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Subject[PATCH] hrtimers: calculate expires_next after all timers are executed
Date
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 <stfomichev@yandex-team.ru>
---
kernel/hrtimer.c | 38 ++++++++++++++++++++++++--------------
1 file changed, 24 insertions(+), 14 deletions(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index 09094361dce5..63805f9f9531 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -1282,6 +1282,9 @@ static void __run_hrtimer(struct hrtimer *timer, ktime_t *now)
*/
void hrtimer_interrupt(struct clock_event_device *dev)
{
+ struct hrtimer_clock_base *base;
+ struct timerqueue_node *node;
+ struct hrtimer *timer;
struct hrtimer_cpu_base *cpu_base = &__get_cpu_var(hrtimer_bases);
ktime_t expires_next, now, entry_time, delta;
int i, retries = 0;
@@ -1304,8 +1307,6 @@ 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;

if (!(cpu_base->active_bases & (1 << i)))
@@ -1315,8 +1316,6 @@ retry:
basenow = ktime_add(now, base->offset);

while ((node = timerqueue_getnext(&base->active))) {
- struct hrtimer *timer;
-
timer = container_of(node, struct hrtimer, node);

/*
@@ -1332,22 +1331,33 @@ retry:
* timer will have to trigger a wakeup anyway.
*/

- 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;
+ if (basenow.tv64 < hrtimer_get_softexpires_tv64(timer))
break;
- }

__run_hrtimer(timer, &basenow);
}
}

+ 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;
+
+ node = timerqueue_getnext(&base->active);
+ if (!node)
+ continue;
+
+ timer = container_of(node, struct hrtimer, node);
+ 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;
+ }
+
/*
* Store the new expiry value so the migration code can verify
* against it.
--
1.8.3.2


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