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

[Ming Lei <ming.lei@redhat.com>]

On Thu, Apr 15, 2021 at 07:21:52PM +0800, JeffleXu wrote:
> 
> 
> On 4/15/21 6:06 PM, Ming Lei wrote:
> > 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@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 out-of-date.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 Thus the outdated queue_flags still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?
> > 
> 
> The IO path has no problem. It is the control path. If you want to turn

Can you explain a bit what the control path is?

> on polling then, you have to check if the device capable of polling,
> while QUEUE_FLAG_POLL has been cleared in this case. IOW you can't rely
> on QUEUE_FLAG_POLL to see if the device has the **ability** of polling.
> QUEUE_FLAG_POLL flag only indicates if polling is turned on or off
> currently.

For bio based driver, I'd suggest to start with do polling simply if
QUEUE_FLAG_POLL is set in bio request queue flag. The flag can be
enabled/disabled during initialization, or via sysfs. That said we
can start with always thinking the bio queue is capable of io polling.


Thanks,
Ming