nohz: prevent tick stop outside of the idle loop

Jack Ren and Eric Miao tracked down the following long standing
problem in the NOHZ code:

	scheduler switch to idle task
	enable interrupts

Window starts here

	----> interrupt happens (does not set NEED_RESCHED)
	      	irq_exit() stops the tick

	----> interrupt happens (does set NEED_RESCHED)

	return from schedule()
	
	cpu_idle(): preempt_disable();

Window ends here

The interrupts can happen at any point inside the race window. The
first interrupt stops the tick, the second one causes the scheduler to
rerun and switch away from idle again and we end up with the tick
disabled.

The fact that it needs two interrupts where the first one does not set
NEED_RESCHED and the second one does made the bug obscure and extremly
hard to reproduce and analyse. Kudos to Jack and Eric.

Solution: Limit the NOHZ functionality to the idle loop to make sure
that we can not run into such a situation ever again.

cpu_idle()
{
	preempt_disable();

	while(1) {
		 tick_nohz_stop_sched_tick(1); <- tell NOHZ code that we
		 			          are in the idle loop

		 while (!need_resched())
		       halt();

		 tick_nohz_restart_sched_tick(); <- disables NOHZ mode
		 preempt_enable_no_resched();
		 schedule();
		 preempt_disable();
	}
}

In hindsight we should have done this forever, but ... 

/me grabs a large brown paperbag.

Debugged-by: Jack Ren <jack.ren@marvell.com>, 
Debugged-by: eric miao <eric.y.miao@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

14 files changed, 26 insertions, 17 deletions

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 46bf2ede6128..84f5a4c778fb 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -164,7 +164,7 @@ void cpu_idle(void)
                 if (!idle)
                         idle = default_idle;
                 leds_event(led_idle_start);
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched())
                         idle();
                 leds_event(led_idle_end);
diff --git a/arch/avr32/kernel/process.c b/arch/avr32/kernel/process.c
index 6cf9df176274..ff820a9e743a 100644
--- a/arch/avr32/kernel/process.c
+++ b/arch/avr32/kernel/process.c
@@ -31,7 +31,7 @@ void cpu_idle(void)
 {
         /* endless idle loop with no priority at all */
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched())
                         cpu_idle_sleep();
                 tick_nohz_restart_sched_tick();
diff --git a/arch/blackfin/kernel/process.c b/arch/blackfin/kernel/process.c
index 53c2cd255441..77800dd83e57 100644
--- a/arch/blackfin/kernel/process.c
+++ b/arch/blackfin/kernel/process.c
@@ -105,7 +105,7 @@ void cpu_idle(void)
 #endif
                 if (!idle)
                         idle = default_idle;
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched())
                         idle();
                 tick_nohz_restart_sched_tick();
diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index 2c09a442e5e5..bdead3aad253 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -53,7 +53,7 @@ void __noreturn cpu_idle(void)
 {
         /* endless idle loop with no priority at all */
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched()) {
 #ifdef CONFIG_SMTC_IDLE_HOOK_DEBUG
                         extern void smtc_idle_loop_hook(void);
diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
index c3cf0e8f3ac1..d308a9f70f1b 100644
--- a/arch/powerpc/kernel/idle.c
+++ b/arch/powerpc/kernel/idle.c
@@ -60,7 +60,7 @@ void cpu_idle(void)
 
         set_thread_flag(TIF_POLLING_NRFLAG);
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched() && !cpu_should_die()) {
                         ppc64_runlatch_off();
 
diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
index b72120751bbe..70b688c1aefb 100644
--- a/arch/powerpc/platforms/iseries/setup.c
+++ b/arch/powerpc/platforms/iseries/setup.c
@@ -561,7 +561,7 @@ static void yield_shared_processor(void)
 static void iseries_shared_idle(void)
 {
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched() && !hvlpevent_is_pending()) {
                         local_irq_disable();
                         ppc64_runlatch_off();
@@ -591,7 +591,7 @@ static void iseries_dedicated_idle(void)
         set_thread_flag(TIF_POLLING_NRFLAG);
 
