6.19-rc5-mm1/include/linux/hrtimer.h
===================================================================
--- linux-2.6.19-rc5-mm1.orig/include/linux/hrtimer.h 2006-11-09 20:16:06.000000000 +0100
+++ linux-2.6.19-rc5-mm1/include/linux/hrtimer.h 2006-11-09 20:16:11.000000000 +0100
@@ -22,6 +22,7 @@
#include <linux/list.h>
#include <linux/wait.h>

+struct seq_file;
struct hrtimer_clock_base;
struct hrtimer_cpu_base;

@@ -178,6 +179,17 @@ struct hrtimer_clock_base {
* in the softirq.
* @sched_timer: hrtimer to schedule the periodic tick in high
* resolution mode
+ * @nr_events: Total number of timer interrupt events
+ * @idle_tick: Store the last idle tick expiry time when the tick
+ * timer is modified for idle sleeps. This is necessary
+ * to resume the tick timer operation in the timeline
+ * when the CPU returns from idle
+ * @tick_stopped: Indicator that the idle tick has been stopped
+ * @idle_jiffies: jiffies at the entry to idle for idle time accounting
+ * @idle_calls: Total number of idle calls
+ * @idle_sleeps: Number of idle calls, where the sched tick was stopped
+ * @idle_entrytime: Time when the idle call was entered
+ * @idle_sleeptime: Sum of the time slept in idle with sched tick stopped
*/
struct hrtimer_cpu_base {
spinlock_t lock;
@@ -189,6 +201,16 @@ struct hrtimer_cpu_base {
unsigned long check_clocks;
struct list_head cb_pending;
struct hrtimer sched_timer;
+ unsigned long nr_events;
+#endif
+#ifdef CONFIG_NO_HZ
+ ktime_t idle_tick;
+ int tick_stopped;
+ unsigned long idle_jiffies;
+ unsigned long idle_calls;
+ unsigned long idle_sleeps;
+ ktime_t idle_entrytime;
+ ktime_t idle_sleeptime;
#endif
};

@@ -295,6 +317,18 @@ extern void hrtimer_run_queues(void);
/* Resume notification */
void hrtimer_notify_resume(void);

+#ifdef CONFIG_NO_HZ
+extern void hrtimer_stop_sched_tick(void);
+extern void hrtimer_restart_sched_tick(void);
+extern void hrtimer_update_jiffies(void);
+extern void show_no_hz_stats(struct seq_file *p);
+#else
+static inline void hrtimer_stop_sched_tick(void) { }
+static inline void hrtimer_restart_sched_tick(void) { }
+static inline void hrtimer_update_jiffies(void) { }
+static inline void show_no_hz_stats(struct seq_file *p) { }
+#endif
+
/* Bootup initialization: */
extern void __init hrtimers_init(void);

Index: linux-2.6.19-rc5-mm1/kernel/hrtimer.c
===================================================================
--- linux-2.6.19-rc5-mm1.orig/kernel/hrtimer.c 2006-11-09 20:16:06.000000000 +0100
+++ linux-2.6.19-rc5-mm1/kernel/hrtimer.c 2006-11-09 20:16:11.000000000 +0100
@@ -44,6 +44,7 @@
#include <linux/profile.h>
#include <linux/seq_file.h>
#include <linux/err.h>
+#include <linux/kernel_stat.h>

#include <asm/uaccess.h>

@@ -482,6 +483,7 @@ static void update_jiffies64(ktime_t now
{
unsigned long seq;
ktime_t delta;
+ unsigned long ticks = 0;

/* Preevaluate to avoid lock contention */
do {
@@ -497,7 +499,6 @@ static void update_jiffies64(ktime_t now

delta = ktime_sub(now, last_jiffies_update);
if (delta.tv64 >= nsec_per_hz.tv64) {
- unsigned long ticks = 1;

delta = ktime_sub(delta, nsec_per_hz);
last_jiffies_update = ktime_add(last_jiffies_update,
@@ -511,13 +512,238 @@ static void update_jiffies64(ktime_t now

last_jiffies_update = ktime_add_ns(last_jiffies_update,
incr * ticks);
- ticks++;
}
+ ticks++;
do_timer(ticks);
}
write_sequnlock(&xtime_lock);
}

