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: Elias Oltmanns <eo@nebensachen.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Elias Oltmanns <eo@nebensachen.de>

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index cb01cd8f919b2752cfb7d97b4dc9ea4e87834a97..f98a1b7b16e942018ffe9e5998756405695da9a4 100644 (file)
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -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
  */

diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
index 469248782c2355c0c2d31cf731dc8c692023268e..b1c05bf75ee0ce36e0664d3e2e71d00ac0e09edd 100644 (file)
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h
@@ -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 */

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7aedf4343539a5078e62d2f060982b5aef7100c2..0581c11fe6c6765121a84f531475e0e3c78809b3 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -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;