[dm-devel] [PATCH] block: introduce QUEUE_FLAG_POLL_CAP flag

[JeffleXu]

On 4/19/21 10:21 AM, Ming Lei wrote:
> On Sat, Apr 17, 2021 at 10:06:53PM +0800, JeffleXu wrote:
>>
>>
>> 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?
> 
> It won't be one issue, see blk_bio_poll_preprocess() which is called
> from submit_bio_checks(), so any bio's HIPRI will be cleared if the
> queue doesn't support POLL, that code does cover underlying bios.

Sorry there may be some confusion in my description. Let's discuss in
the following scenario: MD/DM advertise QUEUE_FLAG_POLL, though **all**
underlying devices are without QUEUE_FLAG_POLL. This scenario is
possible, if you want to enable MD/DM's polling without checking the
capability of underlying devices.

In this case, it seems that REQ_HIPRI is kept for both MD/DM and
underlying blk-mq devices. I used to think that REQ_HIPRI will be
cleared for underlying blk-mq deivces, but now it seems that REQ_HIPRI
of bios submitted to underlying blk-mq deivces won't be cleared, since
submit_bio_checks() is only called in the entry of submit_bio(), not in
the while() loop of __submit_bio_noacct_ctx(). Though these underlying
blk-mq devices don't support IO polling at all, or they all have been
disabled for polling, REQ_HIPRI bios are finally submitted down.

Or do I miss something?


> 
>>
>>
>> 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)) {
> 
> REQ_HIPRI won't be set for underlying bios which queue doesn't support
> poll, so this branch won't be reached. 

Sorry I missed the '(bio->bi_opf & REQ_HIPRI)' condition here. Indeed
bio without REQ_HIPRI won't be enqueued into the poll_context.

> And the submission queue will
> be empty, and blk_poll() for DM/MD(bio based queue) checks nothing, but
> the polling won't be stopped until the iocb is completed. And this
> handling is actually same with current polling on IRQ based queue.
> 
>> 			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).
> 
> I am a bit confused, this patch requires .submit_bio() of DM/MD(bio
> based queue) to return either 0 or pid of the submission task.
> 
> 
> Thanks,
> Ming
> 

-- 
Thanks,
Jeffle