Commit fb02fbc1 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Thomas Gleixner
Browse files

NOHZ: restart tick device from irq_enter() 



We did not restart the tick device from irq_enter() to avoid double
reprogramming and extra events in the return immediate to idle case.

But long lasting softirqs can lead to a situation where jiffies become
stale:

idle()
  tick stopped (reprogrammed to next pending timer)
  halt()
   interrupt
     jiffies updated from irq_enter()
     interrupt handler
     softirq function 1 runs 20ms
     softirq function 2 arms a 10ms timer with a stale jiffies value
     jiffies updated from irq_exit()
     timer wheel has now an already expired timer
     (the one added in function 2)
     timer fires and timer softirq runs

This was discovered when debugging a timer problem which happend only
when the ath5k driver is active. The debugging proved that there is a
softirq function running for more than 20ms, which is a bug by itself.

To solve this we restart the tick timer right from irq_enter(), but do
not go through the other functions which are necessary to return from
idle when need_resched() is set.

Reported-by: default avatarElias Oltmanns <eo@nebensachen.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarElias Oltmanns <eo@nebensachen.de>
parent c34bec5a

kernel/time/tick-broadcast.c

+13 −0
Original line number Diff line number Diff line
@@ -383,6 +383,19 @@ int tick_resume_broadcast_oneshot(struct clock_event_device *bc) 
	return 0;

}



/*

 * Called from irq_enter() when idle was interrupted to reenable the

 * per cpu device.

 */

void tick_check_oneshot_broadcast(int cpu)

{

	if (cpu_isset(cpu, tick_broadcast_oneshot_mask)) {

		struct tick_device *td = &per_cpu(tick_cpu_device, cpu);



		clockevents_set_mode(td->evtdev, CLOCK_EVT_MODE_ONESHOT);

	}

}



/*

 * Handle oneshot mode broadcasting

 */

kernel/time/tick-internal.h

+2 −0
Original line number Diff line number Diff line
@@ -36,6 +36,7 @@ extern void tick_broadcast_switch_to_oneshot(void); 
extern void tick_shutdown_broadcast_oneshot(unsigned int *cpup);

extern int tick_resume_broadcast_oneshot(struct clock_event_device *bc);

extern int tick_broadcast_oneshot_active(void);

extern void tick_check_oneshot_broadcast(int cpu);

# else /* BROADCAST */

static inline void tick_broadcast_setup_oneshot(struct clock_event_device *bc)

{
@@ -45,6 +46,7 @@ static inline void tick_broadcast_oneshot_control(unsigned long reason) { } 
static inline void tick_broadcast_switch_to_oneshot(void) { }

static inline void tick_shutdown_broadcast_oneshot(unsigned int *cpup) { }

static inline int tick_broadcast_oneshot_active(void) { return 0; }

static inline void tick_check_oneshot_broadcast(int cpu) { }

# endif /* !BROADCAST */



#else /* !ONESHOT */

kernel/time/tick-sched.c

+23 −8
Original line number Diff line number Diff line
@@ -508,10 +508,6 @@ static void tick_nohz_handler(struct clock_event_device *dev) 
	update_process_times(user_mode(regs));

	profile_tick(CPU_PROFILING);



	/* Do not restart, when we are in the idle loop */

	if (ts->tick_stopped)

		return;



	while (tick_nohz_reprogram(ts, now)) {

		now = ktime_get();

		tick_do_update_jiffies64(now);
@@ -557,6 +553,27 @@ static void tick_nohz_switch_to_nohz(void) 
	       smp_processor_id());

}



/*

 * When NOHZ is enabled and the tick is stopped, we need to kick the

 * tick timer from irq_enter() so that the jiffies update is kept

 * alive during long running softirqs. That's ugly as hell, but

 * correctness is key even if we need to fix the offending softirq in

 * the first place.

 *

 * Note, this is different to tick_nohz_restart. We just kick the

 * timer and do not touch the other magic bits which need to be done

 * when idle is left.

 */

static void tick_nohz_kick_tick(int cpu)

{

	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);



	if (!ts->tick_stopped)

		return;



	tick_nohz_restart(ts, ktime_get());

}



#else



static inline void tick_nohz_switch_to_nohz(void) { }
@@ -568,9 +585,11 @@ static inline void tick_nohz_switch_to_nohz(void) { } 
 */

void tick_check_idle(int cpu)

{

	tick_check_oneshot_broadcast(cpu);

#ifdef CONFIG_NO_HZ

	tick_nohz_stop_idle(cpu);

	tick_nohz_update_jiffies();

	tick_nohz_kick_tick(cpu);

#endif

}
@@ -627,10 +646,6 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer) 
		profile_tick(CPU_PROFILING);

	}



	/* Do not restart, when we are in the idle loop */

	if (ts->tick_stopped)

		return HRTIMER_NORESTART;



	hrtimer_forward(timer, now, tick_period);



	return HRTIMER_RESTART;

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