nohz: Prevent a timer interrupt storm in tick_nohz_stop_sched_tick()

KAGA-KOKO · KAGA-KOKO · commit 5d62c183f9e9 · 2017-12-29T23:13:10.000+01:00
The conditions in irq_exit() to invoke tick_nohz_irq_exit() which subsequently invokes tick_nohz_stop_sched_tick() are: if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) If need_resched() is not set, but a timer softirq is pending then this is an indication that the softirq code punted and delegated the execution to softirqd. need_resched() is not true because the current interrupted task takes precedence over softirqd. Invoking tick_nohz_irq_exit() in this case can cause an endless loop of timer interrupts because the timer wheel contains an expired timer, but softirqs are not yet executed. So it returns an immediate expiry request, which causes the timer to fire immediately again. Lather, rinse and repeat.... Prevent that by adding a check for a pending timer soft interrupt to the conditions in tick_nohz_stop_sched_tick() which avoid calling get_next_timer_interrupt(). That keeps the tick sched timer on the tick and prevents a repetitive programming of an already expired timer. Reported-by: Sebastian Siewior <bigeasy@linutronix.d> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Acked-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Cc: Anna-Maria Gleixner <anna-maria@linutronix.de> Cc: Sebastian Siewior <bigeasy@linutronix.de> Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1712272156050.2431@nanos
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
@@ -650,6 +650,11 @@ static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
 	ts->next_tick = 0;
 }
 
+static inline bool local_timer_softirq_pending(void)
+{
+	return local_softirq_pending() & TIMER_SOFTIRQ;
+}
+
 static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 					 ktime_t now, int cpu)
 {
@@ -666,8 +671,18 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 	} while (read_seqretry(&jiffies_lock, seq));
 	ts->last_jiffies = basejiff;
 
-	if (rcu_needs_cpu(basemono, &next_rcu) ||
-	    arch_needs_cpu() || irq_work_needs_cpu()) {
+	/*
+	 * Keep the periodic tick, when RCU, architecture or irq_work
+	 * requests it.
+	 * Aside of that check whether the local timer softirq is
+	 * pending. If so its a bad idea to call get_next_timer_interrupt()
+	 * because there is an already expired timer, so it will request
+	 * immeditate expiry, which rearms the hardware timer with a
+	 * minimal delta which brings us back to this place
+	 * immediately. Lather, rinse and repeat...
+	 */
+	if (rcu_needs_cpu(basemono, &next_rcu) || arch_needs_cpu() ||
+	    irq_work_needs_cpu() || local_timer_softirq_pending()) {
 		next_tick = basemono + TICK_NSEC;
 	} else {
 		/*
