[PATCH V3 6/6] nvmet: use inline bio for passthru fast path

[Chaitanya Kulkarni]

On 10/22/20 08:58, Logan Gunthorpe wrote:
>
>
> On 2020-10-21 7:02 p.m., Chaitanya Kulkarni wrote:
>> In nvmet_passthru_execute_cmd() which is a high frequency function
>> it uses bio_alloc() which leads to memory allocation from the fs pool
>> for each I/O.
>>
>> For NVMeoF nvmet_req we already have inline_bvec allocated as a part of
>> request allocation that can be used with preallocated bio when we
>> already know the size of request before bio allocation with bio_alloc(),
>> which we already do.
>>
>> Introduce a bio member for the nvmet_req passthru anon union. In the
>> fast path, check if we can get away with inline bvec and bio from
>> nvmet_req with bio_init() call before actually allocating from the
>> bio_alloc().
>>
>> This will be useful to avoid any new memory allocation under high
>> memory pressure situation and get rid of any extra work of
>> allocation (bio_alloc()) vs initialization (bio_init()) when
>> transfer len is < NVMET_MAX_INLINE_DATA_LEN that user can configure at
>> compile time.
>>
>> Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni at wdc.com>
>> ---
>>  drivers/nvme/target/nvmet.h    |  1 +
>>  drivers/nvme/target/passthru.c | 20 ++++++++++++++++++--
>>  2 files changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/nvme/target/nvmet.h b/drivers/nvme/target/nvmet.h
>> index 559a15ccc322..408a13084fb4 100644
>> --- a/drivers/nvme/target/nvmet.h
>> +++ b/drivers/nvme/target/nvmet.h
>> @@ -330,6 +330,7 @@ struct nvmet_req {
>>  			struct work_struct      work;
>>  		} f;
>>  		struct {
>> +			struct bio		inline_bio;
>>  			struct request		*rq;
>>  			struct work_struct      work;
>>  			bool			use_workqueue;
>> diff --git a/drivers/nvme/target/passthru.c b/drivers/nvme/target/passthru.c
>> index 496ffedb77dc..32498b4302cc 100644
>> --- a/drivers/nvme/target/passthru.c
>> +++ b/drivers/nvme/target/passthru.c
>> @@ -178,6 +178,14 @@ static void nvmet_passthru_req_done(struct request *rq,
>>  	blk_mq_free_request(rq);
>>  }
>>  
>> +static void nvmet_passthru_bio_done(struct bio *bio)
>> +{
>> +	struct nvmet_req *req = bio->bi_private;
>> +
>> +	if (bio != &req->p.inline_bio)
>> +		bio_put(bio);
>> +}
>> +
>>  static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
>>  {
>>  	int sg_cnt = req->sg_cnt;
>> @@ -186,13 +194,21 @@ static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
>>  	int i;
>>  
>>  	bio = bio_alloc(GFP_KERNEL, min(sg_cnt, BIO_MAX_PAGES));
>> -	bio->bi_end_io = bio_put;
>> +	if (req->transfer_len <= NVMET_MAX_INLINE_DATA_LEN) {
>> +		bio = &req->p.inline_bio;
>> +		bio_init(bio, req->inline_bvec, ARRAY_SIZE(req->inline_bvec));
>> +	} else {
>> +		bio = bio_alloc(GFP_KERNEL, min(sg_cnt, BIO_MAX_PAGES));
>> +	}
>> +
>> +	bio->bi_end_io = nvmet_passthru_bio_done;
> I still think it's cleaner to change bi_endio for the inline/alloc'd
> cases by simply setting bi_endi_io to bio_put() only in the bio_alloc
> case. This should also be more efficient as it's one less indirect call
> and condition for the inline case.
>
> Besides that, the entire series looks good to me.
>
> Reviewed-by: Logan Gunthorpe <logang at deltatee.com>
>
> Logan
>
Sagi/Christoph, any comments on this one ?

This series been sitting out for a while now.