Re: [PATCH 0/5] blk-mq: allow to run queue if queue refcount is held

["jianchao.wang" <jianchao.w.wang@oracle.com>]

Hi Ming

On 4/1/19 6:03 PM, Ming Lei wrote:
> On Mon, Apr 01, 2019 at 05:19:01PM +0800, jianchao.wang wrote:
>> Hi Ming
>>
>> On 4/1/19 11:28 AM, Ming Lei wrote:
>>> On Mon, Apr 01, 2019 at 11:25:50AM +0800, jianchao.wang wrote:
>>>> Hi Ming
>>>>
>>>> On 4/1/19 10:52 AM, Ming Lei wrote:
>>>>>> percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead" state.
>>>>>> percpu_ref_kill() changes the state of a per-cpu counter to the "dead"
>>>>>> state. blk_freeze_queue_start() calls percpu_ref_kill(). blk_cleanup_queue()
>>>>>> already calls blk_set_queue_dying() and that last function calls
>>>>>> blk_freeze_queue_start(). So I think that what you wrote is not correct and
>>>>>> that inserting a percpu_ref_tryget_live()/percpu_ref_put() pair in
>>>>>> blk_mq_run_hw_queues() or blk_mq_run_hw_queue() would make a difference and
>>>>>> also that moving the percpu_ref_exit() call into blk_release_queue() makes
>>>>>> sense.
>>>>> If percpu_ref_exit() is moved to blk_release_queue(), we still need to
>>>>> move freeing of hw queue's resource into blk_release_queue() like what
>>>>> the patchset is doing.
>>>>>
>>>>> Then we don't need to get/put q_usage_counter in blk_mq_run_hw_queues() any more,
>>>>> do we?
>>>>
>>>> IMO, if we could get a way to prevent any attempt to run queue, it would be
>>>> better and clearer.
>>>
>>> It is hard to do that way, and not necessary.
>>>
>>> I will post V2 soon for review.
>>>
>>
>> Put percpu_ref_tryget/put pair into blk_mq_run_hw_queues could stop run queue after
>> requet_queue is frozen and drained (run queue is also unnecessary because there is no
>> entered requests). And also percpu_ref_tryget could avoid the io hung issue you mentioned.
>> We have similar one in blk_mq_timeout_work.
> 
> If percpu_ref_tryget() is used, percpu_ref_exit() has to be moved into
> queue's release handler.
> 
> Then we still have to move freeing hctx's resource into hctx or queue's
> release handler, that is exactly what this patch is doing. Then
> percpu_ref_tryget() becomes unnecessary again, right?

I'm not sure about the percpu_ref_exit. Perhaps I have some misunderstanding about it.

From the code of it, it frees the percpu_count and set ref->percpu_count_ptr to __PERCPU_REF_ATOMIC_DEAD.
The comment says 'the caller is responsible for ensuring that @ref is no longer in active use'
But if we use it after kill, does it count a active use ?
Based on the code, the __ref_is_percpu is always false during this, and percpu_ref_tryget will not
touch the freed percpu counter but just the atomic ref->count.

It looks safe.


> 
>>
>> freeze and drain queue to stop new attempt to run queue, blk_sync_queue syncs and stops
>> the started ones, then hctx->run_queue is cleaned totally.
>>
>> IMO, it would be better to have a checkpoint after which there will be no any in-flight
>> asynchronous activities of the request_queue (hctx->run_work, q->requeue_work, q-> timeout_work)
>> and any attempt to start them will fail.
> 
> All are canceled in blk_cleanup_queue(), but not enough, given queue can
> be run in sync mode(such as via plug, direct issue, ...), or driver's
> requeue, such as SCSI's requeue. SCSI's requeue may run other LUN's queue
> just by holding queue's kobject refcount.

Yes, so we need a checkpoint here to ensure the request_queue to enter into a certain state.
We provide a guarantee that all of the activities are stopped after this checkpoint.
It will be convenient for us to do other things following, for example release request_queue's
resource.

Thanks
Jianchao

> 
>>
>> Perhaps, this will be a good change to do this ；）
> 
> However, I don't see it is necessary if we simply move freeing hctx's
> resource into its release handler, just like V2.
> 
> 
> Thanks,
> Ming
>