Re: RCU nocb list not reclaiming causing OOM

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jul 20, 2018 at 11:05:52PM +0000, David Chen wrote:
> Hi Paul,
> 
> We hit an RCU issue on 4.9.37 kernel. One of the nocb_follower list grows too
> large, and not getting reclaimed, causing the system to OOM.
> 
> Printing the culprit rcu_sched_data:
> 
>   nocb_q_count = {
>     counter = 32369635
>   },
>   nocb_follower_head = 0xffff88ae901c0a00,
>   nocb_follower_tail = 0xffff88af1538b8d8,
>   nocb_kthread = 0xffff88b06d290000,
> 
> As you can see here, the nocb_follower_head is not empty, so in theory, the
> nocb_kthread shouldn't go to sleep. However, if dump the stack of the kthread:
> 
> crash> bt 0xffff88b06d290000
> PID: 21     TASK: ffff88b06d290000  CPU: 3   COMMAND: "rcuos/1"
>  #0 [ffffafc9020b7dc0] __schedule at ffffffff8d8789dc
>  #1 [ffffafc9020b7e38] schedule at ffffffff8d878e76
>  #2 [ffffafc9020b7e50] rcu_nocb_kthread at ffffffff8d112337
>  #3 [ffffafc9020b7ec8] kthread at ffffffff8d0c6ce7
>  #4 [ffffafc9020b7f50] ret_from_fork at ffffffff8d87d755
> 
> And if we dis the address at ffffffff8d112337:
> 
> /usr/src/debug/kernel-4.9.37/linux-4.9.37-29.nutanix.07142017.el7.centos.x86_64/kernel/rcu/tree_plugin.h: 2106
> 0xffffffff8d11232d <rcu_nocb_kthread+381>:      test   %rax,%rax
> 0xffffffff8d112330 <rcu_nocb_kthread+384>:      jne    0xffffffff8d112355 <rcu_nocb_kthread+421>
> 0xffffffff8d112332 <rcu_nocb_kthread+386>:      callq  0xffffffff8d878e40 <schedule>
> 0xffffffff8d112337 <rcu_nocb_kthread+391>:      lea    -0x40(%rbp),%rsi
> 
> So the kthread is blocked at swait_event_interruptible in the nocb_follower_wait.
> This contradict with the fact that nocb_follower_head was not empty. So I
> wonder if this is caused by the lack of memory barrier in the place shown below.
> If the head is set to NULL after doing xchg, it will overwrite the head set
> by leader. This caused the kthread to sleep the next iteration, and the leader
> won't wake him up as the tail doesn't point to head.
> 
> Please tell me what do you think.
> 
> Thanks,
> David
> 
> diff -ru linux-4.9.37.orig/kernel/rcu/tree_plugin.h linux-4.9.37/kernel/rcu/tree_plugin.h
> --- linux-4.9.37.orig/kernel/rcu/tree_plugin.h	2017-07-12 06:42:41.000000000 -0700
> +++ linux-4.9.37/kernel/rcu/tree_plugin.h	2018-07-20 15:25:57.311206343 -0700
> @@ -2149,6 +2149,7 @@
>  		BUG_ON(!list);
>  		trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, "WokeNonEmpty");
>  		WRITE_ONCE(rdp->nocb_follower_head, NULL);
> +		smp_mb();
>  		tail = xchg(&rdp->nocb_follower_tail, &rdp->nocb_follower_head);

The xchg() operation implies full memory barriers both before and after,
so adding the smp_mb() before would have no effect.

But let me take a look at post-4.9 changes to this code...

I suggest trying out the following commit:

6b5fc3a13318 ("rcu: Add memory barriers for NOCB leader wakeup")

If that one doesn't help, the following might be worth trying, but probably
a lot harder to backport:

8be6e1b15c54 ("rcu: Use timer as backstop for NOCB deferred wakeups")

Please let me know how it goes!

							Thanx, Paul

------------------------------------------------------------------------

commit 6b5fc3a1331810db407c9e0e673dc1837afdc9d0
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Fri Apr 28 20:11:09 2017 -0700

    rcu: Add memory barriers for NOCB leader wakeup
    
    Wait/wakeup operations do not guarantee ordering on their own.  Instead,
    either locking or memory barriers are required.  This commit therefore
    adds memory barriers to wake_nocb_leader() and nocb_leader_wait().
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Tested-by: Krister Johansen <kjlx@templeofstupid.com>
    Cc: <stable@vger.kernel.org> # 4.6.x

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0b1042545116..573fbe9640a0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1810,6 +1810,7 @@ static void wake_nocb_leader(struct rcu_data *rdp, bool force)
 	if (READ_ONCE(rdp_leader->nocb_leader_sleep) || force) {
 		/* Prior smp_mb__after_atomic() orders against prior enqueue. */
 		WRITE_ONCE(rdp_leader->nocb_leader_sleep, false);
+		smp_mb(); /* ->nocb_leader_sleep before swake_up(). */
 		swake_up(&rdp_leader->nocb_wq);
 	}
 }
@@ -2064,6 +2065,7 @@ static void nocb_leader_wait(struct rcu_data *my_rdp)
 	 * nocb_gp_head, where they await a grace period.
 	 */
 	gotcbs = false;
+	smp_mb(); /* wakeup before ->nocb_head reads. */
 	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_follower) {
 		rdp->nocb_gp_head = READ_ONCE(rdp->nocb_head);
 		if (!rdp->nocb_gp_head)