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

[Ming Lei]

On Thu, Apr 15, 2021 at 05:21:56PM +0800, JeffleXu wrote:
> 
> 
> On 4/15/21 3:43 PM, Ming Lei wrote:
> > On Thu, Apr 15, 2021 at 09:34:36AM +0800, JeffleXu wrote:
> >>
> >>
> >> 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 out-of-date.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 Thus the outdated queue_flags still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);
> >> 	}
> >> ```
> > 
> > Yes, if it is a bio based queue. Or bio queued queue(DM, MD or others) may
> > use freeze_queue to do similar thing.
> > 
> >>
> >> 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.
> > 
> > Not like blk-mq, the refcount is just grabbed during submission for bio based
> > queue. 
> 
> Could you please explain it more detailed ....

Please see __submit_bio(), in which the queue ref is dropped.

> 
> 
> I will research a bit and see if we can extend freeze queue for
> > covering bio based queue. One trouble is that bio is ended before
> > freeing request.
> > 
> >>
> >>>
> >>>>
> >>>> 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?
> > 
> > Just wondering why we can't use QUEUE_FLAG_POLL simply? If user wants to
> > enable it, let's do it for them. And bio driver can start with default poll
> > state by checking underlying queues.
> > 
> 
> Consider the following scenario: QUEUE_FLAG_POLL is set after
> initialization, indicating the device capable of polling; then polling
> is turned off by '/sys/block/<dev>/io_poll', thus QUEUE_FLAG_POLL is
> cleared.

If the flag is cleared, the bio will be submitted to irq queue, what is
the problem?


Thanks, 
Ming