I think I'm hitting particularly subtle issue with NOHZ_IDLE kernel.
The sequence is as follows:
- CPU enters idle, we disable tick
- hrtimer interrupt fires (for hrtimer_wakeup)
- for clock base #1 (REALTIME) we wake up SCHED_RT thread and
start RT period timer (from start_rt_bandwidth) for clock base #0 (MONOTONIC)
- because we already checked expiry time for clock base #0
we end up programming wrong wake up time (minutes, from tick_sched_timer)
- then, we exit idle loop and restart tick;
but because tick_sched_timer is not leftmost (the leftmost one
is RT period timer) we don't program it
So in the end, I see working CPU without tick interrupt.
This eventually leads to RCU stall on that CPU: rcu_gp_kthread
is not woken up because there is no tick (this is the reason
I started digging this up).
The proposed fix runs expired timers first and only then tries to find
next expiry time for all clocks.
Signed-off-by: Stanislav Fomichev <stfomic...@yandex-team.ru>
---
include/linux/hrtimer.h | 2 ++
kernel/hrtimer.c | 41 +++++++++++++++++++++++++++++++----------
2 files changed, 33 insertions(+), 10 deletions(-)
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index d19a5c2d2270..520a671f90ee 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -141,6 +141,7 @@ struct hrtimer_sleeper {
* @get_time: function to retrieve the current time of the clock
* @softirq_time: the time when running the hrtimer queue in the softirq
* @offset: offset of this clock to the monotonic base
+ * @next: time of the next event on this clock base
*/
struct hrtimer_clock_base {
struct hrtimer_cpu_base *cpu_base;
@@ -151,6 +152,7 @@ struct hrtimer_clock_base {
ktime_t (*get_time)(void);
ktime_t softirq_time;
ktime_t offset;
+ ktime_t next;
};
enum hrtimer_base_type {
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 09094361dce5..d284411e6dad 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -882,6 +882,8 @@ EXPORT_SYMBOL_GPL(hrtimer_forward);
static int enqueue_hrtimer(struct hrtimer *timer,
struct hrtimer_clock_base *base)
{
+ ktime_t expires;
+
debug_activate(timer);
timerqueue_add(&base->active, &timer->node);
@@ -893,6 +895,10 @@ static int enqueue_hrtimer(struct hrtimer *timer,
*/
timer->state |= HRTIMER_STATE_ENQUEUED;
+ expires = ktime_sub(hrtimer_get_expires(timer), base->offset);
+ if (ktime_compare(base->next, expires) > 0)
+ base->next = expires;
+
return (&timer->node == base->active.next);
}
@@ -929,8 +935,10 @@ static void __remove_hrtimer(struct hrtimer *timer,
}
#endif
}
- if (!timerqueue_getnext(&base->active))
+ if (!timerqueue_getnext(&base->active)) {
base->cpu_base->active_bases &= ~(1 << base->index);
+ base->next = ktime_set(KTIME_SEC_MAX, 0);
+ }
out:
timer->state = newstate;
}
@@ -1282,6 +1290,7 @@ static void __run_hrtimer(struct hrtimer *timer, ktime_t
*now)
*/
void hrtimer_interrupt(struct clock_event_device *dev)
{
+ struct hrtimer_clock_base *base;
struct hrtimer_cpu_base *cpu_base = &__get_cpu_var(hrtimer_bases);
ktime_t expires_next, now, entry_time, delta;
int i, retries = 0;
@@ -1304,7 +1313,6 @@ retry:
cpu_base->expires_next.tv64 = KTIME_MAX;
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
- struct hrtimer_clock_base *base;
struct timerqueue_node *node;
ktime_t basenow;
@@ -1333,14 +1341,8 @@ retry:
*/
if (basenow.tv64 < hrtimer_get_softexpires_tv64(timer))
{
- ktime_t expires;
-
- expires = ktime_sub(hrtimer_get_expires(timer),
- base->offset);
- if (expires.tv64 < 0)
- expires.tv64 = KTIME_MAX;
- if (expires.tv64 < expires_next.tv64)
- expires_next = expires;
+ base->next =
ktime_sub(hrtimer_get_expires(timer),
+ base->offset);
break;
}
@@ -1349,6 +1351,25 @@ retry:
}
/*
+ * Because timer handler may add new timer on a different clock base,
+ * we need to find next expiry only after we execute all timers.
+ */
+ for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
+ ktime_t expires;
+
+ if (!(cpu_base->active_bases & (1 << i)))
+ continue;
+
+ base = cpu_base->clock_base + i;
+ expires = base->next;
+
+ if (expires.tv64 < 0)
+ expires.tv64 = KTIME_MAX;
+ if (expires.tv64 < expires_next.tv64)
+ expires_next = expires;
+ }
+
+ /*
* Store the new expiry value so the migration code can verify
* against it.
*/
--
1.8.3.2
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