[ClusterLabs] corosync 2.4 CPG config change callback

Wed Mar 7 07:04:39 EST 2018

First thanks for your answer!

On 3/7/18 11:16 AM, Jan Friesse wrote:
> Thomas,
> 
> 
>> Hi,
>>
>> first some background info for my questions I'm going to ask:
>> We use corosync as a basis for our distributed realtime configuration
>> file system (pmxcfs)[1].
> 
> nice
> 
>>
>> We got some reports of a completely hanging FS with the only
>> correlations being high load, often IO, and most times a message that
>> corosync did not got scheduled for longer than the token timeout.
>>
>> See this example from a three node cluster, first:
>>
>>> Mar 01 13:07:56 ceph05-01-public corosync[1638]: warning [MAIN  ] Corosync main process was not scheduled for 3767.3159 ms (threshold is 1320.0000 ms). Consider token timeout increase.
>>
>> then we get a few logs that JOIN or LEAVE messages were thrown away
>> (understandable for this event):
>>
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]: warning [TOTEM ] JOIN or LEAVE message was thrown away during flush operation.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [MAIN  ] Corosync main process was not scheduled for 3767.3159 ms (threshold is 1320.0000 ms). Consider token timeout increase.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] JOIN or LEAVE message was thrown away during flush operation.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] JOIN or LEAVE message was thrown away during flush operation.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] JOIN or LEAVE message was thrown away during flush operation.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] JOIN or LEAVE message was thrown away during flush operation.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] JOIN or LEAVE message was thrown away during flush operation.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]: notice  [TOTEM ] A new membership (192.168.21.51:2324) was formed. Members joined: 2 3 left: 2 3
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]: notice  [TOTEM ] Failed to receive the leave message. failed: 2 3
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] A new membership (192.168.21.51:2324) was formed. Members joined: 2 3 left: 2 3
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [TOTEM ] Failed to receive the leave message. failed: 2 3
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]: notice  [QUORUM] Members[3]: 1 2 3
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]: notice  [MAIN  ] Completed service synchronization, ready to provide service.
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [QUORUM] Members[3]: 1 2 3
>> Mar 01 13:07:56 ceph05-01-public corosync[1638]:  [MAIN  ] Completed service synchronization, ready to provide service.
>>
>> Until recently we stepped really in the dark and had everything from
>> Kernel bugs to our filesystem logic as possible cause in mind...  But
>> then we had the luck to trigger this in our test systems and went to
>> town with gdb on the core dump, finding that we can trigger this by
>> pausing the leader (from our FS POV) for a short moment (may be shorter
>> than the token timeout), so that a new leader get elected, and then
>> resuming our leader node VM again.
>>
>> The problem I saw was that while the leader had a log entry which
>> proved that he noticed his blackout:
>>> [TOTEM ] A new membership (192.168.21.51:2324) was formed. Members joined: 2 3 left: 2 3
> 
> I know it looks weird but it's perfectly fine and expected.
> 

It seemed OK, from this nodes POV, just the missing config change CB
was a bit odd to us.

>>
>> our FS cpg_confchg_fn callback[2] was never called, thus it thought it
> 
> That shouldn't happen
> 

So we really should get a config change CB on the paused node after
unpausing, with all other (online) nodes in both leave and join member
list?
Just asking again to confirm my thinking and that I did not misunderstood
you. :)

>> was still in sync and nothing ever happened, until another member
>> triggered this callback, by either leaving or (re-)joining.
>>
>> Looking in the cpg.c code I saw that there's another callback, namely
>> cpg_totem_confchg_fn, which seemed a bit odd as wew did not set that
> 
> This callback is not necessary to have as long as information about cpg group is enough. cpg_totem_confchg_fn contains information about all processors (nodes).
> 

OK, make sense.

>> one... (I ain't the original author of the FS and it predates at least
>> to 2010, so maybe cpg_initialize was not yet deprecated there, and
>> thus model_initialize was not used then)
> 
>>
>> I switched over to using cpg_model_initialize and set the totem_confchg
>> callback, but for the "blacked out node" it gets called twice after the
>> event, but always shows all members...
>>
>> So to finally get to my questions:
>>
>> * why doesn't get the cpg_confchg_fn CB called when a node has a short
>>   blackout (i.e., corosync not being scheduled for a bit of time)?
>>   having all other nodes in it's leave and join list, as the log
>>   would suggests ("Members joined: 2 3 left: 2 3")
> 
> I believe it was called but not when corosync was paused.
> 

But I never see my registered callback called after unpausing and
also when printing out the dispatch_data->id inside cpg_dispatch
I do not get one with "MESSAGE_RES_CPG_CONFCHG_CALLBACK" after 
unpausing only a "MESSAGE_RES_CPG_TOTEM_CONFCHG_CALLBACK"

>>
>> * If that doesn't seems like a good idea, what can we use to really
>>   detect such a node blackout?
> 
> It's not possible to detect from the affected node, but it must be detected from other nodes.
> 

You mean not directly, as in: we cannot differ between a legitimate fall
out and such a pause, caused artificially or by real high IO.
That makes sense.

>>
>> As a work around I added logic for when through a config change a node
>> with a lower ID joined. The node which was leader until then triggers
>> a CPG leave enforcing a cluster wide config change event to happen,
>> which this time also the blacked out node gets and syncs then again
>> This works, but does not feels really nice, IMO...
> 
> Ok let me explain what exactly happened in your test and simplify it to two nodes:
> 
> - Node A and B are running corosync and same cpg application
> - Node A is paused for time > token_timeout
> - Node B detects that node A is not responding
>   - Creates new membership (with only node B)
>   - Sends notification to cpg app about Node A leave
> - Node A is still paused so it cannot send notification into cpg app
> - Node A is unpaused
>   - Pause is detected and start forming new membership
>   - From node A point of view, node B left - simply because node never considers itself as left
>   - Node A is able to contact node B so final membership is A, B.
>   - Node A can finally deliver two cpg conf changes into cpg application. One about left of node B and second one about join of node B.
> 

Exactly the last point is missing here, and I'd also expect it
to happen... I only get the config change on Node B for the Node
A join, but Node A logs (as stated)... :/

> Now it's really cpg application problem to synchronize its data. Many applications (usually FS) are using quorum together with fencing to find out, which cluster partition is quorate and clean inquorate one.
> 
> Hopefully my explanation help you and feel free to ask more questions!
> 

They help, but I'm still a bit unsure about why the CB could not happen here,
may need to dive a bit deeper into corosync :)

> Regards,
>   Honza
> 
>>
>> help would be appreciated, much thanks!
>>
>> cheers,
>> Thomas
>>
>> [1]: https://git.proxmox.com/?p=pve-cluster.git;a=tree;f=data/src;h=e5493468b456ba9fe3f681f387b4cd5b86e7ca08;hb=HEAD
>> [2]: https://git.proxmox.com/?p=pve-cluster.git;a=blob;f=data/src/dfsm.c;h=cdf473e8226ab9706d693a457ae70c0809afa0fa;hb=HEAD#l1096
>>