linux-kernel - [tip: timers/core] tick: Detect and fix jiffies update stall

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Message-ID: <164725009728.16921.14131316005907895212.tip-bot2@tip-bot2>
Date:   Mon, 14 Mar 2022 09:28:17 -0000
From:   "tip-bot2 for Frederic Weisbecker" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: timers/core] tick: Detect and fix jiffies update stall

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     a1ff03cd6fb9c501fff63a4a2bface9adcfa81cd
Gitweb:        https://git.kernel.org/tip/a1ff03cd6fb9c501fff63a4a2bface9adcfa81cd
Author:        Frederic Weisbecker <frederic@...nel.org>
AuthorDate:    Wed, 02 Feb 2022 01:01:07 +01:00
Committer:     Frederic Weisbecker <frederic@...nel.org>
CommitterDate: Mon, 07 Mar 2022 23:01:19 +01:00

tick: Detect and fix jiffies update stall

On some rare cases, the timekeeper CPU may be delaying its jiffies
update duty for a while. Known causes include:

* The timekeeper is waiting on stop_machine in a MULTI_STOP_DISABLE_IRQ
  or MULTI_STOP_RUN state. Disabled interrupts prevent from timekeeping
  updates while waiting for the target CPU to complete its
  stop_machine() callback.

* The timekeeper vcpu has VMEXIT'ed for a long while due to some overload
  on the host.

Detect and fix these situations with emergency timekeeping catchups.

Original-patch-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/tick-sched.c | 17 +++++++++++++++++
 kernel/time/tick-sched.h |  4 ++++
 2 files changed, 21 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 17a283c..c89f50a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -169,6 +169,8 @@ static ktime_t tick_init_jiffy_update(void)
 	return period;
 }
 
+#define MAX_STALLED_JIFFIES 5
+
 static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 {
 	int cpu = smp_processor_id();
@@ -196,6 +198,21 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 	if (tick_do_timer_cpu == cpu)
 		tick_do_update_jiffies64(now);
 
+	/*
+	 * If jiffies update stalled for too long (timekeeper in stop_machine()
+	 * or VMEXIT'ed for several msecs), force an update.
+	 */
+	if (ts->last_tick_jiffies != jiffies) {
+		ts->stalled_jiffies = 0;
+		ts->last_tick_jiffies = READ_ONCE(jiffies);
+	} else {
+		if (++ts->stalled_jiffies == MAX_STALLED_JIFFIES) {
+			tick_do_update_jiffies64(now);
+			ts->stalled_jiffies = 0;
+			ts->last_tick_jiffies = READ_ONCE(jiffies);
+		}
+	}
+
 	if (ts->inidle)
 		ts->got_idle_tick = 1;
 }
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index d952ae3..5046495 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -49,6 +49,8 @@ enum tick_nohz_mode {
  * @timer_expires_base:	Base time clock monotonic for @timer_expires
  * @next_timer:		Expiry time of next expiring timer for debugging purpose only
  * @tick_dep_mask:	Tick dependency mask - is set, if someone needs the tick
+ * @last_tick_jiffies:	Value of jiffies seen on last tick
+ * @stalled_jiffies:	Number of stalled jiffies detected across ticks
  */
 struct tick_sched {
 	struct hrtimer			sched_timer;
@@ -77,6 +79,8 @@ struct tick_sched {
 	u64				next_timer;
 	ktime_t				idle_expires;
 	atomic_t			tick_dep_mask;
+	unsigned long			last_tick_jiffies;
+	unsigned int			stalled_jiffies;
 };
 
 extern struct tick_sched *tick_get_tick_sched(int cpu);