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

[JeffleXu]

On 4/19/21 9:36 PM, Ming Lei wrote:
> On Mon, Apr 19, 2021 at 01:40:21PM +0800, JeffleXu wrote:
>>
>>
>> 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?
> 
> No matter the loop, the bios are actually submitted to the
> current->bio_list via submit_bio_noacct() or submit_bio().
> 'grep -r submit_bio drivers/md' will show you the point.

Oops. I forgot that. Thanks and sorry for the noise.

So if that's the case, it seems that patch 11/12 are not needed anymore.

-- 
Thanks,
Jeffle