Re: Soft lockup in __slab_free (SLUB)

[Nikolay Borisov <kernel@kyup.com>]

On 09/28/2016 08:31 AM, Joonsoo Kim wrote:
> Hello,
> 
> Ccing Paul, because it looks like RCU problem.
> 
> On Mon, Sep 26, 2016 at 10:46:56AM +0300, Nikolay Borisov wrote:
>> Hello, 
>>
>> On 4.4.14 stable kernel I observed the following soft-lockup, however I
>> also checked that the code is the same in 4.8-rc so the problem is 
>> present there as well: 
>>
>> [434575.862377] NMI watchdog: BUG: soft lockup - CPU#13 stuck for 23s! [swapper/13:0]
>> [434575.866352] CPU: 13 PID: 0 Comm: swapper/13 Tainted: P           O    4.4.14-clouder5 #2
>> [434575.866643] Hardware name: Supermicro X9DRD-iF/LF/X9DRD-iF, BIOS 3.0b 12/05/2013
>> [434575.866932] task: ffff8803714aadc0 ti: ffff8803714c4000 task.ti: ffff8803714c4000
>> [434575.867221] RIP: 0010:[<ffffffff81613f4c>]  [<ffffffff81613f4c>] _raw_spin_unlock_irqrestore+0x1c/0x30
>> [434575.867566] RSP: 0018:ffff880373ce3dc0  EFLAGS: 00000203
>> [434575.867736] RAX: ffff88066e0c9a40 RBX: 0000000000000203 RCX: 0000000000000000
>> [434575.868023] RDX: 0000000000000008 RSI: 0000000000000203 RDI: ffff88066e0c9a40
>> [434575.868311] RBP: ffff880373ce3dc8 R08: ffff8803e5c1d118 R09: ffff8803e5c1d538
>> [434575.868609] R10: 0000000000000000 R11: ffffea000f970600 R12: ffff88066e0c9a40
>> [434575.868895] R13: ffffea000f970600 R14: 000000000046cf3b R15: ffff88036f8e3200
>> [434575.869183] FS:  0000000000000000(0000) GS:ffff880373ce0000(0000) knlGS:0000000000000000
>> [434575.869472] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [434575.869643] CR2: ffffffffff600400 CR3: 0000000367201000 CR4: 00000000001406e0
>> [434575.869931] Stack:
>> [434575.870095]  ffff88066e0c9a40 ffff880373ce3e78 ffffffff8117ea8a ffff880373ce3e08
>> [434575.870567]  000000000046bd03 0000000100170017 ffff8803e5c1d118 ffff8803e5c1d118
>> [434575.871037]  00ff000100000000 0000000000000203 0000000000000000 ffffffff8123d9ac
>> [434575.874253] Call Trace:
>> [434575.874418]  <IRQ> 
>> [434575.874473]  [<ffffffff8117ea8a>] __slab_free+0xca/0x290
>> [434575.874806]  [<ffffffff8123d9ac>] ? ext4_i_callback+0x1c/0x20
>> [434575.874978]  [<ffffffff8117ee3a>] kmem_cache_free+0x1ea/0x200
>> [434575.875149]  [<ffffffff8123d9ac>] ext4_i_callback+0x1c/0x20
>> [434575.875325]  [<ffffffff810ad09b>] rcu_process_callbacks+0x21b/0x620
>> [434575.875506]  [<ffffffff81057337>] __do_softirq+0x147/0x310
>> [434575.875680]  [<ffffffff8105764f>] irq_exit+0x5f/0x70
>> [434575.875851]  [<ffffffff81616a82>] smp_apic_timer_interrupt+0x42/0x50
>> [434575.876025]  [<ffffffff816151e9>] apic_timer_interrupt+0x89/0x90
>> [434575.876197]  <EOI> 
>> [434575.876250]  [<ffffffff81510601>] ? cpuidle_enter_state+0x141/0x2c0
>> [434575.876583]  [<ffffffff815105f6>] ? cpuidle_enter_state+0x136/0x2c0
>> [434575.876755]  [<ffffffff815107b7>] cpuidle_enter+0x17/0x20
>> [434575.876929]  [<ffffffff810949fc>] cpu_startup_entry+0x2fc/0x360
>> [434575.877105]  [<ffffffff810330e3>] start_secondary+0xf3/0x100
>>
>> The ip in __slab_free points to this piece of code (in mm/slub.c): 
>>
>> if (unlikely(n)) {
>> 	spin_unlock_irqrestore(&n->list_lock, flags);
>>         n = NULL;
>> }
>>
>> I think it's a pure chance that the spin_unlock_restore is being shown in this trace, 
>> do you think that a cond_resched is needed in this unlikely if clause? Apparently there 
>> are cases where this loop can take a considerable amount of time.
> 
> I think that __slab_free() doesn't take too long time even if there is
> lock contention. And, cond_resched() is valid on softirq context?

Now that I think of it - it's not valid since it might sleep and softirq
is atomic context. So my suggestion is actually invalid, too bad.

> 
> I think that problem would be caused by too many rcu callback is
> executed without scheduling. Paul?

I don't think it's an RCU problem per-se since ext4_i_callback is being
called from RCU due to the way inodes are being freed.

On a slightly different note - on a different physical server, running
zfsonlinux I experienced a very similar issue but without being in an
RCU context, here what the stacktrace looked there:

 Call Trace:
  [<ffffffff8117ea8a>] __slab_free+0xca/0x290
  [<ffffffff8117ee3a>] ? kmem_cache_free+0x1ea/0x200
  [<ffffffff8117ee3a>] kmem_cache_free+0x1ea/0x200
  [<ffffffffa0245c97>] spl_kmem_cache_free+0x147/0x1d0 [spl]
  [<ffffffffa02cbecc>] dnode_destroy+0x1dc/0x230 [zfs]
  [<ffffffffa02cbf64>] dnode_buf_pageout+0x44/0xc0 [zfs]
  [<ffffffffa0248301>] taskq_thread+0x291/0x4e0 [spl]
  [<ffffffff8107ccd0>] ? wake_up_q+0x70/0x70
  [<ffffffffa0248070>] ? taskq_thread_spawn+0x50/0x50 [spl]
  [<ffffffff810715bf>] kthread+0xef/0x110
  [<ffffffff810714d0>] ? kthread_park+0x60/0x60
  [<ffffffff8161483f>] ret_from_fork+0x3f/0x70
  [<ffffffff810714d0>] ? kthread_park+0x60/0x60

It's hard to believe this is a coincidence. I've inspected the callpaths
in the taskq_thread/dnode_buf_pageout/dnode_destroy and there doesn't
seem to be a loop apart from the one in __slab_free.

[SNIP]

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