Re: [Cluster-devel] [PATCH] DLM: increase socket backlog to avoid hangs with 16 nodes

[Edwin Torok]

> On 2 Feb 2023, at 22:25, Alexander Aring <aahringo@xxxxxxxxxx> wrote:
> 
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> 
> Hi,
> 
> On Thu, Feb 2, 2023 at 10:37 AM Edwin Török <edvin.torok@xxxxxxxxxx> wrote:
>> 
>> On a 16 node virtual cluster with e1000 NICs joining the 12th node prints
>> SYN flood warnings for the DLM port:
>> Dec 21 01:46:41 localhost kernel: [ 2146.516664] TCP: request_sock_TCP: Possible SYN flooding on port 21064. Sending cookies.  Check SNMP counters.
>> 
>> And then joining a DLM lockspace hangs:
>> ```
>> Dec 21 01:49:00 localhost kernel: [ 2285.780913] INFO: task xapi-clusterd:17638 blocked for more than 120 seconds.                                                                     │
>> Dec 21 01:49:00 localhost kernel: [ 2285.786476]       Not tainted 4.4.0+10 #1                                                                                                         │
>> Dec 21 01:49:00 localhost kernel: [ 2285.789043] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.                                                             │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794611] xapi-clusterd   D ffff88001930bc58     0 17638      1 0x00000000                                                                      │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794615]  ffff88001930bc58 ffff880025593800 ffff880022433800 ffff88001930c000                                                                  │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794617]  ffff88000ef4a660 ffff88000ef4a658 ffff880022433800 ffff88000ef4a000                                                                  │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794619]  ffff88001930bc70 ffffffff8159f6b4 7fffffffffffffff ffff88001930bd10
>> Dec 21 01:49:00 localhost kernel: [ 2285.794644]  [<ffffffff811570fe>] ? printk+0x4d/0x4f                                                                                              │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794647]  [<ffffffff810b1741>] ? __raw_callee_save___pv_queued_spin_unlock+0x11/0x20                                                           │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794649]  [<ffffffff815a085d>] wait_for_completion+0x9d/0x110                                                                                  │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794653]  [<ffffffff810979e0>] ? wake_up_q+0x80/0x80                                                                                           │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794661]  [<ffffffffa03fa4b8>] dlm_new_lockspace+0x908/0xac0 [dlm]                                                                             │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794665]  [<ffffffff810aaa60>] ? prepare_to_wait_event+0x100/0x100                                                                             │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794670]  [<ffffffffa0402e37>] device_write+0x497/0x6b0 [dlm]                                                                                  │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794673]  [<ffffffff811834f0>] ? handle_mm_fault+0x7f0/0x13b0                                                                                  │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794677]  [<ffffffff811b4438>] __vfs_write+0x28/0xd0                                                                                           │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794679]  [<ffffffff811b4b7f>] ? rw_verify_area+0x6f/0xd0                                                                                      ┤
>> Dec 21 01:49:00 localhost kernel: [ 2285.794681]  [<ffffffff811b4dc1>] vfs_write+0xb1/0x190                                                                                            │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794686]  [<ffffffff8105ffc2>] ? __do_page_fault+0x302/0x420                                                                                   │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794688]  [<ffffffff811b5986>] SyS_write+0x46/0xa0                                                                                             │
>> Dec 21 01:49:00 localhost kernel: [ 2285.794690]  [<ffffffff815a31ae>] entry_SYSCALL_64_fastpath+0x12/0x71
>> ```
>> 
>> The previous limit of 5 seems like an arbitrary number, that doesn't match any
>> known DLM cluster size upper bound limit.
>> 
>> Signed-off-by: Edwin Török <edvin.torok@xxxxxxxxxx>
>> Cc: Christine Caulfield <ccaulfie@xxxxxxxxxx>
>> Cc: David Teigland <teigland@xxxxxxxxxx>
>> Cc: cluster-devel@xxxxxxxxxx
>> ---
>> Notes from 2023:
>>  This patch was initially developed on 21 Dec 2017, and in production use ever since.
>> I expected to drop out of our patchqueue at the next kernel upgrade, however it
>> hasn't, so I probably forgot to send it.
>> 
>> I haven't noticed this bug again with the patch applied, and the previous value
>> of '5' seems like an arbitrary limit not matching any supported upper bounds
>> on DLM cluster sizes, so this patch has (unintentionally) had a 5 year test
>> cycle.
> 
> Correct, I guess the 128 coming from dlm_controld 128 max node
> boundary [0] and I think it's okay to change it to 128, especially if
> you start a cluster every "mostly" node wants to connect() at the same
> time and the backlog of 5 could be not enough to handle them.
> From my understanding the connect() will return -ECONNREFUSED in the
> case of backlog is full. As I see the code of kernel v4.0 dlm at this
> point will _not_ slow down a reattempt to run connect() again, see
> [1].

Thanks for digging into the code, that explains why 'SYN cookies' is a symptom rather than a bug here.

> 
>> 
>> Although the join hanging forever like that may still be a bug, if the SYN cookies
>> consistently trigger it lets try to avoid the bug by avoiding the SYN cookies.
> 
> right, it should work even with 1 as a backlog... after digging into
> the code. My guess is that [2] ran into the MAX_CONNECT_RETRIES
> condition and we do _not_ reset con->retries to 0 if connect() [3]
> fails. If this happens we are kind of stuck and we would not try to
> connect() again. Means you run into the hung task situation, because
> e.g. ping_members() will not succeed.
> 
> The current upstream code does not have any limitations of trying
> retries and will always wait some time to try a connect() again if it
> fails. It should retry to connect so long the cluster manager tells
> dlm kernel that a specific node is still part of a lockspace resource
> and there is a transmission happening to this particular node. However
> I think pumping this value to the maximum amount of possible nodes in
> a cluster is not a bad idea.


Although there are improvements possible in how connect failures are handled, this situation is quite rare outside of
a stress test under lab conditions, and this one-liner is probably the easiest way to improve robustness in this area.

> 
> There is still a question of "why seeing SYN flooding" warning, but as
> I said there are a lot of connect() happening at the beginning at
> mostly the same time starting a cluster and some of them retrying
> connect() fast.

Yes, I think the SYN flood warning means just that: too many connects in a very short timeframe
and backlog exceeded, i.e. a symptom rather than a cause.

Best regards,
--Edwin

> 
> ...
> 
> - Alex
> 
> [0] https://pagure.io/dlm/blob/c1e2ee574794e0dea68fd45783991ef7b1835901/f/dlm_controld/dlm_daemon.h#_177
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/dlm/lowcomms.c?h=v4.0#n1226
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/dlm/lowcomms.c?h=v4.0#n1168
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/dlm/lowcomms.c?h=v4.0#n1220
>