Re: [PATCH 3/3] nvme: start keep-alive after admin queue setup

[Hannes Reinecke <hare@suse.de>]

On 11/20/23 20:05, Sagi Grimberg wrote:
> 
> 
> On 11/20/23 18:01, Hannes Reinecke wrote:
>> On 11/20/23 14:39, Sagi Grimberg wrote:
>>>
>>>> Setting up I/O queues might take quite some time on larger and/or
>>>> busy setups, so KATO might expire before all I/O queues could be
>>>> set up.
>>>> Fix this by start keep alive from the ->init_ctrl_finish() callback,
>>>> and stopping it when calling nvme_cancel_admin_tagset().
>>>
>>> If this is a fix, the title should describe the issue it is fixing, and
>>> the body should say how it is fixing it.
>>>
>>>> Signed-off-by: Hannes Reinecke <hare@suse.de>
>>>> ---
>>>>   drivers/nvme/host/core.c | 6 +++---
>>>>   drivers/nvme/host/fc.c   | 6 ++++++
>>>>   2 files changed, 9 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
>>>> index 62612f87aafa..f48b4f735d2d 100644
>>>> --- a/drivers/nvme/host/core.c
>>>> +++ b/drivers/nvme/host/core.c
>>>> @@ -483,6 +483,7 @@ EXPORT_SYMBOL_GPL(nvme_cancel_tagset);
>>>>   void nvme_cancel_admin_tagset(struct nvme_ctrl *ctrl)
>>>>   {
>>>> +    nvme_stop_keep_alive(ctrl);
>>>>       if (ctrl->admin_tagset) {
>>>>           blk_mq_tagset_busy_iter(ctrl->admin_tagset,
>>>>                   nvme_cancel_request, ctrl);
>>>
>>> There is a cross dependency here, now nvme_cancel_admin_tagset needs to
>>> have the keep-alive stopped first, which may be waiting on I/O, which
>>> needs to be cancelled...
>>>
>>> Keep in mind that kato can be arbitrarily long, and now this function
>>> may be blocked on this kato period.
>>>
>>> I also think that now the function is doing something that is more
>>> than simply cancelling the inflight admin tagset, as it is named.
>>>
>> I am having a hard time following this reasoning.
>> While I do accept that nvme_stop_keep_alive() might trigger I/O
>> (ie if the work queue has just been started when calling 
>> cancel_delayed_work), nvme_tcp_error_recovery_work() has this:
>>
>>      nvme_stop_keep_alive(ctrl);
>>      flush_work(&ctrl->async_event_work);
>>      nvme_tcp_teardown_io_queues(ctrl, false);
>>      /* unquiesce to fail fast pending requests */
>>      nvme_unquiesce_io_queues(ctrl);
>>      nvme_tcp_teardown_admin_queue(ctrl, false);
>>
>> and nvme_tcp_teardown_admin_queue() calls nvme_cancel_admin_tagset().
>> So by your above reasoning this code should be wrong, too.
>> What am I missing?
> 
> Need to dig through the history, but it can most definitely move to
> after the teardown. It could be from the earlier days where the
> transport fencing was not as reliable particularly for admin requests.

Ah, no. We are both wrong. Colophon of cancel_delayed_work():

  * Note:
  * The work callback function may still be running on return, unless
  * it returns %true and the work doesn't re-arm itself.  Explicitly 
flush or
  * use cancel_delayed_work_sync() to wait on it.

Hence we won't be blocked by running I/O when calling 
nvme_stop_keep_alive().
So the correct syntax would indeed be calling nvme_stop_keep_alive()
first, and then disable/cancel the admin tagset (which will terminate
any outstanding keep-alive requests).

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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