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 if (!need_resched()) {
                         while (!need_resched()) {
                                 ppc64_runlatch_off();
diff --git a/arch/sh/kernel/process_32.c b/arch/sh/kernel/process_32.c
index b98e37a1f54c..921892c351da 100644
--- a/arch/sh/kernel/process_32.c
+++ b/arch/sh/kernel/process_32.c
@@ -86,7 +86,7 @@ void cpu_idle(void)
                 if (!idle)
                         idle = default_idle;
 
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched())
                         idle();
                 tick_nohz_restart_sched_tick();
diff --git a/arch/sparc64/kernel/process.c b/arch/sparc64/kernel/process.c
index 2084f81a76e1..0798928ba361 100644
--- a/arch/sparc64/kernel/process.c
+++ b/arch/sparc64/kernel/process.c
@@ -97,7 +97,7 @@ void cpu_idle(void)
         set_thread_flag(TIF_POLLING_NRFLAG);
 
         while(1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
 
                 while (!need_resched() && !cpu_is_offline(cpu))
                         sparc64_yield(cpu);
diff --git a/arch/um/kernel/process.c b/arch/um/kernel/process.c
index 83603cfbde81..a1c6d07cac3e 100644
--- a/arch/um/kernel/process.c
+++ b/arch/um/kernel/process.c
@@ -243,7 +243,7 @@ void default_idle(void)
                 if (need_resched())
                         schedule();
 
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 nsecs = disable_timer();
                 idle_sleep(nsecs);
                 tick_nohz_restart_sched_tick();
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index f8476dfbb60d..1f5fa1cf16dd 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -166,7 +166,7 @@ void cpu_idle(void)
 
         /* endless idle loop with no priority at all */
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched()) {
                         void (*idle)(void);
 
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index e2319f39988b..c0a5c2a687e6 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -148,7 +148,7 @@ void cpu_idle(void)
         current_thread_info()->status |= TS_POLLING;
         /* endless idle loop with no priority at all */
         while (1) {
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(1);
                 while (!need_resched()) {
                         void (*idle)(void);
 
diff --git a/include/linux/tick.h b/include/linux/tick.h
index a881c652f7e9..d3c02695dc5d 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -49,6 +49,7 @@ struct tick_sched {
         unsigned long                   check_clocks;
         enum tick_nohz_mode             nohz_mode;
         ktime_t                         idle_tick;
+        int                             inidle;
         int                             tick_stopped;
         unsigned long                   idle_jiffies;
         unsigned long                   idle_calls;
@@ -105,14 +106,14 @@ static inline int tick_check_oneshot_change(int allow_nohz) { return 0; }
 #endif /* !CONFIG_GENERIC_CLOCKEVENTS */
 
 # ifdef CONFIG_NO_HZ
-extern void tick_nohz_stop_sched_tick(void);
+extern void tick_nohz_stop_sched_tick(int inidle);
 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_stop_sched_tick(int inidle) { }
 static inline void tick_nohz_restart_sched_tick(void) { }
 static inline void tick_nohz_update_jiffies(void) { }
 static inline ktime_t tick_nohz_get_sleep_length(void)
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 36e061740047..05f248039d77 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -312,7 +312,7 @@ void irq_exit(void)
 #ifdef CONFIG_NO_HZ
         /* Make sure that timer wheel updates are propagated */
         if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
-                tick_nohz_stop_sched_tick();
+                tick_nohz_stop_sched_tick(0);
         rcu_irq_exit();
 #endif
         preempt_enable_no_resched();
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 86baa4f0dfe4..ee962d11107b 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -195,7 +195,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
  * 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(void)
+void tick_nohz_stop_sched_tick(int inidle)
 {
         unsigned long seq, last_jiffies, next_jiffies, delta_jiffies, flags;
         struct tick_sched *ts;
@@ -224,6 +224,11 @@ void tick_nohz_stop_sched_tick(void)
         if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE))
                 goto end;
 
+        if (!inidle && !ts->inidle)
+                goto end;
+
+        ts->inidle = 1;
+
         if (need_resched())
                 goto end;
 
@@ -372,11 +377,14 @@ void tick_nohz_restart_sched_tick(void)
         local_irq_disable();
         tick_nohz_stop_idle(cpu);
 
-        if (!ts->tick_stopped) {
+        if (!ts->inidle || !ts->tick_stopped) {
+                ts->inidle = 0;
                 local_irq_enable();
                 return;
         }
 
+        ts->inidle = 0;
+
         rcu_exit_nohz();
 
         /* Update jiffies first */