[dm-devel] Question about iscsi session block

[michael.christie at oracle.com]

On 2/15/22 7:28 PM, Zhengyuan Liu wrote:
> On Wed, Feb 16, 2022 at 12:31 AM Mike Christie
> <michael.christie at oracle.com> wrote:
>>
>> On 2/15/22 9:49 AM, Zhengyuan Liu wrote:
>>> Hi, all
>>>
>>> We have an online server which uses multipath + iscsi to attach storage
>>> from Storage Server. There are two NICs on the server and for each it
>>> carries about 20 iscsi sessions and for each session it includes about 50
>>>  iscsi devices (yes, there are totally about 2*20*50=2000 iscsi block devices
>>>  on the server). The problem is: once a NIC gets faulted, it will take too long
>>> (nearly 80s) for multipath to switch to another good NIC link, because it
>>> needs to block all iscsi devices over that faulted NIC firstly. The callstack is
>>>  shown below:
>>>
>>>     void iscsi_block_session(struct iscsi_cls_session *session)
>>>     {
>>>         queue_work(iscsi_eh_timer_workq, &session->block_work);
>>>     }
>>>
>>>  __iscsi_block_session() -> scsi_target_block() -> target_block() ->
>>>   device_block() ->  scsi_internal_device_block() -> scsi_stop_queue() ->
>>>  blk_mq_quiesce_queue()>synchronize_rcu()
>>>
>>> For all sessions and all devices, it was processed sequentially, and we have
>>> traced that for each synchronize_rcu() call it takes about 80ms, so
>>> the total cost
>>> is about 80s (80ms * 20 * 50). It's so long that the application can't
>>> tolerate and
>>> may interrupt service.
>>>
>>> So my question is that can we optimize the procedure to reduce the time cost on
>>> blocking all iscsi devices?  I'm not sure if it is a good idea to increase the
>>> workqueue's max_active of iscsi_eh_timer_workq to improve concurrency.
>>
>> We need a patch, so the unblock call waits/cancels/flushes the block call or
>> they could be running in parallel.
>>
>> I'll send a patchset later today so you can test it.
> 
> I'm glad to test once you push the patchset.
> 
> Thank you, Mike.

I forgot I did this recently :)

commit 7ce9fc5ecde0d8bd64c29baee6c5e3ce7074ec9a
Author: Mike Christie <michael.christie at oracle.com>
Date:   Tue May 25 13:18:09 2021 -0500

    scsi: iscsi: Flush block work before unblock
    
    We set the max_active iSCSI EH works to 1, so all work is going to execute
    in order by default. However, userspace can now override this in sysfs. If
    max_active > 1, we can end up with the block_work on CPU1 and
    iscsi_unblock_session running the unblock_work on CPU2 and the session and
    target/device state will end up out of sync with each other.
    
    This adds a flush of the block_work in iscsi_unblock_session.


It was merged in 5.14.