Re: [patch 2/4] nohz: Prevent erroneous tick stop invocations

[Thomas Gleixner]

On Wed, 27 Dec 2017, Thomas Gleixner wrote:
> Bah, no. We need to move that into the nohz logic somehow to prevent that
> repetitive expiry yesterday reprogramming. Lemme think about it some more.

The patch below should be the proper cure.

Thanks,

	tglx

8<-------------------
Subject: nohz: Prevent a timer interrupt storm in tick_nohz_stop_sched_tick()
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date: Fri, 22 Dec 2017 15:51:13 +0100

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

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.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx>

---
 kernel/time/tick-sched.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -650,6 +650,11 @@ static void tick_nohz_restart(struct tic
 	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 +671,8 @@ static ktime_t tick_nohz_stop_sched_tick
 	} while (read_seqretry(&jiffies_lock, seq));
 	ts->last_jiffies = basejiff;
 
-	if (rcu_needs_cpu(basemono, &next_rcu) ||
-	    arch_needs_cpu() || irq_work_needs_cpu()) {
+	if (rcu_needs_cpu(basemono, &next_rcu) || arch_needs_cpu() ||
+	    irq_work_needs_cpu() || local_timer_softirq_pending()) {
 		next_tick = basemono + TICK_NSEC;
 	} else {
 		/*