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Date:   Tue, 20 Sep 2022 09:49:33 +0800
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Subject: Re: [PATCH V3 5/7] ublk_drv: consider recovery feature in aborting
 mechanism
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To:     Ming Lei <ming.lei@redhat.com>
Cc:     axboe@kernel.dk, xiaoguang.wang@linux.alibaba.com,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        joseph.qi@linux.alibaba.com
References: <20220913041707.197334-1-ZiyangZhang@linux.alibaba.com>
 <20220913041707.197334-6-ZiyangZhang@linux.alibaba.com>
 <Yyg3KLfQaxbS1miq@T590>
 <9a682fac-f022-1f4d-5c2c-e1f0a84746d8@linux.alibaba.com>
 <YyhhnbrHTJpW4Xcm@T590>
From:   Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
In-Reply-To: <YyhhnbrHTJpW4Xcm@T590>
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Precedence: bulk

On 2022/9/19 20:33, Ming Lei wrote:
>>>> +
>>>> +static void ublk_quiesce_queue(struct ublk_device *ub,
>>>> +		struct ublk_queue *ubq)
>>>> +{
>>>> +	int i;
>>>> +
>>>> +	for (i = 0; i < ubq->q_depth; i++) {
>>>> +		struct ublk_io *io = &ubq->ios[i];
>>>> +
>>>> +		if (!(io->flags & UBLK_IO_FLAG_ACTIVE)) {
>>>> +			struct request *rq = blk_mq_tag_to_rq(
>>>> +					ub->tag_set.tags[ubq->q_id], i);
>>>> +
>>>> +			WARN_ON_ONCE(!rq);
>>>> +			pr_devel("%s: %s rq: qid %d tag %d io_flags %x\n", __func__,
>>>> +					ublk_queue_can_use_recovery_reissue(ubq) ?
>>>> +					"requeue" : "abort",
>>>> +					ubq->q_id, i, io->flags);
>>>> +			if (ublk_queue_can_use_recovery_reissue(ubq))
>>>> +				blk_mq_requeue_request(rq, false);
>>>
>>> This way is too violent.
>>>
>>> There may be just one queue dying, but you requeue all requests
>>> from any queue. I'd suggest to take the approach in ublk_daemon_monitor_work(),
>>> such as, just requeuing requests in dying queue.
>>
>> If we want to start a new process after a crash for USER_RECOVERY, all old ubq_daemons
>> must exit and rqs of all queues have to be requeued/aborted. We cannot let live
>> ubq_daemons run any more because they do not belong to the new process.
> 
> IMO, the old process really can exist, and recently even I got such
> requirement for switching queue from one thread to another.

For now, only one process can open /dev/ublkcX, so a new process is necessary now.

If you think "per ubq_daemon" recovery is reasonable, I can do that in the future
if multiple processes is supported. But I really suggest that we can keep current
design as the first step which assumes all ubq_daemons are exited and a new process
is started, and that really meets our requirement.

BTW, START_USER_RECOVERY has to be reconsidered because we may need to pass a ubq_id
with it.

> 
> What we should do is to get all inflight requests done, and cancel all io
> commands, no matter if the ubq pthread is dead or live.
> 
>>
>> BTW, I really wonder why there could be just one queue dying? All queues must be dying
>> shortly after any ubq_daemon is dying since they are all pthreads in the same process.
> 
> You can't assume it is always so. Maybe one pthread is dead first, and
> others are dying later, maybe just one is dead.

Yes, I know there may be only one pthread is dead while others keep running, but now
ublk_drv only support one process opening the same /dev/ublkcX, so other pthreads
must dead(no matter they are aborted by signal or themselves) later.

> 
> If one queue's pthread is live, you may get trouble by simply requeuing
> the request, that is why I suggest to re-use the logic of
> ublk_daemon_monitor_work/ublk_abort_queue().

Actually, if any ubq_daemon is live, no rqs are requeued, please see the check in
ublk_quiesce_dev(). It always makes sure that ALL ubq_daemons are dying, then it
starts quiesce jobs.

> 
> For stopping device, request queue is frozen in del_gendisk() and all
> in-flight requests are drained, and monitor work provides such
> guarantee.
> 
> For user recovery, monitor work should help you too by aborting one
> queue if it is dying until all requests are drained.

Monitor work can schedule quiesce_work if it finds a dying ubq_daemon.
Then quiesce_work calls ublk_quiesce_dev(). I do this because ublk_quiesce_dev()
has to wait all inflight rqs with ACTIVE set being requeued.

> 
>>
>>>
>>> That said you still can re-use the logic in ublk_abort_queue()/ublk_daemon_monitor_work()
>>> for making progress, just changing aborting request with requeue in
>>> ublk_abort_queue().
>>
>> I get your point, but it may be hard to reuse the logic in ublk_daemon_monitor_work()
>> because:
>> (1) we have to quiesce request queue in ublk_quiesce_dev(). This has to be done with
>>     ub_mutex.
>> (2) ublk_quiesce_dev() cannot be run after rqs are requeued/aborted.
> 
> I don't get your point, the request queue needs to be quiesced once, then
> either inflight requests are requeued if the queue is dying, or completed by
> the queue's pthread if it is live. As you mentioned, in reality, most times,
> all pthreads will be killed, but timing can be different, and I think
> you can not requeue one request if the ubq pthread isn't dying.

I do not requeue rqs with a live ubq_daemon. ublk_quiesce_dev() always starts
after all ubq_daemons are dying.

Regards,
Zhang.