Re: [PATCH] block: introduce QUEUE_FLAG_POLL_CAP flag

[JeffleXu <jefflexu@linux.alibaba.com>]

On 4/16/21 5:07 PM, Ming Lei wrote:
> On Fri, Apr 16, 2021 at 04:00:37PM +0800, Jeffle Xu wrote:
>> Hi,
>> How about this patch to remove the extra poll_capable() method?
>>
>> And the following 'dm: support IO polling for bio-based dm device' needs
>> following change.
>>
>> ```
>> +       /*
>> +        * Check for request-based device is remained to
>> +        * dm_mq_init_request_queue()->blk_mq_init_allocated_queue().
>> +        * For bio-based device, only set QUEUE_FLAG_POLL when all underlying
>> +        * devices supporting polling.
>> +        */
>> +       if (__table_type_bio_based(t->type)) {
>> +               if (dm_table_supports_poll(t)) {
>> +                       blk_queue_flag_set(QUEUE_FLAG_POLL_CAP, q);
>> +                       blk_queue_flag_set(QUEUE_FLAG_POLL, q);
>> +               }
>> +               else {
>> +                       blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
>> +                       blk_queue_flag_clear(QUEUE_FLAG_POLL_CAP, q);
>> +               }
>> +       }
>> ```
> 
> Frankly speaking, I don't see any value of using QUEUE_FLAG_POLL_CAP for
> DM, and the result is basically subset of treating DM as always being capable
> of polling.
> 
> Also underlying queue change(either limits or flag) won't be propagated
> to DM/MD automatically. Strictly speaking it doesn't matter if all underlying
> queues are capable of supporting polling at the exact time of 'write sysfs/poll',
> cause any of them may change in future.
> 
> So why not start with the simplest approach(always capable of polling)
> which does meet normal bio based polling requirement?
> 

I find one scenario where this issue may matter. Consider the scenario
where HIPRI bios are submitted to DM device though **all** underlying
devices has been disabled for polling. In this case, a **valid** cookie
(pid of current submitting process) is still returned. Then if @spin of
the following blk_poll() is true, blk_poll() will get stuck in dead loop
because blk_mq_poll() always returns 0, since previously submitted bios
are all enqueued into IRQ hw queue.

Maybe you need to re-remove the bio from the poll context if the
returned cookie is BLK_QC_T_NONE?


Something like:

-static blk_qc_t __submit_bio_noacct(struct bio *bio)
+static blk_qc_t __submit_bio_noacct_ctx(struct bio *bio, struct
io_context *ioc)
 {
 	struct bio_list bio_list_on_stack[2];
 	blk_qc_t ret = BLK_QC_T_NONE;
@@ -1047,7 +1163,15 @@ static blk_qc_t __submit_bio_noacct(struct bio *bio)
 		bio_list_on_stack[1] = bio_list_on_stack[0];
 		bio_list_init(&bio_list_on_stack[0]);

		if (ioc && queue_is_mq(q) && (bio->bi_opf & REQ_HIPRI)) {
			bool queued = blk_bio_poll_prep_submit(ioc, bio);

			ret = __submit_bio(bio);
+			if (queued && !blk_qc_t_valid(ret))
				/* TODO:remove bio from poll_context */
				
				bio_set_private_data(bio, ret);
		} else {
			ret = __submit_bio(bio);
		}


Then if you'd like fix this in this way, the returned value of
.submit_bio() of DM/MD also needs to return BLK_QC_T_NONE now. Currently
.submit_bio() of DM actually returns the cookie of the last split bio
(to underlying mq deivice).

-- 
Thanks,
Jeffle