From: Frederic Weisbecker frederic@kernel.org
mainline inclusion from mainline-v5.16-rc4 commit 53e87e3cdc155f20c3417b689df8d2ac88d79576 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I6WCC1 CVE: NA
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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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@kernel.org Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Thomas Gleixner tglx@linutronix.de Tested-by: Paul E. McKenney paulmck@kernel.org Link: https://lore.kernel.org/r/20211026141055.57358-2-frederic@kernel.org
Conflicts: kernel/softirq.c
Signed-off-by: Yu Liao liaoyu15@huawei.com Reviewed-by: Xiongfeng Wang wangxiongfeng2@huawei.com Signed-off-by: Jialin Zhang zhangjialin11@huawei.com --- 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 19668d614f47..4196b9f84690 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -350,7 +350,8 @@ asmlinkage __visible void do_softirq(void) */ void irq_enter_rcu(void) { - if (is_idle_task(current) && !in_interrupt()) { + if (tick_nohz_full_cpu(smp_processor_id()) || + (is_idle_task(current) && !in_interrupt())) { /* * Prevent raise_softirq from needlessly waking up ksoftirqd * here, as softirq will be serviced on return from interrupt. diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 33750db5b564..aed5d6b6ca71 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -1347,6 +1347,13 @@ 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); }