+#ifdef CONFIG_NO_HZ
+/**
+ * hrtimer_update_jiffies - update jiffies when idle was interrupted
+ *
+ * Called from interrupt entry when the CPU was idle
+ *
+ * In case the sched_tick was stopped on this CPU, we have to check if jiffies
+ * must be updated. Otherwise an interrupt handler could use a stale jiffy
+ * value.
+ */
+void hrtimer_update_jiffies(void)
+{
+ struct hrtimer_cpu_base *cpu_base = &__get_cpu_var(hrtimer_bases);
+ unsigned long flags;
+ ktime_t now;
+
+ if (!cpu_base->tick_stopped || !cpu_base->hres_active)
+ return;
+
+ now = ktime_get();
+
+ local_irq_save(flags);
+ update_jiffies64(now);
+ local_irq_restore(flags);
+}
+
+/**
+ * hrtimer_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 a idle period was
+ * just interrupted by a interrupt which did not cause a reschedule.
+ */
+void hrtimer_stop_sched_tick(void)
+{
+ struct hrtimer_cpu_base *cpu_base = &__get_cpu_var(hrtimer_bases);
+ unsigned long seq, last_jiffies, next_jiffies;
+ ktime_t last_update, expires, now;
+ unsigned long delta_jiffies;
+ unsigned long flags;
+
+ if (unlikely(!cpu_base->hres_active))
+ return;
+
+ local_irq_save(flags);
+
+ now = ktime_get();
+ /*
+ * When called from irq_exit we need to account the idle sleep time
+ * correctly.
+ */
+ if (cpu_base->tick_stopped) {
+ ktime_t delta = ktime_sub(now, cpu_base->idle_entrytime);
+
+ cpu_base->idle_sleeptime = ktime_add(cpu_base->idle_sleeptime,
+ delta);
+ }
+
+ cpu_base->idle_entrytime = now;
+ cpu_base->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 ((long)delta_jiffies >= 1) {
+ /*
+ * hrtimer_stop_sched_tick can be called several times before
+ * the hrtimer_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 hrtimer_restart_sched_tick.
+ */
+ if (!cpu_base->tick_stopped) {
+ cpu_base->idle_tick = cpu_base->sched_timer.expires;
+ cpu_base->tick_stopped = 1;
+ cpu_base->idle_jiffies = last_jiffies;
+ }
+ /* calculate the expiry time for the next timer wheel timer */
+ expires = ktime_add_ns(last_update,
+ nsec_per_hz.tv64 * delta_jiffies);
+ hrtimer_start(&cpu_base->sched_timer, expires,
+ HRTIMER_MODE_ABS);
+ cpu_base->idle_sleeps++;
+ } else {
+ /* Raise the softirq if the timer wheel is behind jiffies */
+ if ((long) delta_jiffies < 0)
+ raise_softirq_irqoff(TIMER_SOFTIRQ);
+ }
+
+ local_irq_restore(flags);
+}
+
+/**
+ * hrtimer_restart_sched_tick - restart the idle tick from the idle task
+ *
+ * Restart the idle tick when the CPU is woken up from idle
+ */
+void hrtimer_restart_sched_tick(void)
+{
+ struct hrtimer_cpu_base *cpu_base = &__get_cpu_var(hrtimer_bases);
+ unsigned long ticks;
+ ktime_t now, delta;
+
+ if (!cpu_base->hres_active || !cpu_base->tick_stopped)
+ return;
+
+ /* Update jiffies first */
+ now = ktime_get();
+
+ local_irq_disable();
+ update_jiffies64(now);
+
+ /* Account the idle time */
+ delta = ktime_sub(now, cpu_base->idle_entrytime);
+ cpu_base->idle_sleeptime = ktime_add(cpu_base->idle_sleeptime, delta);
+
+ /*
+ * 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 - cpu_base->idle_jiffies;
