Re: mm,numad,rcu: hang on OOM

From: Sasha Levin
Date: Tue Jul 17 2012 - 04:15:31 EST

Hi Paul,

I've been running with your patch below for a while now, and haven't encountered the issue again.

On 07/01/2012 03:15 AM, Paul E. McKenney wrote:
> On Fri, Jun 29, 2012 at 06:44:41PM +0200, Sasha Levin wrote:
>> Hi all,
>>
>> While fuzzing using trinity on a KVM tools guest with todays linux-next, I've hit the following lockup:
>>
>> [ 362.261729] INFO: task numad/2:27 blocked for more than 120 seconds.
>> [ 362.263974] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [ 362.271684] numad/2 D 0000000000000001 5672 27 2 0x00000000
>> [ 362.280052] ffff8800294c7c58 0000000000000046 ffff8800294c7c08 ffffffff81163dba
>> [ 362.294477] ffff8800294c6000 ffff8800294c6010 ffff8800294c7fd8 ffff8800294c6000
>> [ 362.306631] ffff8800294c6010 ffff8800294c7fd8 ffff88000d5c3000 ffff8800294c8000
>> [ 362.315395] Call Trace:
>> [ 362.318556] [<ffffffff81163dba>] ? __lock_release+0x1ba/0x1d0
>> [ 362.325411] [<ffffffff8372ab75>] schedule+0x55/0x60
>> [ 362.328844] [<ffffffff8372b965>] rwsem_down_failed_common+0xf5/0x130
>> [ 362.332501] [<ffffffff8115d38e>] ? put_lock_stats+0xe/0x40
>> [ 362.334496] [<ffffffff81160135>] ? __lock_contended+0x1f5/0x230
>> [ 362.336723] [<ffffffff8372b9d5>] rwsem_down_read_failed+0x15/0x17
>> [ 362.339297] [<ffffffff81985e34>] call_rwsem_down_read_failed+0x14/0x30
>> [ 362.341768] [<ffffffff83729a29>] ? down_read+0x79/0xa0
>> [ 362.343669] [<ffffffff8122d262>] ? lazy_migrate_process+0x22/0x60
>> [ 362.345616] [<ffffffff8122d262>] lazy_migrate_process+0x22/0x60
>> [ 362.347464] [<ffffffff811453c0>] process_mem_migrate+0x10/0x20
>> [ 362.349340] [<ffffffff81145090>] move_processes+0x190/0x230
>> [ 362.351398] [<ffffffff81145b7a>] numad_thread+0x7a/0x120
>> [ 362.353245] [<ffffffff81145b00>] ? find_busiest_node+0x310/0x310
>> [ 362.355396] [<ffffffff81119e82>] kthread+0xb2/0xc0
>> [ 362.356996] [<ffffffff8372ea34>] kernel_thread_helper+0x4/0x10
>> [ 362.359253] [<ffffffff8372ccb4>] ? retint_restore_args+0x13/0x13
>> [ 362.361168] [<ffffffff81119dd0>] ? __init_kthread_worker+0x70/0x70
>> [ 362.363277] [<ffffffff8372ea30>] ? gs_change+0x13/0x13
>>
>> I've hit sysrq-t to see what might be the cause, and it appears that an OOM was in progress, and was stuck on RCU:
>>
>> [ 578.086230] trinity-child69 D ffff8800277a54c8 3968 6658 6580 0x00000000
>> [ 578.086230] ffff880022c5f518 0000000000000046 ffff880022c5f4c8 ffff88001b9d6e00
>> [ 578.086230] ffff880022c5e000 ffff880022c5e010 ffff880022c5ffd8 ffff880022c5e000
>> [ 578.086230] ffff880022c5e010 ffff880022c5ffd8 ffff880023c08000 ffff880022c33000
>> [ 578.086230] Call Trace:
>> [ 578.086230] [<ffffffff8372ab75>] schedule+0x55/0x60
>> [ 578.086230] [<ffffffff837285c8>] schedule_timeout+0x38/0x2c0
>> [ 578.086230] [<ffffffff81161d16>] ? mark_held_locks+0xf6/0x120
>> [ 578.086230] [<ffffffff81163dba>] ? __lock_release+0x1ba/0x1d0
>> [ 578.086230] [<ffffffff8372c67b>] ? _raw_spin_unlock_irq+0x2b/0x80
>> [ 578.086230] [<ffffffff8372a06f>] wait_for_common+0xff/0x170
>> [ 578.086230] [<ffffffff81132c10>] ? try_to_wake_up+0x290/0x290
>> [ 578.086230] [<ffffffff8372a188>] wait_for_completion+0x18/0x20
>> [ 578.086230] [<ffffffff811a5de7>] _rcu_barrier+0x4a7/0x4e0
>
> Hmmm... Perhaps a blocking operation is not appropriate here. I have
> substituted a nonblocking approach, which is at -rcu (thus soon -next)
> at 1ee4c09d (Provide OOM handler to motivate lazy RCU callbacks).
> Patch below.
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> rcu: Provide OOM handler to motivate lazy RCU callbacks
>
> In kernels built with CONFIG_RCU_FAST_NO_HZ=y, CPUs can accumulate a
> large number of lazy callbacks, which as the name implies will be slow
> to be invoked. This can be a problem on small-memory systems, where the
