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Message-ID: <163845435662.11128.144783670493021780.tip-bot2@tip-bot2>
Date: Thu, 02 Dec 2021 14:12:36 -0000
From: "tip-bot2 for Frederic Weisbecker" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: timers/urgent] timers/nohz: Last resort update jiffies on
nohz_full IRQ entry
The following commit has been merged into the timers/urgent branch of tip:
Commit-ID: 53e87e3cdc155f20c3417b689df8d2ac88d79576
Gitweb: https://git.kernel.org/tip/53e87e3cdc155f20c3417b689df8d2ac88d79576
Author: Frederic Weisbecker <frederic@...nel.org>
AuthorDate: Tue, 26 Oct 2021 16:10:54 +02:00
Committer: Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Thu, 02 Dec 2021 15:07:22 +01:00
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
actually matter.
Reported-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Paul E. McKenney <paulmck@...nel.org>
Link: https://lore.kernel.org/r/20211026141055.57358-2-frederic@kernel.org
---
kernel/softirq.c | 3 ++-
kernel/time/tick-sched.c | 7 +++++++
2 files changed, 9 insertions(+), 1 deletion(-)
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 322b65d..41f4709 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -595,7 +595,8 @@ void irq_enter_rcu(void)
{
__irq_enter_raw();
- 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)))
tick_irq_enter();
account_hardirq_enter(current);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6bffe5a..17a283c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1375,6 +1375,13 @@ static inline void tick_nohz_irq_enter(void)
now = ktime_get();
if (ts->idle_active)
tick_nohz_stop_idle(ts, now);
+ /*
+ * 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.
+ */
if (ts->tick_stopped)
tick_nohz_update_jiffies(now);
}
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