From: Thomas Gleixner Date: Fri, 22 Dec 2017 15:51:13 +0100 Subject: [PATCH 2/4] nohz: Prevent erroneous tick stop invocations Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.14/older/patches-4.14.8-rt9.tar.xz The conditions in irq_exit() to invoke tick_nohz_irq_exit() are: if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) This is too permissive in various aspects: 1) If need_resched() is set, then the tick cannot be stopped whether the CPU is idle or in nohz full mode. 2) If need_resched() is not set, but softirqs are pending then this is an indication that the softirq code punted and delegated the execution to softirqd. need_resched() is not true because the current interrupted task takes precedence over softirqd. Invoking tick_nohz_irq_exit() in these cases can cause an endless loop of timer interrupts because the timer wheel contains an expired timer, but softirqs are not yet executed. So it returns an immediate expiry request, which causes the timer to fire immediately again. Lather, rinse and repeat.... Prevent that by making the conditions proper and only allow invokation when in idle or nohz full mode and neither need_resched() nor local_softirq_pending() are set. Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Signed-off-by: Sebastian Andrzej Siewior --- kernel/softirq.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index 4e09821f9d9e..6d260b1229a1 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -381,7 +381,8 @@ static inline void tick_irq_exit(void) int cpu = smp_processor_id(); /* Make sure that timer wheel updates are propagated */ - if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) { + if ((idle_cpu(cpu) || tick_nohz_full_cpu(cpu)) && + !need_resched() && !local_softirq_pending()) { if (!in_interrupt()) tick_nohz_irq_exit(); } -- 2.15.1