+ /*
+ * We might be one off. Do not randomly account a huge number of ticks!
+ */
+ if (ticks && ticks < LONG_MAX) {
+ add_preempt_count(HARDIRQ_OFFSET);
+ account_system_time(current, HARDIRQ_OFFSET,
+ jiffies_to_cputime(ticks));
+ sub_preempt_count(HARDIRQ_OFFSET);
+ }
+
+ /*
+ * Cancel the scheduled timer and restore the tick
+ */
+ cpu_base->tick_stopped = 0;
+ hrtimer_cancel(&cpu_base->sched_timer);
+ cpu_base->sched_timer.expires = cpu_base->idle_tick;
+
+ while (1) {
+ /* Forward the time to expire in the future */
+ hrtimer_forward(&cpu_base->sched_timer, now, nsec_per_hz);
+ hrtimer_start(&cpu_base->sched_timer,
+ cpu_base->sched_timer.expires, HRTIMER_MODE_ABS);
+
+ /* Check, if the timer was already in the past */
+ if (hrtimer_active(&cpu_base->sched_timer))
+ break;
+ /* Update jiffies and reread time */
+ update_jiffies64(now);
+ now = ktime_get();
+ }
+ local_irq_enable();
+}
+
+/**
+ * show_no_hz_stats - print out the no hz statistics
+ *
+ * The no_hz statistics are appended at the end of /proc/stats
+ *
+ * I: total number of idle calls
+ * S: number of idle calls which stopped the sched tick
+ * T: Summed up sleep time in idle with sched tick stopped (unit is seconds)
+ * A: Average sleep time: T/S (unit is seconds)
+ * E: Total number of timer interrupt events
+ */
+void show_no_hz_stats(struct seq_file *p)
+{
+ unsigned long calls = 0, sleeps = 0, events = 0;
+ struct timeval tsum, tavg;
+ ktime_t totaltime = { .tv64 = 0 };
+ int cpu;
+
+ for_each_online_cpu(cpu) {
+ struct hrtimer_cpu_base *base = &per_cpu(hrtimer_bases, cpu);
+
+ calls += base->idle_calls;
+ sleeps += base->idle_sleeps;
+ totaltime = ktime_add(totaltime, base->idle_sleeptime);
+ events += base->nr_events;
+
+#ifdef CONFIG_SMP
+ tsum = ktime_to_timeval(base->idle_sleeptime);
+ if (base->idle_sleeps) {
+ uint64_t nsec = ktime_to_ns(base->idle_sleeptime);
+
+ do_div(nsec, base->idle_sleeps);
+ tavg = ns_to_timeval(nsec);
+ } else
+ tavg.tv_sec = tavg.tv_usec = 0;
+
+ seq_printf(p, "nohz cpu%d I:%lu S:%lu T:%d.%06d A:%d.%06d E: %lu\n",
+ cpu, base->idle_calls, base->idle_sleeps,
+ (int) tsum.tv_sec, (int) tsum.tv_usec,
+ (int) tavg.tv_sec, (int) tavg.tv_usec,
+ base->nr_events);
+#endif
+ }
+
+ tsum = ktime_to_timeval(totaltime);
+ if (sleeps) {
+ uint64_t nsec = ktime_to_ns(totaltime);
+
+ do_div(nsec, sleeps);
+ tavg = ns_to_timeval(nsec);
+ } else
+ tavg.tv_sec = tavg.tv_usec = 0;
+
+ seq_printf(p, "nohz total I:%lu S:%lu T:%d.%06d A:%d.%06d E: %lu\n",
+ calls, sleeps,
+ (int) tsum.tv_sec, (int) tsum.tv_usec,
+ (int) tavg.tv_sec, (int) tavg.tv_usec,
+ events);
+}
+
+#endif
+
/*
* We rearm the timer until we get disabled by the idle code
* Called with interrupts disabled.
@@ -527,12 +753,30 @@ static enum hrtimer_restart hrtimer_sche
struct hrtimer_cpu_base *cpu_base =
container_of(timer, struct hrtimer_cpu_base, sched_timer);
struct pt_regs *regs = get_irq_regs();
+ ktime_t now = ktime_get();
+
+ /* Check, if the jiffies need an update */
+ update_jiffies64(now);

