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SubjectRe: RCU nocb list not reclaiming causing OOM
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)
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