[dm-devel] Question about iscsi session block

[Mike Christie]

On 2/15/22 8:19 PM, michael.christie at oracle.com wrote:
> 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.

Hey, I found one more bug when max_active > 1. While fixing it I decided to just
fix this so we can do the sessions recoveries in parallel and the user doesn't have
to worry about setting max_active.

I'll send a patchset and cc you.