On Wed, Jul 11, 2018 at 01:58:22PM +0100, David Woodhouse wrote: > On Wed, 2018-07-11 at 05:51 -0700, Paul E. McKenney wrote: > > > > Interesting. (I am assuming that the guest is printing these messages, > > not the host, but please let me know if my assumption is incorrect.) > > No, this is all in the host. When the VMM (qemu, etc.) opens more files > and has to expand its fd_table, the threads which are currently in > KVM's vcpu_run() are making synchronize_sched() take multiple seconds. > > > Are the CPUs saturated? If so, could you please try booting with > > rcutree.kthread_prio=2? If that prevents the messages from happening, > > then I need to put some work into guaranteeing forward progress. > > Otherwise, I need to figure out why the setting of rcu_urgent_qs is > > being ignored. > > The CPUs shouldn't be saturated. The guest is fairly much idle. I can > best reproduce this by starting up the guest and then assigning a new > PCI device. At that point fairly much nothing is happening at all.
OK, thank you for the information and again apologies for the hassle. I will do what I should have done long ago and make the relevant addition to rcutorture. In the meantime, one workaround is to export rcu_momentary_dyntick_idle() and to invoke it from within your loop, for example, as enabled by the (untested, probably does not even build) patch below. This approach is quite a bit heavier weight than the hoped-for eventual fix, but it should get this out of the way to allow you to find other problems in your testing. ;-) Thanx, Paul > > I will assume the latter for the moment and see if I can spot the > > problem. ------------------------------------------------------------------------ diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h index b3dbf9502fd0..bbf23e1318a9 100644 --- a/include/linux/rcutiny.h +++ b/include/linux/rcutiny.h @@ -115,6 +115,7 @@ static inline bool rcu_irq_enter_disabled(void) { return false; } static inline void rcu_irq_exit_irqson(void) { } static inline void rcu_irq_enter_irqson(void) { } static inline void rcu_irq_exit(void) { } +static inline void rcu_momentary_dyntick_idle(void) { } static inline void exit_rcu(void) { } #ifdef CONFIG_SRCU void rcu_scheduler_starting(void); diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index 37d6fd3b7ff8..1bec142720dd 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -86,6 +86,7 @@ void rcu_irq_exit(void); void rcu_irq_enter_irqson(void); void rcu_irq_exit_irqson(void); bool rcu_irq_enter_disabled(void); +void rcu_momentary_dyntick_idle(void); void exit_rcu(void); diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f9c0ca2ccf0c..da06a52e5e60 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -439,11 +439,12 @@ bool rcu_eqs_special_set(int cpu) * * The caller must have disabled interrupts. */ -static void rcu_momentary_dyntick_idle(void) +void rcu_momentary_dyntick_idle(void) { raw_cpu_write(rcu_dynticks.rcu_need_heavy_qs, false); rcu_dynticks_momentary_idle(); } +EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle); /* * Note a context switch. This is a quiescent state for RCU-sched,