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; } -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
