[PATCH 4/5] nohz_full: tracking the full dynticks cpu number

[Alex Shi]

This patch try to track the full dynticks cpu number with a global
varible nr_nohz_busy_cpus.

When ts->tick_stopped and !ts->inidle, it is time for full dyntick.
To achieve this, the ts->inidle = 0 was delayed in tick_nohz_idle_exit
safely.

But the patch failed, it may increases 2 or 3 times continuously without
 a decrease. so nr_nohz_busy_cpus increased to 28 in 7200 seconds. :(

---
 include/linux/tick.h     |  3 +++
 kernel/time/tick-sched.c | 19 +++++++++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 5128d33..3784f9f 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -161,6 +161,7 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
 
 #ifdef CONFIG_NO_HZ_FULL
 extern bool tick_nohz_full_running;
+extern atomic_t nr_nohz_busy_cpus;
 extern cpumask_var_t tick_nohz_full_mask;
 
 static inline bool tick_nohz_full_enabled(void)
@@ -184,6 +185,7 @@ extern void __tick_nohz_full_check(void);
 extern void tick_nohz_full_kick(void);
 extern void tick_nohz_full_kick_all(void);
 extern void __tick_nohz_task_switch(struct task_struct *tsk);
+extern bool has_nohz_busy_cpus(void);
 #else
 static inline void tick_nohz_init(void) { }
 static inline bool tick_nohz_full_enabled(void) { return false; }
@@ -192,6 +194,7 @@ static inline void __tick_nohz_full_check(void) { }
 static inline void tick_nohz_full_kick(void) { }
 static inline void tick_nohz_full_kick_all(void) { }
 static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
+static inline bool has_nohz_busy_cpus(void) { return true; }
 #endif
 
 static inline void tick_nohz_full_check(void)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b55d1c0..37f2784 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -148,9 +148,13 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
 	profile_tick(CPU_PROFILING);
 }
 
+atomic_t nr_nohz_busy_cpus = ATOMIC_INIT(0);
 #ifdef CONFIG_NO_HZ_FULL
 cpumask_var_t tick_nohz_full_mask;
 bool tick_nohz_full_running;
+inline bool has_nohz_busy_cpus(void) {
+	return !!atomic_read(&nr_nohz_busy_cpus);
+}
 
 static bool can_stop_full_tick(void)
 {
@@ -642,6 +646,11 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 			ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
 			ts->tick_stopped = 1;
 			trace_tick_stop(1, " ");
+			if (!ts->inidle) {
+				if (atomic_inc_return(&nr_nohz_busy_cpus) != 1) {
+					printk(KERN_ERR "inc cpu %d as %d\n", cpu, test);
+				}
+			}
 		}
 
 		/*
@@ -881,6 +890,12 @@ static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
 	ts->tick_stopped  = 0;
 	ts->idle_exittime = now;
 
+	if (!ts->inidle) {
+		if (atomic_dec_return(&nr_nohz_busy_cpus)) {
+			printk(KERN_ERR "dec cpu %d to %d\n",
+				smp_processor_id(), atomic_read(&nr_nohz_busy_cpus));
+		}
+	}
 	tick_nohz_restart(ts, now);
 }
 
@@ -922,8 +937,6 @@ void tick_nohz_idle_exit(void)
 
 	WARN_ON_ONCE(!ts->inidle);
 
-	ts->inidle = 0;
-
 	if (ts->idle_active || ts->tick_stopped)
 		now = ktime_get();
 
@@ -935,6 +948,8 @@ void tick_nohz_idle_exit(void)
 		tick_nohz_account_idle_ticks(ts);
 	}
 
+	ts->inidle = 0;
+
 	local_irq_enable();
 }
 EXPORT_SYMBOL_GPL(tick_nohz_idle_exit);
-- 
1.8.1.2

-- 
Thanks
    Alex
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