Re: [PATCH 1/5] block: enable batched allocation for blk_mq_alloc_request()

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 9/24/22 05:54, Jens Axboe wrote:
> On 9/23/22 9:13 AM, Pankaj Raghav wrote:
>> On 2022-09-23 16:52, Pankaj Raghav wrote:
>>> On Thu, Sep 22, 2022 at 12:28:01PM -0600, Jens Axboe wrote:
>>>> The filesystem IO path can take advantage of allocating batches of
>>>> requests, if the underlying submitter tells the block layer about it
>>>> through the blk_plug. For passthrough IO, the exported API is the
>>>> blk_mq_alloc_request() helper, and that one does not allow for
>>>> request caching.
>>>>
>>>> Wire up request caching for blk_mq_alloc_request(), which is generally
>>>> done without having a bio available upfront.
>>>>
>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>> ---
>>>>  block/blk-mq.c | 80 ++++++++++++++++++++++++++++++++++++++++++++------
>>>>  1 file changed, 71 insertions(+), 9 deletions(-)
>>>>
>>> I think we need this patch to ensure correct behaviour for passthrough:
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index c11949d66163..840541c1ab40 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -1213,7 +1213,7 @@ void blk_execute_rq_nowait(struct request *rq, bool at_head)
>>>         WARN_ON(!blk_rq_is_passthrough(rq));
>>>  
>>>         blk_account_io_start(rq);
>>> -       if (current->plug)
>>> +       if (blk_mq_plug(rq->bio))
>>>                 blk_add_rq_to_plug(current->plug, rq);
>>>         else
>>>                 blk_mq_sched_insert_request(rq, at_head, true, false);
>>>
>>> As the passthrough path can now support request caching via blk_mq_alloc_request(),
>>> and it uses blk_execute_rq_nowait(), bad things can happen at least for zoned
>>> devices:
>>>
>>> static inline struct blk_plug *blk_mq_plug( struct bio *bio)
>>> {
>>> 	/* Zoned block device write operation case: do not plug the BIO */
>>> 	if (bdev_is_zoned(bio->bi_bdev) && op_is_write(bio_op(bio)))
>>> 		return NULL;
>>> ..
>>
>> Thinking more about it, even this will not fix it because op is
>> REQ_OP_DRV_OUT if it is a NVMe write for passthrough requests.
>>
>> @Damien Should the condition in blk_mq_plug() be changed to:
>>
>> static inline struct blk_plug *blk_mq_plug( struct bio *bio)
>> {
>> 	/* Zoned block device write operation case: do not plug the BIO */
>> 	if (bdev_is_zoned(bio->bi_bdev) && !op_is_read(bio_op(bio)))
>> 		return NULL;
> 
> That looks reasonable to me. It'll prevent plug optimizations even
> for passthrough on zoned devices, but that's probably fine.

Could do:

	if (blk_op_is_passthrough(bio_op(bio)) ||
	    (bdev_is_zoned(bio->bi_bdev) && op_is_write(bio_op(bio))))
		return NULL;

Which I think is way cleaner. No ?
Unless you want to preserve plugging with passthrough commands on regular
(not zoned) drives ?

-- 
Damien Le Moal
Western Digital Research