timers/nohz: Last resort update jiffies on nohz_full IRQ entry
When at least one CPU runs in nohz_full mode, a dedicated timekeeper CPU
is guaranteed to stay online and to never stop its tick.
Meanwhile on some rare case, the dedicated timekeeper may be running
with interrupts disabled for a while, such as in stop_machine.
If jiffies stop being updated, a nohz_full CPU may end up endlessly
programming the next tick in the past, taking the last jiffies update
monotonic timestamp as a stale base, resulting in an tick storm.
Here is a scenario where it matters:
0) CPU 0 is the timekeeper and CPU 1 a nohz_full CPU.
1) A stop machine callback is queued to execute somewhere.
2) CPU 0 reaches MULTI_STOP_DISABLE_IRQ while CPU 1 is still in
MULTI_STOP_PREPARE. Hence CPU 0 can't do its timekeeping duty. CPU 1
can still take IRQs.
3) CPU 1 receives an IRQ which queues a timer callback one jiffy forward.
4) On IRQ exit, CPU 1 schedules the tick one jiffy forward, taking
last_jiffies_update as a base. But last_jiffies_update hasn't been
updated for 2 jiffies since the timekeeper has interrupts disabled.
5) clockevents_program_event(), which relies on ktime_get(), observes
that the expiration is in the past and therefore programs the min
delta event on the clock.
6) The tick fires immediately, goto 3)
7) Tick storm, the nohz_full CPU is drown and takes ages to reach
MULTI_STOP_DISABLE_IRQ, which is the only way out of this situation.
Solve this with unconditionally updating jiffies if the value is stale
on nohz_full IRQ entry. IRQs and other disturbances are expected to be
rare enough on nohz_full for the unconditional call to ktime_get() to
Reported-by: Paul E. McKenney <email@example.com>
Signed-off-by: Frederic Weisbecker <firstname.lastname@example.org>
Signed-off-by: Thomas Gleixner <email@example.com>
Tested-by: Paul E. McKenney <firstname.lastname@example.org>
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 322b65d..41f4709 100644
@@ -595,7 +595,8 @@
- if (is_idle_task(current) && (irq_count() == HARDIRQ_OFFSET))
+ if (tick_nohz_full_cpu(smp_processor_id()) ||
+ (is_idle_task(current) && (irq_count() == HARDIRQ_OFFSET)))
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6bffe5af..17a283c 100644
@@ -1375,6 +1375,13 @@
now = ktime_get();
+ * If all CPUs are idle. We may need to update a stale jiffies value.
+ * Note nohz_full is a special case: a timekeeper is guaranteed to stay
+ * alive but it might be busy looping with interrupts disabled in some
+ * rare case (typically stop machine). So we must make sure we have a
+ * last resort.