> default 6-second sleep for CPUs having only lazy RCU callbacks could well
> be fatal. This commit therefore installs an OOM hander that ensures that
> every CPU with non-lazy callbacks has at least one non-lazy callback,
> in turn ensuring timely advancement for these callbacks.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index 4b47fbe..dab279f 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -314,8 +314,11 @@ struct rcu_data {
> unsigned long n_rp_need_fqs;
> unsigned long n_rp_need_nothing;
>
> - /* 6) _rcu_barrier() callback. */
> + /* 6) _rcu_barrier() and OOM callbacks. */
> struct rcu_head barrier_head;
> +#ifdef CONFIG_RCU_FAST_NO_HZ
> + struct rcu_head oom_head;
> +#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
>
> int cpu;
> struct rcu_state *rsp;
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 81e53eb..1908847 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -25,6 +25,7 @@
> */
>
> #include <linux/delay.h>
> +#include <linux/oom.h>
>
> #define RCU_KTHREAD_PRIO 1
>
> @@ -2128,6 +2129,90 @@ static void rcu_idle_count_callbacks_posted(void)
> __this_cpu_add(rcu_dynticks.nonlazy_posted, 1);
> }
>
> +/*
> + * Data for flushing lazy RCU callbacks at OOM time.
> + */
> +static atomic_t oom_callback_count;
> +static DECLARE_WAIT_QUEUE_HEAD(oom_callback_wq);
> +
> +/*
> + * RCU OOM callback -- decrement the outstanding count and deliver the
> + * wake-up if we are the last one.
> + */
> +static void rcu_oom_callback(struct rcu_head *rhp)
> +{
> + if (atomic_dec_and_test(&oom_callback_count))
> + wake_up(&oom_callback_wq);
> +}
> +
> +/*
> + * Post an rcu_oom_notify callback on the current CPU if it has at
> + * least one lazy callback. This will unnecessarily post callbacks
> + * to CPUs that already have a non-lazy callback at the end of their
> + * callback list, but this is an infrequent operation, so accept some
> + * extra overhead to keep things simple.
> + */
> +static void rcu_oom_notify_cpu(void *flavor)
> +{
> + struct rcu_state *rsp = flavor;
> + struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
> +
> + if (rdp->qlen_lazy != 0) {
> + atomic_inc(&oom_callback_count);
> + rsp->call(&rdp->oom_head, rcu_oom_callback);
> + }
> +}
> +
> +/*
> + * If low on memory, ensure that each CPU has a non-lazy callback.
> + * This will wake up CPUs that have only lazy callbacks, in turn
> + * ensuring that they free up the corresponding memory in a timely manner.
> + */
> +static int rcu_oom_notify(struct notifier_block *self,
> + unsigned long notused, void *nfreed)
> +{
> + int cpu;
> +
> + /* Wait for callbacks from earlier instance to complete. */
> + wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0);
> +
> + /*
> + * Prevent premature wakeup: ensure that all increments happen
> + * before there is a chance of the counter reaching zero.
> + */
> + atomic_set(&oom_callback_count, 1);
> +
> + get_online_cpus();
> + for_each_online_cpu(cpu) {
> +#ifdef CONFIG_PREEMPT_RCU
> + smp_call_function_single(cpu, rcu_oom_notify_cpu,
> + &rcu_preempt_state, 1);
> +#endif /* #ifdef CONFIG_PREEMPT_RCU */
> + smp_call_function_single(cpu, rcu_oom_notify_cpu,
> + &rcu_bh_state, 1);
> + smp_call_function_single(cpu, rcu_oom_notify_cpu,
> + &rcu_sched_state, 1);
> + }
> + put_online_cpus();
> +
> + /* Unconditionally decrement: no need to wake ourselves up. */
> + atomic_dec(&oom_callback_count);
> +
> + *(unsigned long *)nfreed = 1;
> + return NOTIFY_OK;
> +}
> +
> +static struct notifier_block rcu_oom_nb = {
> + .notifier_call = rcu_oom_notify
> +};
> +
> +static int __init rcu_register_oom_notifier(void)
> +{
> + register_oom_notifier(&rcu_oom_nb);
> + return 0;
> +}
> +early_initcall(rcu_register_oom_notifier);
> +
> #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
>
> #ifdef CONFIG_RCU_CPU_STALL_INFO
>
> --
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