kernel crashes with btrfs and busy database IO - how to debug?

[Tomasz Chmielewski <tch@virtall.com>]

I have a server where I've installed a couple of LXC guests, btrfs - so 
easy to test things with snapshots. Or so it seems.

Unfortunately the box crashes when I put "too much IO load" - with too 
much load being these two running at the same time:

- quite busy MySQL database (doing up to 100% IO wait when running 
alone)
- busy mongo database (doing up to 100% IO wait when running alone)

With both mongo and mysql running at the same time, it crashes after 1-2 
days (tried kernels 4.0.4, 4.0.5, 4.1-rc7 from Ubuntu "kernel-ppa"). It 
does not crash if I only run mongo, or only mysql. There is plenty of 
memory available (just around 2-4 GB used out of 32 GB) when it crashes.

As the box is only reachable remotely, I'm not able to catch a crash.
Sometimes, I'm able to get a bit of it printed via remote SSH, like 
here:

[162276.341030] BUG: unable to handle kernel NULL pointer dereference at 
0000000000000008
[162276.341069] IP: [<ffffffff810c06cd>] 
prepare_to_wait_event+0xcd/0x100
[162276.341096] PGD 80a15e067 PUD 6e08c2067 PMD 0
[162276.341116] Oops: 0002 [#1] SMP
[162276.341133] Modules linked in: xfs libcrc32c xt_conntrack veth 
xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4 
iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat 
nf_conntrack xt_tcpudp iptable_filter ip_tables x_tables bridge stp llc 
intel_rapl iosf_mbi x86_pkg_temp_thermal intel_powerclamp coretemp 
kvm_intel kvm crct10dif_pclmul crc32_pclmul ghash_clmulni_intel 
aesni_intel aes_x86_64 lrw eeepc_wmi gf128mul asus_wmi glue_helper 
sparse_keymap ablk_helper cryptd ie31200_edac shpchp lpc_ich edac_core 
mac_hid 8250_fintek tpm_infineon wmi serio_raw video lp parport btrfs 
raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor 
async_tx xor raid6_pq e1000e raid1 raid0 ahci ptp libahci multipath 
pps_core linear [last unloaded: xfs]
[162276.341394] CPU: 6 PID: 12853 Comm: mysqld Not tainted 
4.1.0-040100rc7-generic #201506080035
[162276.341428] Hardware name: System manufacturer System Product 
Name/P8B WS, BIOS 0904 10/24/2011
[162276.341463] task: ffff8800730d8a10 ti: ffff88047a0f8000 task.ti: 
ffff88047a0f8000
[162276.341495] RIP: 0010:[<ffffffff810c06cd>]  [<ffffffff810c06cd>] 
prepare_to_wait_event+0xcd/0x100
[162276.341532] RSP: 0018:ffff88047a0fbcd8  EFLAGS: 00010046
[162276.341583] RDX: ffff88047a0fbd48 RSI: ffff8800730d8a10 RDI: 
ffff8801e2f96ee8
[162276.341615] RBP: ffff88047a0fbd08 R08: 0000000000000000 R09: 
0000000000000001
[162276.341646] R10: 0000000000000001 R11: 0000000000000000 R12: 
ffff8801e2f96ee8
[162276.341678] R13: 0000000000000002 R14: ffff8801e2f96e60 R15: 
ffff8806b513f248
[162276.341709] FS:  00007f9f2bbd3700(0000) GS:ffff88082fb80000(0000) 
knlGS:0000000000000000

Remote syslog does not capture anything.

The above crash does not point at btrfs - although the box does not 
crash with the same tests done on ext4. The box passes memtests and is 
generally stable otherwise.

How can I debug this further?


"prepare_to_wait_event" can be found here in 4.1-rc7 kernel:

include/linux/wait.h:           long __int = prepare_to_wait_event(&wq, 
&__wait, state);\
include/linux/wait.h:long prepare_to_wait_event(wait_queue_head_t *q, 
wait_queue_t *wait, int state);
kernel/sched/wait.c:long prepare_to_wait_event(wait_queue_head_t *q, 
wait_queue_t *wait, int state)
kernel/sched/wait.c:EXPORT_SYMBOL(prepare_to_wait_event);



-- 
Tomasz Chmielewski
http://wpkg.org