[dm-devel] [PATCH V5 11/12] block: add poll_capable method to support bio-based IO polling

[JeffleXu]

On 4/14/21 7:24 PM, Ming Lei wrote:
> On Wed, Apr 14, 2021 at 04:38:25PM +0800, JeffleXu wrote:
>>
>>
>> On 4/12/21 5:38 PM, Christoph Hellwig wrote:
>>> On Thu, Apr 01, 2021 at 10:19:26AM +0800, Ming Lei wrote:
>>>> From: Jeffle Xu <jefflexu at linux.alibaba.com>
>>>>
>>>> This method can be used to check if bio-based device supports IO polling
>>>> or not. For mq devices, checking for hw queue in polling mode is
>>>> adequate, while the sanity check shall be implementation specific for
>>>> bio-based devices. For example, dm device needs to check if all
>>>> underlying devices are capable of IO polling.
>>>>
>>>> Though bio-based device may have done the sanity check during the
>>>> device initialization phase, cacheing the result of this sanity check
>>>> (such as by cacheing in the queue_flags) may not work. Because for dm
>>>> devices, users could change the state of the underlying devices through
>>>> '/sys/block/<dev>/io_poll', bypassing the dm device above. In this case,
>>>> the cached result of the very beginning sanity check could be
>>>> out-of-date. Thus the sanity check needs to be done every time 'io_poll'
>>>> is to be modified.
>>>
>>> I really don't think thi should be a method, and I really do dislike
>>> how we have all this "if (is_mq)" junk.  Why can't we have a flag on
>>> the gendisk that signals if the device can support polling that
>>> is autoamtically set for blk-mq and as-needed by bio based drivers?
>>
>> That would consume one more bit of queue->queue_flags.
>>
>> Besides, DM/MD is somehow special here that when one of the underlying
>> devices is disabled polling through '/sys/block/<dev>/io_poll',
>> currently there's no mechanism notifying the above MD/DM to clear the
>> previously set queue_flags. Thus the outdated queue_flags still
>> indicates this DM/MD is capable of polling, while in fact one of the
>> underlying device has been disabled for polling.
> 
> Right, just like there isn't queue limit progagation.
> 
> Another blocker could be that bio based queue doesn't support queue
> freezing.

Do you mean the queue freezing is called in the following code snippet?

```
static ssize_t queue_poll_store(struct request_queue *q, const char
*page, size_t count)
{
	...
	if (poll_on) {
		blk_queue_flag_set(QUEUE_FLAG_POLL, q);
	} else {
		blk_mq_freeze_queue(q);
		blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
		blk_mq_unfreeze_queue(q);
	}
```

And I can't understand how bio-based queue doesn't support queue freezing.

```
submit_bio_noacct
	__submit_bio_noacct
		bio_queue_enter
```

Every time submitting a bio, bio_queue_enter() will be called, and once
the queue has been frozen, bio_queue_enter() will wait there until the
queue is unfrozen.

> 
>>
>> Mike had ever suggested that we can trust the queue_flag, and clear the
>> outdated queue_flags when later the IO submission or polling routine
>> finally finds that the device is not capable of polling. Currently
>> submit_bio_checks() will silently clear the REQ_HIPRI flag and still
>> submit the bio when the device is actually not capable of polling. To
>> fix the issue, could we break the submission and return an error code in
>> submit_bio_checks() if the device is not capable of polling when
>> submitting HIPRI bio?
> 
> I think we may just leave it alone, if underlying queue becomes not pollable,
> the bio still can be submitted & completed via IRQ, just not efficient enough.

Yes it still works. I agree if there's no better solution...

And what about the issue Christoph originally concerned? Do we use one
more flag bit indicating if the queue capable of polling, or the
poll_capable() method way?

-- 
Thanks,
Jeffle