Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Paul E. McKenney
2018-07-11T14:23:32Z

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,