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

Jens Axboe
2022-09-24T01:01:24Z

On 9/23/22 6:59 PM, Damien Le Moal wrote:
> 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 ?

We most certainly do, without plugging this whole patchset is not
functional. Nor is batched dispatch, for example.

--
Jens Axboe