/*
* Do not call, when we are not in irq context and have
* no valid regs pointer
*/
if (regs) {
+#ifdef CONFIG_NO_HZ
+ /*
+ * When we are idle and the tick is stopped, we have to touch
+ * the watchdog as we might not schedule for a really long
+ * time. This happens on complete idle SMP systems while
+ * waiting on the login prompt. We also increment the "start of
+ * idle" jiffy stamp so the idle accounting adjustment we do
+ * when we go busy again does not account too much ticks.
+ */
+ if (cpu_base->tick_stopped) {
+ touch_softlockup_watchdog();
+ cpu_base->idle_jiffies++;
+ }
+#endif
/*
* update_process_times() might take tasklist_lock, hence
* drop the base lock. sched-tick hrtimers are per-CPU and
@@ -544,7 +788,13 @@ static enum hrtimer_restart hrtimer_sche
spin_lock(&cpu_base->lock);
}

- hrtimer_forward(timer, hrtimer_cb_get_time(timer), nsec_per_hz);
+#ifdef CONFIG_NO_HZ
+ /* Do not restart, when we are in the idle loop */
+ if (cpu_base->tick_stopped)
+ return HRTIMER_NORESTART;
+#endif
+
+ hrtimer_forward(timer, now, nsec_per_hz);

return HRTIMER_RESTART;
}
@@ -1106,13 +1356,11 @@ void hrtimer_interrupt(struct pt_regs *r
int i, raise = 0;

BUG_ON(!cpu_base->hres_active);
+ cpu_base->nr_events++;

retry:
now = ktime_get();

- /* Check, if the jiffies need an update */
- update_jiffies64(now);
-
expires_next.tv64 = KTIME_MAX;

base = cpu_base->clock_base;
Index: linux-2.6.19-rc5-mm1/kernel/softirq.c
===================================================================
--- linux-2.6.19-rc5-mm1.orig/kernel/softirq.c 2006-11-09 20:14:41.000000000 +0100
+++ linux-2.6.19-rc5-mm1/kernel/softirq.c 2006-11-09 20:16:11.000000000 +0100
@@ -278,9 +278,11 @@ EXPORT_SYMBOL(do_softirq);
*/
void irq_enter(void)
{
- account_system_vtime(current);
- add_preempt_count(HARDIRQ_OFFSET);
- trace_hardirq_enter();
+ __irq_enter();
+#ifdef CONFIG_NO_HZ
+ if (idle_cpu(smp_processor_id()))
+ hrtimer_update_jiffies();
+#endif
}

#ifdef __ARCH_IRQ_EXIT_IRQS_DISABLED
@@ -299,6 +301,12 @@ void irq_exit(void)
sub_preempt_count(IRQ_EXIT_OFFSET);
if (!in_interrupt() && local_softirq_pending())
invoke_softirq();
+
+#ifdef CONFIG_NO_HZ
+ /* Make sure that timer wheel updates are propagated */
+ if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
+ hrtimer_stop_sched_tick();
+#endif
preempt_enable_no_resched();
}

Index: linux-2.6.19-rc5-mm1/kernel/time/Kconfig
===================================================================
--- linux-2.6.19-rc5-mm1.orig/kernel/time/Kconfig 2006-11-09 20:16:06.000000000 +0100
+++ linux-2.6.19-rc5-mm1/kernel/time/Kconfig 2006-11-09 20:16:33.000000000 +0100
@@ -9,3 +9,10 @@ config HIGH_RES_TIMERS
hardware is not capable then this option only increases
the size of the kernel image.

+config NO_HZ
+ bool "Tickless System (Dynamic Ticks)"
+ depends on HIGH_RES_TIMERS
+ help
+ This option enables a tickless system: timer interrupts will
+ only trigger on an as-needed basis both when the system is
+ busy and when the system is idle.
Index: linux-2.6.19-rc5-mm1/kernel/timer.c
===================================================================
--- linux-2.6.19-rc5-mm1.orig/kernel/timer.c 2006-11-09 20:16:06.000000000 +0100
+++ linux-2.6.19-rc5-mm1/kernel/timer.c 2006-11-09 20:16:11.000000000 +0100
@@ -462,7 +462,7 @@ static inline void __run_timers(tvec_bas
spin_unlock_irq(&base->lock);
}

-#ifdef CONFIG_NO_IDLE_HZ
+#if defined(CONFIG_NO_IDLE_HZ) || defined(CONFIG_NO_HZ)
/*
* Find out when the next timer event is due to happen. This
* is used on S/390 to stop all activity when a cpus is idle.
--

-
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