Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Mar 27, 2016 at 02:06:41PM -0700, Paul E. McKenney wrote:

> > But, you need hotplug for this to happen, right?
> 
> I do, but Ross Green is seeing something that looks similar, and without
> CPU hotplug.

Yes, but that's two differences so far, you need hotplug and he's on ARM
(which doesn't have TIF_POLLING_NR).

So either we're all looking at the wrong thing or these really are two
different issues.

> > We should not be migrating towards, or waking on, CPUs no longer present
> > in cpu_active_map, and there is a rcu/sched_sync() after clearing that
> > bit. Furthermore, migration_call() does a sched_ttwu_pending() (waking
> > any remaining stragglers) before we migrate all runnable tasks off the
> > dying CPU.
> 
> OK, so I should instrument migration_call() if I get the repro rate up?

Can do, maybe try the below first. (yes I know how long it all takes :/)

> > The other interesting case would be resched_cpu(), which uses
> > set_nr_and_not_polling() to kick a remote cpu to call schedule(). It
> > atomically sets TIF_NEED_RESCHED and returns if TIF_POLLING_NRFLAG was
> > not set. If indeed not, it will send an IPI.
> > 
> > This assumes the idle 'exit' path will do the same as the IPI does; and
> > if you look at cpu_idle_loop() it does indeed do both
> > preempt_fold_need_resched() and sched_ttwu_pending().
> > 
> > Note that one cannot rely on irq_enter()/irq_exit() being called for the
> > scheduler IPI.
> 
> OK, thank you for the info!  Any specific debug actions?

Dunno, something like the below should bring visibility into the
(lockless) wake_list thingy.

So these trace_printk()s should happen between trace_sched_waking() and
trace_sched_wakeup() (I've not fully read the thread, but ISTR you had
some traces with these here thingies on).

---
 arch/x86/include/asm/bitops.h | 6 ++++--
 kernel/sched/core.c           | 9 +++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 7766d1cf096e..5345784d5e41 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -112,11 +112,13 @@ clear_bit(long nr, volatile unsigned long *addr)
 	if (IS_IMMEDIATE(nr)) {
 		asm volatile(LOCK_PREFIX "andb %1,%0"
 			: CONST_MASK_ADDR(nr, addr)
-			: "iq" ((u8)~CONST_MASK(nr)));
+			: "iq" ((u8)~CONST_MASK(nr))
+			: "memory");
 	} else {
 		asm volatile(LOCK_PREFIX "btr %1,%0"
 			: BITOP_ADDR(addr)
-			: "Ir" (nr));
+			: "Ir" (nr)
+			: "memory");
 	}
 }
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0b21e7a724e1..b446f73c530d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1669,6 +1669,7 @@ void sched_ttwu_pending(void)
 	while (llist) {
 		p = llist_entry(llist, struct task_struct, wake_entry);
 		llist = llist_next(llist);
+		trace_printk("waking %d\n", p->pid);
 		ttwu_do_activate(rq, p, 0);
 	}
 
@@ -1719,6 +1720,7 @@ static void ttwu_queue_remote(struct task_struct *p, int cpu)
 	struct rq *rq = cpu_rq(cpu);
 
 	if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list)) {
+		trace_printk("queued %d for waking on %d\n", p->pid, cpu);
 		if (!set_nr_if_polling(rq->idle))
 			smp_send_reschedule(cpu);
 		else
@@ -5397,10 +5399,17 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 		migrate_tasks(rq);
 		BUG_ON(rq->nr_running != 1); /* the migration thread */
 		raw_spin_unlock_irqrestore(&rq->lock, flags);
+
+		/* really bad m'kay */
+		WARN_ON(!llist_empty(&rq->wake_list));
+
 		break;
 
 	case CPU_DEAD:
 		calc_load_migrate(rq);
+
+		/* more bad */
+		WARN_ON(!llist_empty(&rq->wake_list));
 		break;
 #endif
 	}