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

[Ming Lei]

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 out-of-date.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 Thus the outdated queue_flags still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.

> 
> 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 outdated queue_flags when later the IO submission or polling routine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.


Thanks,
Ming