Messages in this thread Patch in this message | | | Date | Mon, 28 Mar 2016 08:25:47 +0200 | From | Peter Zijlstra <> | Subject | Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17 |
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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 }
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