Re: INFO: rcu_sched detected stalls on CPUs/tasks with `kswapd` and `mem_cgroup_shrink_node`

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

On Mon, Nov 21, 2016 at 02:41:31PM +0100, Michal Hocko wrote:
> On Wed 16-11-16 09:30:36, Paul E. McKenney wrote:
> > On Wed, Nov 16, 2016 at 06:01:19PM +0100, Paul Menzel wrote:
> > > Dear Linux folks,
> > > 
> > > 
> > > On 11/08/16 19:39, Paul E. McKenney wrote:
> > > >On Tue, Nov 08, 2016 at 06:38:18PM +0100, Paul Menzel wrote:
> > > >>On 11/08/16 18:03, Paul E. McKenney wrote:
> > > >>>On Tue, Nov 08, 2016 at 01:22:28PM +0100, Paul Menzel wrote:
> > > >>
> > > >>>>Could you please help me shedding some light into the messages below?
> > > >>>>
> > > >>>>With Linux 4.4.X, these messages were not seen. When updating to
> > > >>>>Linux 4.8.4, and Linux 4.8.6 they started to appear. In that
> > > >>>>version, we enabled several CGROUP options.
> > > >>>>
> > > >>>>>$ dmesg -T
> > > >>>>>[…]
> > > >>>>>[Mon Nov  7 15:09:45 2016] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > >>>>>[Mon Nov  7 15:09:45 2016]     3-...: (493 ticks this GP) idle=515/140000000000000/0 softirq=5504423/5504423 fqs=13876
> > > >>>>>[Mon Nov  7 15:09:45 2016]     (detected by 5, t=60002 jiffies, g=1363193, c=1363192, q=268508)
> > > >>>>>[Mon Nov  7 15:09:45 2016] Task dump for CPU 3:
> > > >>>>>[Mon Nov  7 15:09:45 2016] kswapd1         R  running task        0    87      2 0x00000008
> > > >>>>>[Mon Nov  7 15:09:45 2016]  ffffffff81aabdfd ffff8810042a5cb8 ffff88080ad34000 ffff88080ad33dc8
> > > >>>>>[Mon Nov  7 15:09:45 2016]  ffff88080ad33d00 0000000000003501 0000000000000000 0000000000000000
> > > >>>>>[Mon Nov  7 15:09:45 2016]  0000000000000000 0000000000000000 0000000000022316 000000000002bc9f
> > > >>>>>[Mon Nov  7 15:09:45 2016] Call Trace:
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff81aabdfd>] ? __schedule+0x21d/0x5b0
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff81106dcf>] ? shrink_node+0xbf/0x1c0
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff81107865>] ? kswapd+0x315/0x5f0
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff81107550>] ? mem_cgroup_shrink_node+0x90/0x90
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff8106c614>] ? kthread+0xc4/0xe0
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff81aaf64f>] ? ret_from_fork+0x1f/0x40
> > > >>>>>[Mon Nov  7 15:09:45 2016]  [<ffffffff8106c550>] ? kthread_worker_fn+0x160/0x160
> > > >>>>
> > > >>>>Even after reading `stallwarn.txt` [1], I don’t know what could
> > > >>>>cause this. All items in the backtrace seem to belong to the Linux
> > > >>>>kernel.
> > > >>>>
> > > >>>>There is also nothing suspicious in the monitoring graphs during that time.
> > > >>>
> > > >>>If you let it be, do you get a later stall warning a few minutes later?
> > > >>>If so, how does the stack trace compare?
> > > >>
> > > >>With Linux 4.8.6 this is the only occurrence since yesterday.
> > > >>
> > > >>With Linux 4.8.3, and 4.8.4 the following stack traces were seen.
> > > >
> > > >Looks to me like one or both of the loops in shrink_node() need
> > > >an cond_resched_rcu_qs().
> > > 
> > > Thank you for the pointer. I haven’t had time yet to look into it.
> > 
> > In theory, it is quite straightforward, as shown by the patch below.
> > In practice, the MM guys might wish to call cond_resched_rcu_qs() less
> > frequently, but I will leave that to their judgment.  My guess is that
> > the overhead of the cond_resched_rcu_qs() is way down in the noise,
> > but I have been surprised in the past.
> > 
> > Anyway, please give this patch a try and let me know how it goes.
> 
> I am not seeing the full thread in my inbox but I am wondering what is
> actually going on here. The reclaim path (shrink_node_memcg resp.
> shrink_slab should have preemption points and there is not done much
> except of iterating over all memcgs other than that. Are there
> gazillions of memcgs configured (most of them with the low limit
> configured)? In other words is the system configured properly?
> 
> To the patch. I cannot say I would like it. cond_resched_rcu_qs sounds
> way too lowlevel for this usage. If anything cond_resched somewhere inside
> mem_cgroup_iter would be more appropriate to me.

Like this?

							Thanx, Paul

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

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ae052b5e3315..81cb30d5b2fc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -867,6 +867,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 out:
 	if (prev && prev != root)
 		css_put(&prev->css);
+	cond_resched_rcu_qs();
 
 	return memcg;
 }