time: track accurate idle time with tick_sched.idle_sleeptime

Current idle time in kstat is based on jiffies and is coarse grained.
tick_sched.idle_sleeptime is making some attempt to keep track of idle time
in a fine grained manner.  But, it is not handling the time spent in
interrupts fully.

Make tick_sched.idle_sleeptime accurate with respect to time spent on
handling interrupts and also add tick_sched.idle_lastupdate, which keeps
track of last time when idle_sleeptime was updated.

This statistics will be crucial for cpufreq-ondemand governor, which can
shed some conservative gaurd band that is uses today while setting the
frequency.  The ondemand changes that uses the exact idle time is coming
soon.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/tick.h

@@ -51,8 +51,10 @@ struct tick_sched {
 	unsigned long			idle_jiffies;
 	unsigned long			idle_calls;
 	unsigned long			idle_sleeps;
+	int				idle_active;
 	ktime_t				idle_entrytime;
 	ktime_t				idle_sleeptime;
+	ktime_t				idle_lastupdate;
 	ktime_t				sleep_length;
 	unsigned long			last_jiffies;
 	unsigned long			next_jiffies;
@@ -103,6 +105,8 @@ extern void tick_nohz_stop_sched_tick(void);
 extern void tick_nohz_restart_sched_tick(void);
 extern void tick_nohz_update_jiffies(void);
 extern ktime_t tick_nohz_get_sleep_length(void);
+extern void tick_nohz_stop_idle(int cpu);
+extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
 # else
 static inline void tick_nohz_stop_sched_tick(void) { }
 static inline void tick_nohz_restart_sched_tick(void) { }
@@ -113,6 +117,8 @@ static inline ktime_t tick_nohz_get_sleep_length(void)
 
 	return len;
 }
+static inline void tick_nohz_stop_idle(int cpu) { }
+static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return 0; }
 # endif /* !NO_HZ */
 
 #endif

kernel/softirq.c

@@ -280,9 +280,14 @@ asmlinkage void do_softirq(void)
  */
 void irq_enter(void)
 {
+#ifdef CONFIG_NO_HZ
+	int cpu = smp_processor_id();
+	if (idle_cpu(cpu) && !in_interrupt())
+		tick_nohz_stop_idle(cpu);
+#endif
 	__irq_enter();
 #ifdef CONFIG_NO_HZ
-	if (idle_cpu(smp_processor_id()))
+	if (idle_cpu(cpu))
 		tick_nohz_update_jiffies();
 #endif
 }

kernel/time/tick-sched.c

@@ -143,6 +143,44 @@ void tick_nohz_update_jiffies(void)
 	local_irq_restore(flags);
 }
 
+void tick_nohz_stop_idle(int cpu)
+{
+	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+
+	if (ts->idle_active) {
+		ktime_t now, delta;
+		now = ktime_get();
+		delta = ktime_sub(now, ts->idle_entrytime);
+		ts->idle_lastupdate = now;
+		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
+		ts->idle_active = 0;
+	}
+}
+
+static ktime_t tick_nohz_start_idle(int cpu)
+{
+	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	ktime_t now, delta;
+
+	now = ktime_get();
+	if (ts->idle_active) {
+		delta = ktime_sub(now, ts->idle_entrytime);
+		ts->idle_lastupdate = now;
+		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
+	}
+	ts->idle_entrytime = now;
+	ts->idle_active = 1;
+	return now;
+}
+
+u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
+{
+	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+
+	*last_update_time = ktime_to_us(ts->idle_lastupdate);
+	return ktime_to_us(ts->idle_sleeptime);
+}
+
 /**
  * tick_nohz_stop_sched_tick - stop the idle tick from the idle task
  *
@@ -155,13 +193,14 @@ void tick_nohz_stop_sched_tick(void)
 	unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
 	unsigned long rt_jiffies;
 	struct tick_sched *ts;
-	ktime_t last_update, expires, now, delta;
+	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();
+	now = tick_nohz_start_idle(cpu);
 	ts = &per_cpu(tick_cpu_sched, cpu);
 
 	/*
@@ -193,19 +232,7 @@ void tick_nohz_stop_sched_tick(void)
 		}
 	}
 
-	now = ktime_get();
-	/*
-	 * When called from irq_exit we need to account the idle sleep time
-	 * correctly.
-	 */
-	if (ts->tick_stopped) {
-		delta = ktime_sub(now, ts->idle_entrytime);
-		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
-	}
-
-	ts->idle_entrytime = now;
 	ts->idle_calls++;
-
 	/* Read jiffies and the time when jiffies were updated last */
 	do {
 		seq = read_seqbegin(&xtime_lock);
@@ -337,23 +364,22 @@ void tick_nohz_restart_sched_tick(void)
 	int cpu = smp_processor_id();
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 	unsigned long ticks;
-	ktime_t now, delta;
+	ktime_t now;
 
-	if (!ts->tick_stopped)
+	local_irq_disable();
+	tick_nohz_stop_idle(cpu);
+
+	if (!ts->tick_stopped) {
+		local_irq_enable();
 		return;
+	}
 
 	/* Update jiffies first */
-	now = ktime_get();
-
-	local_irq_disable();
 	select_nohz_load_balancer(0);
+	now = ktime_get();
 	tick_do_update_jiffies64(now);
 	cpu_clear(cpu, nohz_cpu_mask);
 
-	/* Account the idle time */
-	delta = ktime_sub(now, ts->idle_entrytime);
-	ts->idle_sleeptime = ktime_add(ts->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